Electric motors: Wikis


Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.


(Redirected to Electric motor article)

From Wikipedia, the free encyclopedia

Electric motors

An electric motor uses electrical energy to produce mechanical energy, very typically through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on vehicles often perform both tasks. Many types of electric motors can be run as generators, and vice versa.

Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current (for example a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give.

The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as 1821. Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electrical distribution networks.

Some devices, such as magnetic solenoids and loudspeakers, although they generate some mechanical power, are not generally referred to as electric motors, and are usually termed actuators[1] and transducers,[2] respectively.


History and development

Electromagnetic experiment of Faraday, ca. 1821.[3]

The principle

The conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821. A free-hanging wire was dipped into a pool of mercury, on which a permanent magnet was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire.[4] This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of devices called homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices only, unsuited to practical applications due to their primitive construction.[citation needed]

Jedlik's "lightning-magnetic self-rotor", 1827. (Museum of Applied Arts, Budapest.)

In 1827, Hungarian Ányos Jedlik started experimenting with electromagnetic rotating devices he called "lightning-magnetic self-rotors". He used them for instructive purposes in universities, and in 1828 demonstrated the first device which contained the three main components of practical direct current motors: the stator, rotor and commutator. Both the stationary and the revolving parts were electromagnetic, employing no permanent magnets.[5][6][7][8][9][10] Again, the devices had no practical application.[citation needed]

The first electric motors

The first commutator-type direct current electric motor capable of turning machinery was invented by the British scientist William Sturgeon in 1832.[11] Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by Americans Emily and Thomas Davenport and patented in 1837. Their motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press.[12] Due to the high cost of the zinc electrodes required by primary battery power, the motors were commercially unsuccessful and the Davenports went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.[citation needed]

In 1855 Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work.[5][7] He built a model electric motor-propelled vehicle that same year.[13] There is no evidence that this experimentation was communicated to the wider scientific world at that time, or that it influenced the development of electric motors in the following decades.[citation needed]

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.[citation needed]

In 1886 Frank Julian Sprague invented the first practical DC motor, a non-sparking motor capable of constant speed under variable loads. Other Sprague electric inventions about this time greatly improved grid electric distribution [prior work done while employed by Edison], allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided controls systems for electric operations. This allowed Sprague to use electric motors to invent the first electric trolley system in 1887-88 in Richmond VA, the electric elevator and control system in 1892, and the electric subway with independently powered centrally controlled cars, which was first installed in 1892 in Chicago by the South Side Elevated Railway where it became popularly known as the "L". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry, while almost simultaneously another great inventor was developing its primary competitor, which would become much more widespread.

In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.[citation needed]

The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of a relatively-small air gap between rotor and stator. Early motors, for some rotor positions, had comparatively huge air gaps which constituted a very high reluctance magnetic circuit. They produced far-lower torque than an equivalent amount of power would produce with efficient designs. The cause of the lack of understanding seems to be that early designs were based on familiarity of distant attraction between a magnet and a piece of ferromagnetic material, or between two electromagnets. Efficient designs, as this article describes, are based on a rotor with a comparatively small air gap, and flux patterns that create torque.[14]

Note that the armature bars are at some distance (unknown) from the field pole pieces when power is fed to one of the field magnets; the air gap is likely to be considerable. The text tells of the inefficiency of the design. (Electricity was created, as a practical matter, by consuming zinc in wet primary cells!)

In his workshops Froment had an electromotive engine of one-horse power. But, though an interesting application of the transformation of energy, these machines will never be practically applied on the large scale in manufactures, for the expense of the acids and the zinc which they use very far exceeds that of the coal in steam-engines of the same force. [...] motors worked by electricity, independently of any question as to the cost of construction, or of the cost of the acids, are at least sixty times as dear to work as steam-engines.

Although Gramme's design was comparatively much more efficient, apparently the Froment motor was still considered illustrative, years later. It is of some interest that the St. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor. Photo of a traditional form of the motor: [3] Note the prominent bar magnets, and the huge air gap at the ends opposite the rotor. Even modern versions still have big air gaps if the rotor poles are not aligned.

Application of electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using shaft, belts, compressed air or hydraulic pressure. Instead every machine could be equipped with its own electric motor, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of all electric energy produced.

Categorization of electric motors

The classic division of electric motors has been that of Alternating Current (AC) types vs Direct Current (DC) types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run on AC power, these motors being referred to as universal motors.

Rated output power is also used to categorise motors, those of less than 746 Watts, for example, are often referred to as fractional horsepower motors (FHP) in reference to the old imperial measurement.

The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation thereof. The two best examples are: the brushless DC motor and the stepping motor, both being poly-phase AC motors requiring external electronic control, although historically, stepping motors (such as for maritime and naval gyrocompass repeaters) were driven from DC switched by contacts.

Considering all rotating (or linear) electric motors require synchronism between a moving magnetic field and a moving current sheet for average torque production, there is a clearer distinction between an asynchronous motor and synchronous types. An asynchronous motor requires slip between the moving magnetic field and a winding set to induce current in the winding set by mutual inductance; the most ubiquitous example being the common AC induction motor which must slip to generate torque. In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production (e.g. permanent magnet motors, synchronous brush-less wound-rotor doubly-fed electric machine).

Comparison of motor types

Comparison of motor types[15]
Type Advantages Disadvantages Typical Application Typical Drive
AC Induction
(Shaded Pole)
Least expensive
Long life
high power
Rotation slips from frequency
Low starting torque
Fans Uni/Poly-phase AC
AC Induction
(split-phase capacitor)
High power
high starting torque
Rotation slips from frequency Appliances Uni/Poly-phase AC
AC Synchronous Rotation in-sync with freq
long-life (alternator)
More expensive Industrial motors
Audio turntables
tape drives
Uni/Poly-phase AC
Stepper DC Precision positioning
High holding torque
Requires a controller Positioning in printers and floppy drives DC
Brushless DC Long lifespan
low maintenance
High efficiency
High initial cost
Requires a controller
Hard drives
CD/DVD players
electric vehicles
Brushed DC Low initial cost
Simple speed control
High maintenance (brushes)
Low lifespan
Treadmill exercisers
automotive starters
Direct DC or PWM
Pancake DC Compact design
Simple speed control
Medium cost
Medium lifespan
Office Equip
Direct DC or PWM

Servo motor

A servomechanism, or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.

Synchronous electric motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip to produce torque. A synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to rotor. The rotor poles connect to each other and move at the same speed hence the name synchronous motor.

Induction motor

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. Another commonly used name is squirrel cage motor because the rotor bars with short circuit rings resemble a squirrel cage (hamster wheel). An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an induction motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Electrostatic motor (capacitor motor)

An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high voltage power supply, although very small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon. Today the electrostatic motor finds frequent use in micro-mechanical (MEMS) systems where their drive voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. Also, the molecular machinery which runs living cells is often based on linear and rotary electrostatic motors.

DC Motors

A DC motor is designed to run on DC electric power. Two examples of pure DC designs are Michael Faraday's homopolar motor (which is uncommon), and the ball bearing motor, which is (so far) a novelty. By far the most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to create an oscillating AC current from the DC source—so they are not purely DC machines in a strict sense.

Brushed DC motors

The classic DC motor design generates an oscillating current in a wound rotor, or armature, with a split ring commutator, and either a wound or permanent magnet stator. A rotor consists of one or more coils of wire wound around a core on a shaft; an electrical power source is connected to the rotor coil through the commutator and its brushes, causing current to flow in it, producing electromagnetism. The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does) but rather keeps rotating indefinitely (as long as power is applied and is sufficient for the motor to overcome the shaft torque load and internal losses due to friction, etc.)

Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. (Sparks are also created inevitably by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections—and hence coil ends—momentarily while crossing the gaps. Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes.) This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the output of the motor. The making and breaking of electric contact also causes electrical noise, and the sparks additionally cause RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance (on larger motors) or replacement (on small motors). The commutator assembly on a large machine is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.

Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking (comparable to the problem of "valve float" in internal combustion engines). (Small brushes are also desirable for lower cost.) Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear.

A: shunt
B: series
C: compound
f = field coil

There are five types of brushed DC motor:

A. DC shunt wound motor

B. DC series wound motor

C. DC compound motor (two configurations):

  • Cumulative compound
  • Differentially compounded

D. Permanent Magnet DC Motor (not shown)

E. Separately-excited (sepex) (not shown).

Brushless DC motors

Some of the problems of the brushed DC motor are eliminated in the brushless design. In this motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switch synchronised to the rotor's position. Brushless motors are typically 85-90% efficient or more (higher efficiency for a brushless electric motor of up to 96.5% were reported by researchers at the Tokai University in Japan in 2009),[16] whereas DC motors with brushgear are typically 75-80% efficient.

Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect sensors to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from either Hall effect sensors or from the back EMF (electromotive force) of the undriven coils. In effect, they act as three-phase synchronous motors containing their own variable-frequency drive electronics. A specialized class of brushless DC motor controllers utilize EMF feedback through the main phase connections instead of Hall effect sensors to determine position and velocity. These motors are used extensively in electric radio-controlled vehicles. When configured with the magnets on the outside, these are referred to by modelists as outrunner motors.

Brushless DC motors are commonly used where precise speed control is necessary, as in computer disk drives or in video cassette recorders, the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products such as fans, laser printers and photocopiers. They have several advantages over conventional motors:

  • Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to much-improved life of the fan's bearings.
  • Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers.
  • The same Hall effect sensors that provide the commutation can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal.
  • The motor can be easily synchronized to an internal or external clock, leading to precise speed control.
  • Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels. Also, sparking generates ozone which can accumulate in poorly ventilated buildings risking harm to occupants' health.
  • Brushless motors are usually used in small equipment such as computers and are generally used to get rid of unwanted heat.
  • They are also very quiet motors which is an advantage if being used in equipment that is affected by vibrations.

Modern DC brushless motors range in power from a fraction of a watt to many kilowatts. Larger brushless motors up to about 100 kW rating are used in electric vehicles. They also find significant use in high-performance electric model aircraft.

Coreless or ironless DC motors

Nothing in the design of any of the motors described above requires that the iron (steel) portions of the rotor actually rotate; torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless or ironless DC motor, a specialized form of a brush or brushless DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only the magnet wire and the bonding material. The rotor can fit inside the stator magnets; a magnetically-soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux. A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically-soft cylinder that can serve as the housing for the motor, and likewise provides a return path for the flux.

Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air.

Related limited-travel actuators have no core and a bonded coil placed between the poles of high-flux thin permanent magnets. These are the fast head positioners for rigid-disk ("hard disk") drives.

Printed Armature or Pancake DC Motors

A rather unique motor design the pancake/printed armature motor has the windings shaped as a disc running between arrays of high-flux magnets, arranged in a circle, facing the rotor and forming an axial air gap. This design is commonly known the pancake motor because of its extremely flat profile, although the technology has had many brand names since it's inception, such as ServoDisc.

The printed armature (originally formed on a printed circuit board) in a printed armature motor is made from punched copper sheets that are laminated together using advanced composites to form a thin rigid disc. The printed armature has a unique construction, in the brushed motor world, in that is does not have a separate ring commutator. The brushes run directly on the armature surface making the whole design very compact.

An alternative manufacturing method is to use wound copper wire laid flat with a central conventional commutator, in a flower and petal shape. The windings are typically stabilized by being impregnated with electrical epoxy potting systems. These are filled epoxies that have moderate mixed viscosity and a long gel time. They are highlighted by low shrinkage and low exotherm, and are typically UL 1446 recognized as a potting compound for use up to 180°C (Class H) (UL File No. E 210549).

The unique advantage of ironless DC motors is that there is no cogging (vibration caused by attraction between the iron and the magnets) and parasitic eddy currents cannot form in the rotor as it is totally ironless. This can greatly improve efficiency, but variable-speed controllers must use a higher switching rate (>40 kHz) or direct current because of the decreased electromagnetic induction.

These motors were originally invented to drive the capstan(s) of magnetic tape drives, in the burgeoning computer industry. Pancake motors are still widely used in high-performance servo-controlled systems, humanoid robotic systems, industrial automation and medical devices. Due to the variety of constructions now available the technology is used in applications from high temperature military to low cost pump and basic servo applications.

Universal motors

A series-wound motor is referred to as a universal motor when it has been designed to operate on either AC or DC power. The ability to operate on AC is because the current in both the field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) in synchronism, and hence the resulting mechanical force will occur in a constant direction.

Operating at normal power line frequencies, universal motors are very rarely larger than one kilowatt (about 1.3 horsepower). Universal motors also form the basis of the traditional railway traction motor in electric railways. In this application, to keep their electrical efficiency high, they were operated from very low frequency AC supplies, with 25 and 16.7 hertz (Hz) operation being common. Because they are universal motors, locomotives using this design were also commonly capable of operating from a third rail powered by DC.

An advantage of the universal motor is that AC supplies may be used on motors which have some characteristics more common in DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. As a result, such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently, and often have high starting-torque demands. Continuous speed control of a universal motor running on AC is easily obtained by use of a thyristor circuit, while (imprecise) stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC).

Universal motors generally run at high speeds, making them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high RPM operation is desirable. They are also commonly used in portable power tools, such as drills, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed 10,000 RPM, while Dremel and other similar miniature grinders will often exceed 30,000 RPM.

Motor damage may occur due to overspeeding (running at an RPM in excess of design limits) if the unit is operated with no significant load. On larger motors, sudden loss of load is to be avoided, and the possibility of such an occurrence is incorporated into the motor's protection and control schemes. In some smaller applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a safe value, as well as a means to circulate cooling airflow over the armature and field windings.

AC motors

In 1882, Nikola Tesla discovered the rotating magnetic field, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, Galileo Ferraris independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Tesla had suggested that the commutators from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine.[17] Tesla would later attain U.S. Patent 0,416,194, Electric Motor (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.

Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor is now used for the vast majority of commercial applications.


A typical AC motor consists of two parts:

  • An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;
  • An inside rotor attached to the output shaft that is given a torque by the rotating field.

Torque motors

A torque motor (also known as a limited torque motor) is a specialized form of induction motor which is capable of operating indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor will apply a steady torque to the load (hence the name).

A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In this application, driven from a low voltage, the characteristics of these motors allow a relatively-constant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, (and so delivering a higher torque), the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches. In the computer gaming world, torque motors are used in force feedback steering wheels.

Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup [18] and, depending on the speed, makes small adjustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being applied to the motor, causing the return spring to pull back and close the throttle.

Slip ring

The slip ring is a component of the wound rotor motor as an induction machine (best evidenced by the construction of the common automotive alternator), where the rotor comprises a set of coils that are electrically terminated in slip rings. These are metal rings rigidly mounted on the rotor, and combined with brushes (as used with commutators), provide continuous unswitched connection to the rotor windings.

In the case of the wound-rotor induction motor, external impedances can be connected to the brushes. The stator is excited similarly to the standard squirrel cage motor. By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered.

(Slip rings are most-commonly used in automotive alternators as well as in synchro angular data-transmission devices, among other applications.)

The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero speed to full speed. This type of motor also offers controllable speed.

Motor speed can be changed because the torque curve of the motor is effectively modified by the amount of resistance connected to the rotor circuit. Increasing the value of resistance will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced.

When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation and net efficiency is also very poor.

