Brushless DC electric motor: Wikis

  

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A brushless DC motor (BLDC) is a synchronous electric motor which is powered by direct-current electricity (DC) and which has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In such motors, current and torque, voltage and rpm are linearly related.

A BLDC motor powering a micro remote-controlled airplane. The motor is connected to a microprocessor-controlled BLDC controller. This 5-gram motor is approximately 11 watts (15 millihorsepower) and produces about two times more thrust than the weight of the plane. Being an outrunner, the rotor-can containing the magnets spins around the coil windings on the stator.

Two subtypes exist:

In a conventional (brushed) DC motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature coil closest to the stationary stator.

In a BLDC motor, the electromagnets do not move; instead, the permanent magnets rotate and the armature remains static. This gets around the problem of how to transfer current to a moving armature. In order to do this, the brush-system/commutator assembly is replaced by an electronic controller. The controller performs the same power distribution found in a brushed DC motor, but using a solid-state circuit rather than a commutator/brush system.

Contents

Comparison with brushed DC motors

Because of induction of the windings, power requirements, and temperature management, some glue circuitry is necessary between digital controller and motor.

BLDC motors offer several advantages over brushed DC motors, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, more power, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the electromagnets are attached to the casing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter. The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can damage the magnets. BLDC's main disadvantage is higher cost, which arises from two issues. First, BLDC motors require complex electronic speed controllers to run. Brushed DC motors can be regulated by a comparatively simple controller, such as a rheostat (variable resistor). Second, many practical uses have not been well developed in the commercial sector. For example, in the RC hobby scene, even commercial brushless motors are often hand-wound while brushed motors use armature coils which can be inexpensively machine-wound.

BLDC motors are often more efficient at converting electricity into mechanical power than brushed DC motors. This improvement is largely due to the absence of electrical and friction losses due to brushes. The enhanced efficiency is greatest in the no-load and low-load region of the motor's performance curve. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency.

Controller implementations

Because the controller must direct the rotor rotation, the controller needs some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called sensorless controllers. Like an AC motor, the voltage on the undriven coils is sinusoidal, but over an entire commutation the output appears trapezoidal because of the DC output of the controller.

The controller contains 3 bi-directional drivers to drive high-current DC power, which are controlled by a logic circuit. Simple controllers employ comparators to determine when the output phase should be advanced, while more advanced controllers employ a microcontroller to manage acceleration