Stepper motors

Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a magnetically-soft rotor with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the stepper motor may not rotate continuously; instead, it "steps" — starts and then quickly stops again — from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards, and it may change direction, stop, speed up or slow down arbitrarily at any time.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position between the cog points and thereby rotate extremely smoothly. This mode of operation is often called microstepping. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.

Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for read/write head positioning in computer floppy diskette drives. They were used for the same purpose in pre-gigabyte era computer disk drives, where the precision and speed they offered was adequate for the correct positioning of the read/write head of a hard disk drive. As drive density increased, the precision and speed limitations of stepper motors made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it refers to the structure in a typical (cone type) loudspeaker. This structure was used for a while to position the heads. Modern drives have a pivoted coil mount; the coil swings back and forth, something like a blade of a rotating fan. Nevertheless, like a voice coil, modern actuator coil conductors (the magnet wire) move perpendicular to the magnetic lines of force.)

Stepper motors were and still are often used in computer printers, optical scanners, and digital photocopiers to move the optical scanning element, the print head carriage (of dot matrix and inkjet printers), and the platen. Likewise, many computer plotters (which since the early 1990s have been replaced with large-format inkjet and laser printers) used rotary stepper motors for pen and platen movement; the typical alternatives here were either linear stepper motors or servomotors with complex closed-loop control systems.

So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one coil, draw very little power, and have a permanent-magnet rotor. The same kind of motor drives battery-powered quartz clocks. Some of these watches, such as chronographs, contain more than one stepping motor.

Stepper motors were upscaled to be used in electric vehicles under the term SRM (Switched Reluctance Motor).

Linear motors

A linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing a torque (rotation), it produces a straight-line force along its length by setting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground, and in many roller-coasters where the rapid motion of the motorless railcar is controlled by the rail. On a smaller scale, at least one letter-size (8.5" x 11") computer graphics X-Y pen plotter made by Hewlett-Packard (in the late 1970s to mid 1980's) used two linear stepper motors to move the pen along the two orthogonal axes.

Feeding and windings

Doubly-fed electric motor

Doubly-fed electric motors have two independent multiphase windings that actively participate in the energy conversion process with at least one of the winding sets electronically controlled for variable speed operation. Two is the most active multiphase winding sets possible without duplicating singly-fed or doubly-fed categories in the same package. As a result, doubly-fed electric motors are machines with an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as singly-fed electric machines, which have only one active winding set.

A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings may offset the saving in the power electronics components. Difficulties with controlling speed near synchronous speed limit applications.[19]

Singly-fed electric motor

Singly-fed electric motors incorporate a single multiphase winding set that is connected to a power supply. Singly-fed electric machines may be either induction or synchronous. The active winding set can be electronically controlled. Induction machines develop starting torque at zero speed and can operate as standalone machines. Synchronous machines must have auxiliary means for startup, such as a starting induction squirrel-cage winding or an electronic controller. Singly-fed electric machines have an effective constant torque speed range up to synchronous speed for a given excitation frequency.

The induction (asynchronous) motors (i.e., squirrel cage rotor or wound rotor), synchronous motors (i.e., field-excited, permanent magnet or brushless DC motors, reluctance motors, etc.), which are discussed on this page, are examples of singly-fed motors. By far, singly-fed motors are the predominantly installed type of motors.

Nanotube nanomotor

Researchers at University of California, Berkeley, recently developed rotational bearings based upon multiwall carbon nanotubes. By attaching a gold plate (with dimensions of the order of 100 nm) to the outer shell of a suspended multiwall carbon nanotube (like nested carbon cylinders), they are able to electrostatically rotate the outer shell relative to the inner core. These bearings are very robust; devices have been oscillated thousands of times with no indication of wear. These nanoelectromechanical systems (NEMS) are the next step in miniaturization and may find their way into commercial applications in the future.

See also:


To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:

\eta = \frac{P_m}{P_e},

where η is energy conversion efficiency, Pe is electrical input power, and Pm is mechanical output power.

In simplest case Pe = VI, and Pm = Tω, where V is input voltage, I is input current, T is output torque, and ω is output angular frequency.

In theory, this would imply that the efficiency of a DC motor would peak at 1/2 of the stall torque. However, in practice, the efficiency of a DC motor peaks at a torque that is less than 1/2 of the stall torque.


Because a DC motor operates most efficiently at less than 1/2 its stall torque, an "oversized" motor runs with the highest efficiency. IE: using a bigger motor than is necessary enables the motor to operate closest to no load, or peak operating conditions.

Torque capability of motor types

When optimally designed for a given active current (i.e., torque current), voltage, pole-pair number, excitation frequency (i.e., synchronous speed), and core flux density, all categories of electric motors or generators will exhibit virtually the same maximum continuous shaft torque (i.e., operating torque) within a given physical size of electromagnetic core. Some applications require bursts of torque beyond the maximum operating torque, such as short bursts of torque to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or safe operating temperature rise and voltage, the capacity for torque bursts beyond the maximum operating torque differs significantly between categories of electric motors or generators.

Note: Capacity for bursts of torque should not be confused with Field Weakening capability inherent in fully electromagnetic electric machines (Permanent Magnet (PM) electric machine are excluded). Field Weakening, which is not readily available with PM electric machines, allows an electric machine to operate beyond the designed frequency of excitation without electrical damage.

Electric machines without a transformer circuit topology, such as Field-Wound (i.e., electromagnet) or Permanent Magnet (PM) Synchronous electric machines cannot realize bursts of torque higher than the maximum designed torque without saturating the magnetic core and rendering any increase in current as useless. Furthermore, the permanent magnet assembly of PM synchronous electric machines can be irreparably damaged, if bursts of torque exceeding the maximum operating torque rating are attempted.

Electric machines with a transformer circuit topology, such as Induction (i.e., asynchronous) electric machines, Induction Doubly-Fed electric machines, and Induction or Synchronous Wound-Rotor Doubly-Fed (WRDF) electric machines, exhibit very high bursts of torque because the active current (i.e., Magneto-Motive-Force or the product of current and winding-turns) induced on either side of the transformer oppose each other and as a result, the active current contributes nothing to the transformer coupled magnetic core flux density, which would otherwise lead to core saturation.

Electric machines that rely on Induction or Asynchronous principles short-circuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active (i.e., real) current. Still, bursts of torque that are two to three times higher than the maximum design torque are realizable.

The Synchronous WRDF electric machine is the only electric machine with a truly dual ported transformer circuit topology (i.e., both ports independently excited with no short-circuited port). The dual ported transformer circuit topology is known to be unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the rotor winding set. If a precision means were available to instantaneously control torque angle and slip for synchronous operation during motoring or generating while simultaneously providing brushless power to the rotor winding set (see Brushless wound-rotor doubly-fed electric machine), the active current of the Synchronous WRDF electric machine would be independent of the reactive impedance of the transformer circuit and bursts of torque significantly higher than the maximum operating torque and far beyond the practical capability of any other type of electric machine would be realizable. Torque bursts greater than eight times operating torque have been calculated.


There is an impending shortage of many rare raw materials used in the manufacture of hybrid and electric cars (Nishiyama 2007) (Cox 2008). For example, the rare earth element dysprosium is required to fabricate many of the advanced electric motors used in hybrid cars (Cox 2008). However, over 95% of the world's rare earth elements are mined in China (Haxel et al. 2005), and domestic Chinese consumption is expected to consume China's entire supply by 2012 (Cox 2008).[citation needed]

While permanent magnet motors, favored in hybrids such as those made by Toyota, often use rare earth materials in their magnets, AC traction motors used in production electric vehicles such as the GM EV1, Toyota RAV4 EV and Tesla Roadster do not use permanent magnets or the associated rare earth materials. AC motors typically use conventional copper wire for their stator coils and copper or aluminum rods or bars for their rotor. AC motors do not significantly use rare earth materials.

Motor standards

The following are major design and manufacturing standards covering electric motors:


Electric motors are used in many, if not most, modern machines. Obvious uses would be in rotating machines such as fans, turbines, drills, the wheels on electric cars, locomotives and conveyor belts. Also, in many vibrating or oscillating machines, an electric motor spins an irregular figure with more area on one side of the axle than the other, causing it to appear to be moving up and down.

Electric motors are also popular in robotics. They are used to turn the wheels of vehicular robots, and servo motors are used to turn arms and legs in humanoid robots. In flying robots, along with helicopters, a motor causes a propeller or wide, flat blades to spin and create lift force, allowing vertical motion.

Electric motors are replacing hydraulic cylinders in airplanes and military equipment.[20][21]

In industrial and manufacturing businesses, electric motors are used to turn saws and blades in cutting and slicing processes, and to spin gears and mixers (the latter very common in food manufacturing). Linear motors are often used to push products into containers horizontally.

Many kitchen appliances also use electric motors to accomplish various jobs. Food processors and grinders spin blades to chop and break up foods. Blenders use electric motors to mix liquids, and microwave ovens use motors to turn the tray food sits on. Toaster ovens also use electric motors to turn a conveyor to move food over heating elements.

References and further reading

  1. ^ "What is an Actuator?", wiseGEEK. Conjecture Corp., 2010. Retrieved 2010-03-13.
  2. ^ Schoenherr, Steven E. (2001), "Loudspeaker History". Recording Technology History. Retrieved 2010-03-13.
  3. ^ Faraday, Michael (1844). Experimental Researches in Electricity. 2.  See plate 4.
  4. ^ spark museum
  5. ^ a b Electricity and magnetism, translated from the French of Amédée Guillemin. Rev. and ed. by Silvanus P. Thompson. London, MacMillan, 1891
  6. ^ Nature 53. (printed in 1896) page: 516
  7. ^ a b http://www.mpoweruk.com/timeline.htm
  8. ^ http://www.fh-zwickau.de/mbk/kfz_ee/praesentationen/Elma-Gndl-Generator%20-%20Druckversion.pdf
  9. ^ http://www.uni-regensburg.de/Fakultaeten/phil_Fak_I/Philosophie/Wissenschaftsgeschichte/Termine/E-Maschinen-Lexikon/Chronologie.htm
  10. ^ http://www.mpoweruk.com/history.htm
  11. ^ Gee, William (2004). "Sturgeon, William (1783–1850)". Oxford Dictionary of National Biography. Oxford, England: Oxford University Press. doi:10.1093/ref:odnb/26748. 
  12. ^ [1] Garrison, Ervan G., "A history of engineering and technology". CRC Press, 1998. ISBN 084939810X, 9780849398100. Retrieved May 7, 2009.
  13. ^ http://www.frankfurt.matav.hu/angol/magytud.htm
  14. ^ For a description and superb illustration of one such early electric motor designed by Froment, see a Google Books PDF online version of Ganot's Physics, 14th Edition, N.Y., 1893 translated by Atkinson, pp. 907 and 908. (Section 899, and Figure 888). (Note to readers using Google: This is not Ganon's Physics.) [2]
  15. ^ http://www.circuitcellar.com/ Motor Comparison, Circuit Cellar Magazine, July 2008, Issue 216, Bachiochi, p.78
  16. ^ [JSAP] Tokai University Unveils 100W DC Motor with 96% Efficiency http://techon.nikkeibp.co.jp/english/NEWS_EN/20090403/168295/
  17. ^ "Tesla's Early Years". PBS.
  18. ^ http://www.daytronic.com/products/trans/t-magpickup.htm
  19. ^ Cyril W. Lander, Power Electronics 3rd Edition, Mc Graw Hill International UK Limited, London 1993 ISBN 0-07-707714-8 Chapter 9-8 Slip Ring Induction Motor Control
  20. ^ Briere D. and Traverse, P. (1993) “Airbus A320/A330/A340 Electrical Flight Controls: A Family of Fault-Tolerant Systems” Proc. FTCS, pp. 616-623.
  21. ^ North, David. (2000) "Finding Common Ground in Envelope Protection Systems". Aviation Week & Space Technology, Aug 28, pp. 66–68.
General references
Further reading
  • Shanefield D. J., Industrial Electronics for Engineers, Chemists, and Technicians,William Andrew Publishing, Norwich, NY, 2001.
  • Fitzgerald/Kingsley/Kusko (Fitzgerald/Kingsley/Umans in later years), Electric Machinery, classic text for junior and senior electrical engineering students. Originally published in 1952, 6th edition published in 2002.
  • Bedford, B. D.; Hoft, R. G. et al. (1964). Principles of Inverter Circuits. New York: John Wiley & Sons, Inc.. ISBN 0 471 06134 4.  (Inverter circuits are used for variable-frequency motor speed control)
  • B. R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters: Operation, Control, and Performance" (New York: John Wiley, 1971).
  • John N. Chiasson, Modeling and High Performance Control of Electric Machines, Wiley-IEEE Press, New York, 2005, ISBN 0-471-68449-X.

See also

External links

Study guide

Up to date as of January 14, 2010

From Wikiversity

Why Electric Machines?

Electricity is neither found plenty in nature nor it can be used directly(Except for electric chairs!). Then the question arises, why are we going for electricity for the transmission of power. The answer is very simple - very efficient mode of transmission. Also,

(wrong, Most mining machines use electric motors. E.g Draglines, Shovels and underground boring machines. It is more cost effective and cleaner.)


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ELECTRIC. MOTORS Fundamentally, electric motors are electric generators reversed in function: they convert into mechanical energy the continued stresses between two electro magnetic fields relatively movable, just as generators convert into electromagnetic stresses the mechanical energy applied to them. Since no transformation of energy is ever absolutely quantitative, the conversions just considered are not accomplished without loss of energy to about the same extent in both cases. The sources of this loss are ohmic loss in the conductors, hysteresis, friction of bearings and brushes, air friction and eddy currents; the sum of these losses in large modern machines does not exceed 5 or 6%. The torque of the motor is the dynamical result of the electromagnetic stresses between the magnetic field of the motor and that due to the armature currents, the latter field being proportional to the strength of the current sheet due to the numerical strength of the current and the number of its effective convolutions. This applies to all types of motors, if one remembers that whenever either of these two stress factors is a periodic variable, as in the case of alternating motors, the torque is proportional to their geometrical co-directed product and not merely to their numerical product. At this point it will be convenient to distinguish between the various types of motors. The first broad distinction is between continuous-current and alternating-current motors, a distinction rather of convenience than of necessity, for in point of fact the two depend upon the same broad principles and can be considered on precisely the same lines.

Electric motors may be conveniently divided as follows: - _ (A) Continuous Current. 1. Separately excited.

2. Series-wound constant current.

3. Series-wound constant potential.

4. Series-wound interdependent current and potential.

5. Shunt-wound constant potential.

(B) Alternating Current. I. Synchronous constant potential.

2. Induction-polyphase constant potential.

3. Induction-monophase constant potential.

4. Repulsion-commutating.

5. Series-commutating.

Of these, the series-wound constant potential, shunt-wound constant potential, and polyphase induction motors do a very large proportion of the active work of power transmission: the first mentioned furnish power for electric railways; the second chiefly power distribution from public electric supply stations; while the third are mainly relied upon in long-distance transmission systems. The fourth and fifth groups of class (B) are old in principle but have been slow in practical development. They include many modifications and transition forms not involving radical changes in the principles or properties of the machines. Their chief use has been for electrical traction, with reference to which they have, in the main, been developed, and their performance is best at low frequency, 15 to 25 cycles per second.

In class (A) in general, for a certain value of the torque current must be forced through the armature against the motor electromotive force which results from the rotation of the armature in a given field. This demands a certain greater applied electromotive force to produce the current required, which is determined by the effective electromotive force, equal to the geometrical difference between the applied and motor electromotive forces, and by the impedance of the armature. For steady currents this last is of course the same as the ohmic resistance, just as for steady electromotive force the geometrical and the numerical difference of the applied and motor electromotive forces are coincident. The torque depends, as heretofore noted, on the field strength and the strength of the current sheet due to the current thus determined. For small values of the torque the speed practically depends upon the applied electromotive force and the field, so that if the former and the latter be constant the speed is also sensibly constant. This is likewise the case if the armature resistance be very small; and in general the variations of speed at constant potential are determined by the product of this resistance and the torque, while the absolute speed depends essentially upon the field strength. Motors for low speed or high electromotive force must have both a strong field and many turns upon the armature, so that both the fundamental stresses may be large. As the field is generally strong - to secure economy of iron - low-voltage and high-voltage machines differ principally in the number of armature turns. For variable speed, this latter factor being fixed, field strength and applied electromotive force are the factors easily altered, and most of the speed variation is accomplished by changing one or both of them. Torque, neglecting field distortion, is at a maximum when the current is the greatest possible at the given applied voltage - that is, when the motor is at rest. With a small armature resistance this current is generally far too great for convenience; hence the motors are usually started with a rheostat in series with the winding if the current is not limited by the generator itself. The torque then depends on the sum of the resistances in circuit, and can be made just sufficient to start the motor under the required load. By the same device the motor can run at reduced speed, although with a considerable loss of energy in the rheostat; it is indeed, as a rule, difficult to get effective speed variation in motors of any kind without serious loss of energy. The field can be changed within wide limits only by a considerable increase of the iron in the magnetic circuit, the applied electromotive force cannot usually be varied except by increasing the resistances in circuit, and the number of armature turns cannot be varied without complication, although the effective number can be modified by shifting the brushes, probably at the expense of sparking. Altogether, if the speed variation demanded be more than 15 or 20%, it causes, in one way or another, considerable expense and trouble, particularly if each speed must be closely held irrespective of load. No large change in absolute speed can readily be made without considerable change in the percentage variation of speeds at various loads. Practically, the best results are obtained from motors of very low armature resistance, in which the field or the applied electromotive force, or both, are varied. The whole problem is nearly identical with the production of constant potential or constant current from generators driven at constant speed, and is solved by similar means. For any one absolute speed a generator can be made to give constant potential, nearly irrespective of load, by compound winding. Similarly, a motor may give a very nearly constant speed at constant potential by a differential winding in series with the armature, weakening the field as the armature current rises. This device, however, obviously increases the energy required for magnetization, and decreases the effective torque at starting. Practically, the best continuous-current motors can be made to hold their speed to within T or 2% from no load to full load. Commercial machines, however, generally vary from 5 to To% in speed. With respect to the direction or rotation of a motor, the torque changes sign with a change of sign in either field or armature current, but not with a change of sign in both. The input of the motor is numerically equal to the product of the current and the applied electromotive force, while the output is determined by the product of the current and motor electromotive force; hence the efficiency of the motor as a transformer of energy is the ratio between these two quantities. The output is a maximum when the applied electromotive force is double the motor electromotive force, and the efficiency is a maximum when the motor and applied electromotive forces are substantially equal. At the point of maximum output the speed is that sufficient to reduce the current to one-half its static value. No motor is worked at or near this point, except momentarily, on account of the low efficiency and severe heating in the armature. These theoretical values are slightly modified in practical machines by the small miscellaneous losses subject to independent variations.

The practical output of electric motors is limited in machines of normal design by the temperature they can safely endure. As a rule the working temperature, which is commonly reached only after six hours or more of continuous running, should not rise more than 40° to 50° F. above the temperature of the surrounding air. In case of traction motors and others subjected to occasional severe overloads, separated by periods of rest or of subnormal load, the temporary rise of temperature tolerated may be much higher, say 60° to 75° F., after a run of an hour or so. The temperature of the air is assumed at 70° F. in most cases, and the temperature of the motor-windings is preferably ascertained by the rise in electrical resistance due to the heating. Thermometers can seldom be so applied as to measure the full heating effect.

The actual output obtainable from a motor structure of given dimensions under these conditions with respect to heating depends chiefly upon the practicable rotative speed of the armature, since the chief losses are proportional to the torque, while the mechanical output at given torque is approximately proportional to the speed. Most makers utilize a single structure for several standard motors varying in speed and output, a 15 h.p. machine at, say, 1200 r.p.m. becoming a 10 h.p. at 800 r.p.m. or a 20 h.p. at 1600 r.p.m. There is no practically fixed relation between the rating and the speed, although it is approximately linear, for in winding the same carcass for different speeds the ratings are settled rather by commercial convenience than by exact determinations. Motors generally have approximately the same efficiencies as the corresponding sizes of generators. Small motors, say from 1 to 5 h.p., are commonly of 70-80% efficiency at full load, medium sized machines of 5 to 50 h.p. about 80 to 90%, and the larger sizes run up to 95% or thereabouts. In the effort to get low-speed motors without immoderately increasing the cost they are generally dropped a little in efficiency and allowed to run hotter than if wound for higher speeds.

The weight of motors per h.p. of output is therefore very variable. In machines of medium size and speed it is likely to be 50 to 75 lb per h.p., falling to 30 or 40 in large or specially high speed machines, and rising to 80 or Too lb in small or very low speed motors. High-voltage motors, particularly if small, lose somewhat in relative output on account of the space taken up by the necessary insulation.

In all ordinary motors the magnetization of the iron is, for economy of material, pushed high; and hence the field, even at heavy loads, is fairly stable and the conditions of commutation remain good. When, however, motors are designed to stand severe overloads, or to admit of a wide range of speed regulation by varying the field strength, the commutation is likely to be unstable, and severe sparking may result. To meet this condition the commutating-pole motor - really a recrudescence of an old idea - has been introduced on a considerable scale. In this construction auxiliary pole pieces, excited by series coils from the motor circuit, are set midway between the ordinary field poles. The office of these poles is to neutralize the magnetomotive force due to the armature winding, thus checking field distortion, and also to ensure the proper reversal of the current in the armature coil directly under the brush. Of the total magneto-motive force due to the windings of the commutating pole, the major part, perhaps three-fourths, is devoted to the former work and the remainder to the latter, the proportion varying widely according to the design of the motor. The result of this construction is excellent, sparkless commutation being ensured over a wide range of load and field strength. The commutating-pole motor is intrinsically more expensive and slightly less efficient than the ordinary type, but for the particular kind of service it is designed to perform is extremely effective. It gives promise of especial value in high-voltage traction motors.

(A) 1. Separately excited Motors are interesting principally on account of the very efficient method of speed regulation possible by their use. In this method the field of the motor is excited from the supply mains, and the armature current is furnished by a motorgenerator running at constant speed. A rheostat in the shunt field of the latter element enables the applied electromotive force to be varied to any desired extent, and hence the working motor can be given full torque at any speed up to that assigned by the maximum value of the electromotive force which can be applied to the armature. Moreover, if the armature resistance be small, the motor is fairly self-regulating at all speeds. The effect is rather startling, since the motor may be giving a very great torque when it is merely turning over at a few revolutions per minute; and although the process is complicated, it leads to excellent results, and is widely used where delicate speed regulation is required.

(A) 2. Series-wound Constant-current Motors were early worked to a considerable extent on arc-lights circuits, but have now passed out of use save in a small number of constant-current power-transmission systems on the continent of Europe. In these motors the motor electromotive force is directly proportional to the output, the torque being constant. They will not start with more than a certain definite load, but once started the speed will increase until added work (internal or external) balances the torque. The type is intrinsically bad in speed regulation, and must be treated by the same methods as are adopted to secure constant current in arc machines. The most successful device in most cases is to vary the field strength by shunting the field coils or to vary the number of effective armature conductors by shifting the brushes. Both methods are carried out mechanically rather than by purely electrical means - in the first case by an automatic rheostat, and in the second by an automatic brush shifter, but neither is wholly satisfactory. Nevertheless, such motors have proved capable of excellent commercial service in some of the European plants, especially in the larger sizes.

(A) 3. Series-wound Constant-potential Motors comprise nearly all motors used for electric traction - aggregating not less, probably, than one and a half million horse-power; hence they are of great practical importance. These traction motors are usually highly specialized machines with very powerful armatures and fields strongly saturated at all working values of the current. The brushes have an invariable position. Such motors behave much like separatelyexcited motors, having a rather large armature resistance. Speed regulation has to be obtained by varying the applied electromotive force. In early traction motors this variation depended upon inserting a rheostat; in modern practice it is customary to employ two, or even four, identical motors on each car, operated in series for low speeds and in parallel for full speed. In practice, however, resistances are inserted when necessary, to prevent too sudden changes of speed and to secure intermediate steps between those obtained by the series-parallel connexions. In rare instances a still further variation is secured by the use of a field only partially saturated at ordinary loads.

(A) 4. Series-wound Motors with Interdependent Current and Potential are used only in connexion with generators of similar design, motor and generator forming a dynamical unit. This system is occasionally used with good results in power transmission. Assuming the motor field to be saturated, if the speed is to be constant the applied electromotive force must rise with the load to an amount depending on the resistances in circuit. If the corresponding generator has a field less fully saturated, the increase in current demanded by the increment of torque in the motor can be made not only to raise the applied electromotive force enough to compensate for armature resistance, but for the total resistances in circuit, including the line. With this difference in saturation the motor will automatically maintain constant speed. The fields of the machines need not be designed for a given saturation, since shunting them with a suitable resistance will give the same result.

(A) 5. Shunt-wound Motors at Constant Potential are the mainstay of continuous-current distributions for industrial purposes. At constant potential the field remains sensibly constant and the torque is directly proportional to the current. The motor then behaves much like a separately-excited motor, and the armature resistance being generally very small, the speed is very nearly constant, varying less than 5% from no load to full load in the best commercial machines. Operating on a compound-wound generator, a single motor of this type can be made to regulate with great precision, as in the previous case. If the motor field be only moderately saturated, its strength, and hence the motor electromotive force, rises and falls with the applied electromotive force; and therefore at constant load these motors run at very nearly constant speed, in spite of small variations of voltage. If speed variation be required, it can be obtained to a moderate extent by a rheostat in the field circuit. At starting a rheostat is necessary in the armature circuit. The differentially wound modification is now seldom used.

(B) t. Synchronous Alternating-current Motors. - The simplest starting point in the consideration of this class is the continuouscurrent generator. This machine actually generates within the armature alternating currents; and if the commutator be replaced by two or more slip-rings connected symmetrically to two or more points on the armature winding, alternating currents, monophase or polyphase, according to the number of connexions and the points touched, can be withdrawn therefrom. The simplest case involves only two slip-rings, joined to the winding at diametrically opposite points. Consider two such modified machines as motor and generator. The condition of complete reversibility is that the instantaneous values of the currents, and the instantaneous values of the angular displacements between poles and armature coils, shall be equal throughout. This evidently requires that the rotation of the motor should be synchronous, pole for pole, with that of the generator. Here, as before, the torque depends on the two fundamental stresses, but the torque has no determinate sign in the absence of an initial rotation. The instantaneous value of the torque depends on the instantaneous value of the current and on its angular displacement. The speed of the motor being invariable, its motor electromotive force depends only on the effective excitation, including the armature reactions, and it may or may not, according to the conditions of load, be in phase with the impressed electromotive force. In the case of the continuous-current motor, the motor output is numerically equal to the product of current and motor electromotive force; and since, in the alternating circuit, these quantities are usually not in phase, in alternating motors the activity is determined by the co-directed part of their product. The current in the alternating motor depends, not on the ohmic resistance alone, but upon the impedance and upon the geometrical difference between the applied and motor electromotive forces. At a given applied electromotive force, and an armature impedance assumed constant, the fundamental variables in the motor are the output, motor electromotive force, and motor current. The two last factors are interdependent, so that the current may have a wide range of values, according to the excitation, while the output remains constant, or, itself remaining constant, may cover a variety of values of the power corresponding to different excitations. These changes involve changes in the phase angle between the motor electromotive force and the current, so that at given output the power-factor of the motor - that is, the ratio between the numerical and geometrical products of current and electromotive force - may be given various values at will by changing the field excitation of the motor, a most unique and valuable property. If the motor electromotive force be fixed and the output varied, the phase angle between current and motor electromotive force varies by reason of the armature taking up a new angular position with respect to the field, backward for increasing load, forward for decreasing load. The minimum value of the current for a given load is reached when the excitation is such that the applied electromotive force and current are in phase, at which point the real and the apparent energy in the circuit coincide. The input can then be accurately measured by voltmeter and ammeter readings, and the motor is working at its best efficiency for the given load. For greater values of the motor electromotive force the current leads in phase with respect to the applied electromotive force; for less values it lags. The former condition is accompanied by the rising of the electromotive force at the motor terminals, the latter by its fall. It therefore becomes possible to use a synchronous motor, if the necessary current due to the load be not too great, as a voltage and phase regulator upon an alternating circuit, a function very valuable in power-transmission work. If the excitation be set to produce leading phase at small loads, the phase angle will gradually diminish as the load rises, and then, passing through zero, increase again with the lagging current, thus holding the power-factor near to unity at all working loads. In a well-designed synchronous motor, by proper initial adjustment of the field, the power-factor can easily be kept between 0.95 and i from quarter load to full load, and very close to unity within the ordinary working range. Save for its inability to start independently, the synchronous motor is a highly desirable addition to a transmission system. Starting is generally accomplished by the help of an induction motor or other auxiliary power, and the motor is treated exactly like an alternator, to be thrown in parallel with the supply circuit. A synchronous motor will pull itself up to synchronism if brought near to its synchronous speed, but this requires a very large amount of current. Operating from a generator of its own, it can be brought to speed by giving it a small initial rotation and raising the generator speed very carefully and gradually, when the two machines will accelerate in synchronism. Polyphase synchronous motors obey these same general laws; they can, however, be started as quasi-induction motors with an open field circuit, the pole faces serving as secondary conductors, but require so large currents in thus starting themselves that it is better practice to bring them to speed by extraneous means.

Synchronous motors sometimes cause serious trouble by "pumping," a phenomenon closely allied to the surging of current between alternators in parallel, and due to similar causes. If not due to defective governing of the prime mover, it usually starts with a change of load or of phase, producing fluctuations in the electromotive force in the system great enough to interfere seriously with incandescent lighting, and continuing with nearly uniform amplitude and frequency for hours if unchecked. The amplitude varies with the conditions, but in the same machine the frequency is nearly constant. The fluctuation affects both the armature and the field circuits, the latter inductively by changes in the armature magnetomotive force, but it can as a rule be controlled by varying the excitation until a neutral point is found, usually when the phase angle is near to zero. Motors with solid pole pieces give little trouble of this sort, the oscillations being rapidly damped by the eddy currents. In motors with laminated fields the most effective remedy is chamfering away the edges of the pole pieces so as to admit heavy copper shoes running along and under the edges, and even bridging the spaces between the pole pieces. The eddy currents in these shoes completely check the "pumping." Synchronous and other Converters. - It seems here appropriate to refer to these converting devices, not in their general functions, but merely in so far as they are directly related to motor practice. The synchronous converter proper is in effect a synchronous motor, in spite of its commutating function. Owing to the fact that the direct current voltage is dependent on the alternating current voltage of supply, the converter cannot advantageously be used to control the power factor by variation of the field strength, but the field can be adjusted once for all to hold the power factor reasonably near unity, provided independent means are available for so adjusting the applied alternating voltage as to give the required result at the commutator. If close regulation of the direct-current voltage is not demanded the converter field can be used more freely. As a matter of fact the synchronous converter finds its chief use in electric traction where close regulation is not important, and motor-generators in one form or another have been found more suitable for electric-lighting work. The synchronous converters have the liability to "pumping" or "hunting," to which reference has already been made, sometimes even of sufficient amplitude to throw the machine out of step, and are often provided with the shoes or bridges found useful with ordinary synchronous motors.

Synchronous motor-generators, so far as the motor function is concerned, present no peculiarities at all. Synchronous commutators, "permutators," and the like, usually have motor-parts of very moderate capacity, and must be kept rigorously free of hunting in order to preserve the conditions of commutation.

In many instances, particularly in American practice, motor generators with induction motors have been used for ease of starting and to secure immunity from hunting. A modification of interest from the motor standpoint is found in the "cascade converter." In this machine the rotor of an induction motor is directly coupled to the armature of a commuting converter of equal output, the windings of the two being in series and approximately equivalent. In this case the normal motor-electromotive force is reached at approximately half synchronous speed, and half the energy is delivered to the output end of the machine by the rotor acting as frequency changer, the rest by torque on the shaft. Commutation takes place therefore at half the initial frequency, which is often a great advantage.

(B) 2. Polyphase Induction Motors. - Speaking broadly, an induction motor is one in which the armature current is introduced into the armature windings by electromagnetic induction instead of by brushes. It is at once an alternating current transformer and an alternating current motor, operating in the latter function by virtue of the current received from the former. In the commonest form the alternating currents are of two or more phases interacting in carrying on these duplicate functions. Induction motors consist of two concentric masses of laminated iron taking the form of short hollow cylinders, of which the outer is fixed and the inner fitted to revolve. The outer surface of the inner drum and the inner surface of the outer drum are slotted or perforated to receive the primary and secondary windings of the apparatus. The outer winding is usually the primary, and the inner (or armature) winding the secondary. The primary winding is almost universally a multipolar drum in character; the secondary is, in the most highly developed motors, of the same character, but very often consists merely of numerous insulated armature bars united at each end of the drum by a common end-plate or end-ring, forming the structure usually known as a "squirrel-cage" winding. In polyphase motors of the usual type the primary drum winding is in duplicate or triplicate, resembling very closely the armature winding in a twoor threephase generator. The actions which go on in these motors have been the subject of much debate; most of the theoretical discussions of the matter have been based upon the concept of a rotary magnetization produced by two simple sinusoidal magnetisms superimposed in quadrature upon the same core, or, in the case of a three-phase motor, three superimposed in a similar symmetrical manner. This hypothesis is often most convenient, being merely an application of the general physical thesis that two equal simple harmonic motions in quadrature produce circular motion, as in the case of the conical pendulum. All the results of this hypothesis follow, however, from the introduction of two alternating magnetizations, acting in quadrature in time but independently; and one or the other view of the matter is convenient according as, in the structure considered, the effective magnetizations do or do not produce a definite physical resultant. There is no discrepancy between the two hypotheses; they are merely two points of view of the same phenomena. In the general case, one need make no supposition as to the existence or non-existence of the physical resultant rotary magnetization; it is merely necessary to note that if one phase-winding predominately produce a magnetic field, and the other a current in the rotary member fitted to react with that field, torque will result, whether the two phase-windings act upon the same magnetic structure or upon two entirely separate magnetic structures merely connected by the leads which deliver current from one to the other.

Induction motors having both these forms of structure are in successful use. If one considers the latter case, the two-phasewindings have exchanged functions every 90° in the two-phase structure, each phase-winding serving to produce a magnetic field and to deliver, almost as if it were merely a pair of brushes, current to react with this field alternately, and the two halves of the motor structure exchange functions every 90°. Considering the motor in which the two-phase-windings are superimposed on the same core, there is a virtual magnetic resultant rotating at a speed determined by the frequency of the current and the number of poles, and setting up induced currents in the secondary member, which currents are so disposed as to react with the field to produce rotary motion. At rest, the secondary electromotive force produced by the machine as a transformer is a maximum; when the motor is running at speed, unloaded, it is a minimum, and an increment of load causes the secondary member merely to slip behind synchronous speed far enough to receive an increment of transformed energy sufficient to carry the new load. If the secondary member is of very low resistance, the slip behind synchronism is very small, even at full load - less than 2% in motors developed for this particular property. An increase of secondary resistance produces increased falling behind from synchronous speed; and if resistance be added to the secondary member by interpolating rheostats in its circuits, the motor can be made to produce uniform torque over a very wide range of speed, as is the case with continuous current motors. The percentage of slip is the percentage of energy lost in the secondary member, as likewise in continuous-current motors if one regards their synchronous speed as that at which the motor electromotive force would equal that impressed. Polyphase induction motors start, when properly designed, with a very powerful torque, even up to three or four times the full load running torque of the same motor. With a very low-resistance secondary member this torque demands an immensely large current, the structure acting almost like a shortcircuited transformer, and the lag in the secondary circuit is considerable. In motors in which this large starting current is objectionable, it may be reduced very greatly by interpolating resistances in the secondary circuits at starting, the effect of these being to diminish the lag in the secondary circuit and to decrease the demand for primary current. A certain critical value of this resistance gives a maximum torque per ampere in the primary circuit with a given motor, being approximately that total secondary resistance which equals the secondary reactance. For maximum torque obviously both resistance and reactance should be equal and as small as possible. Where a small primary current in starting is of considerable importance, this extra resistance is frequently introduced at starting and cut out afterwards, particularly in cases where large torque is necessary. If great starting torque is not necessary, the primary electromotive force is often diminished by inductive resistances, or a change in the connexions of the transformer from which the motor is fed. Both methods of starting are in commercial use on a very large scale.

In efficiency and closeness of speed regulation and good general running properties polyphase induction motors approximate very closely to the best continuous-current practice. They produce, however, a certain amount of lag between primary electromotive force and current, which causes the apparent input to be larger than the real input, as generally happens in alternating-current work. The ratio between the real and the apparent watts input is the power factor of the motor. In well-designed modern machines this is usually from 85 to 90% at rated load; it should seldom fall below the former figure, and rarely rises more than I or 2% above the latter, though in rare instances power-factors as high as 94 or 95% have been obtained. Condensers have sometimes been employed in connexion with such motors to increase the power-factor, and with considerable success, particularly in maintaining the power-factor at low and moderate loads; but their use is generally unnecessary, and condensers of sufficient capacity at any reasonable value of the voltage have proved troublesome to build and maintain. The weakest point in these polyphase induction motors is the importance of employing a very small clearance between armature and field, in order to increase the power-factor by !making the structure more efficient, considered merely as a transformer. The clearances in ordinary use are seldom greater than 1 1 6 in., even in motors as large as too h.p., and in smaller machines are frequently not more than 3 1 3 in. Induction motors, however, possess many valuable properties, and are the mainstay of long-distance power-transmission work at the present time.

(B) 3. Monophase Induction Motors closely resemble the polyphase motor in construction, but have only a single-phase winding in the primary. The theories of their action are very similar to those of polyphase motors. The essential point of difference is that the stable angular displacement between the field magnetization and the armature currents which co-act with it is obtained in the polyphase motor by the time-displacements in the several phase windings, while in the single-phase motor it is obtained by the angular space-displacement of the armature, which has to be set up by an initial rotation. Single-phase motors therefore are not inherently self-starting, and run in either direction equally well when once started. The torque is always in the direction of the initial rotation. This rotation is sometimes given by hand and sometimes by auxiliary phase-windings supplied by derived current from the main circuit, or merely short-circuited on themselves and receiving induced currents from the main winding. Both these devices give a small initial torque in a definite direction, setting up a so-called elliptical rotary field, i.e. one produced by the composition of two unequal magnetizations, in this case at some indeterminate angle, seldom large. Once up to speed, the single-phase motors act much like the polyphase. They are conspicuously weak in the matter of power-factor, however, as well as in that of startingtorque, and have as yet not come into very extensive commercial use, although under special conditions they have been and are successfully employed. A theoretically interesting form of induction motor is a modification which runs at absolutely synchronous speed, receiving the necessary energy in the secondary not in virtue of slip behind synchronous speed, but from great difference in wave form between the primary and secondary circuits, so that energy due to harmonics of the fundamental frequency is periodically received by the armature in spite of synchronism in speed. Such motors are not employed commercially, but sometimes find a field for usefulness in the laboratory.

(B) 4. Repulsion-commutating Motors constitute a class of singlephase alternating-current motors which has risen to considerable commercial importance. They are fundamentally induction motors in the sense that the armature currents are supplied by the inductive action of the field. The armature winding is, however, provided with a commutator and (for a two-pole motor) two diametrically opposite brushes, which are short-circuited on each other and placed at an angle with the line of field magnetization. By this device the magnetic axis of the armature is held at a fixed angle with the field flux, so that the condition for steady torque is always fulfilled, its amount depending on the position of the brushes. Were these either in line with, or exactly at right angles to, the field poles, the torque would be zero - in the first case from lack of angular displacement, in the second from lack of secondary current. The brushes being skewed, however, the secondary current is maintained at a suitable value, and the motor runs in a definite direction. The general principle is merely that of a transformer with a movable secondary under magnetic thrust. During reversal of the current the torque relation remains fixed, since the primary and secondary currents both change sign, preserving the magnetic relations as in a series-wound continuous-current machine.

If such a motor is of moderate reactance, the currents are large and the torque very considerable. The repulsion-commutating connexion is considerably used as a starting device for single-phase induction motors, the commutator being short-circuited as a whole when the armature reaches synchronous speed. Thereafter the machine operates as a pure induction motor of the sort just described. The advantage of this change is that the commutator is eliminated, save at starting, and the motor becomes practically a constant speed machine like any other properly-designed simple induction motor. Such motors can be made to start if necessary with several times the normal running torque and a nearly proportionate increase of current. The short-circuiting of the commutator is generally performed automatically by a centrifugal governor. When at speed, efficiency and power factor are those of the typical motor of class(B)3.

The pure repulsion-commutating motor, worked as such, on the other hand, resembles a series-wound motor in its characteristics, having no fixed speed and being capable of running far above nominal synchronism. This results from the fixed angular relation maintained by the brushes between the armature and field magnetizations, whereby the torque conditions are preserved. Above the nominal synchronous speed, however, difficulties of commutation set in, so that some modifications of this simple type are desirable for wide ranges of speed. The power factors of these motors compare well, both in starting and in running, with those of the best pure induction motors, and their efficiencies are similar. These machines are reversible, serving as alternating generators when driven mechanically at "negative" speed.

Instead of simply skewing the brush line in the repulsion motor, an entirely analogous effect may be produced by dividing the field coils into pairs placed in quadrature, the brush line being parallel to one pair and at right angles to the other. This merely amounts to dividing the function of the original field physically into its components, a change which sometimes tends to improve the stability of the running conditions.

A more radical departure is found in the group of so-called "compensated-repulsion" motors, of which there are several members, due to various inventors, all material improvements on the pure repulsion type just described. Their common characteristic is that while possessing like simple commutator-repulsion motors, a transformer field acting upon the armature as secondary, and a pair of short-circuiting brushes holding the resulting armature magnetization in definite alignment, they also send the primary current in series through the armature via a second pair of brushes in quadrature with the first. The substantial effect of this series connexion is to cut down the virtual reactance of the armature as the speed rises, practically annulling it at synchronous speed. In alternating motors the motor-electromotive force is not merely that due to the motion of the armature conductors but the geometrical resultant of this and the reactance E.M.F.'s. In the motor here considered and analogous machines an auxiliary E.M.F. is applied either as here, conductively or inductively, in such direction as to compensate more or less perfectly the armature reactance E.M.F. The result is to secure, at least for a certain speed, a power factor near unity, as in the motor under discussion, although the starting conditions are not particularly good and the performance deteriorates above synchronism. In some motors of this type the compensating E.M.F. is introduced by an auxiliary winding in series and in quadrature with the main field, instead of by supplementary brushes. The modifications of the general scheme are rather numerous, and out of them have come some excellent single-phase motors now widely used for traction purposes.

(B) 5. Series Commutating Motors. - This important and interesting type is derived directly from the ordinary series motor for continuous current. The torque in these does not change sign with reversal of the current in both field and armature, and consequently alternating current can still produce in them unidirectional torque. Practically the first step toward an alternating current series motor is lamination of the field to reduce parasitic currents; the second is to keep down the reactance. A laminated field motor performs fairly well at a frequency of io periods or thereabouts, but to render it useful at ordinary frequencies requires modification in design. The motor E.M.F. being as before the geometrical sum of the reactance E.M.F. and that due to motion of the armature conductors, the first improvement can be made by making the latter dominant, i.e. by making the armature relatively very powerful. The plain series commutating motor has then a relatively weak laminated field'and a powerful armature. To check trouble with commutation due to short-circuiting coils under a brush, it usually has high resistance commutator leads, and thus equipped is capable of very fair performance, having the same general characteristics as the continuous-current series motor. Even so the armature reactance is somewhat excessive, so that with this simple construction the power factor is apt to be bad. Practically the plain series commutating motor is hardly used at all, but rather modifications of it corresponding very closely to those mentioned in connexion with the repulsion motor. In other words, an auxiliary electromotive force tending to annul the reactance E.M.F. of the armature is imposed upon the armature circuit. This is accomplished generally by a "compensating coil" in series and in space-quadrature with the main field. In another modification the compensating coil is closed upon itself, forming a short-circuited secondary, to which the armature itself acts as primary. The end to be attained is the addition of an E.M.F. such that the vector sum of the E.M.F.'s in the armature shall reduce as nearly as may be to the E.M.F. due to the motion of the armature conductors, as in a continuous-current motor. Obviously it is difficult to secure full compensation for all loads and speeds, but it can be made nearly complete for some particular load and speed.

These "series-compensated" motors behave much like continuouscurrent series motors, and, when properly designed, run well on continuous current. They have been developed particularly for heavy traction purposes, to which they are well adapted, owing to their ability to work well at all speeds. They give a very high maximum power factor and a reasonably good one over a considerable range of speed and load. Obviously both the field proper and the compensating field can be made subject to regulation to increase the range of successful action. Motors of this type have already come into successful use for fast and heavy railway service. Commutation appears to be reasonably good, although it is a far more difficult problem than with continuous-current machines.

The efficiency and output for unit weight in all alternatingcurrent motors is a little less favourable than with continuouscurrent motors. In the last resort the supply of energy to a singlephase motor is essentially discontinuous, and there is inevitable extra loss from hysteresis and parasitic currents, whether the motor is single phase or polyphase. The result is that an alternatingcurrent motor requires, other things being similar, more or better material, and loses a little more energy than a continuous-current motor of equal output. Motor design is a compromise, and while any one property can be exaggerated, it will be at the expense of others. One could probably build, for instance, a series-compensated motor of as high efficiency or as large output per unit weight as any commercial motor, but there would be sacrifice somewhere, in cost if not conspicuously elsewhere. As a matter of fact, the difference in efficiency usually amounts only to a very few per cents., and the difference in output per unit weight to a few more. The gain in the use of alternating-current motors is in facility and economy of distribution, which in many cases is far more than enough to overweigh any inherent disabilities in the machines themselves. Hence they are coming steadily into extended use. (L. BL.)

<< Motmot

Motor Vehicles >>


Got something to say? Make a comment.
Your name
Your email address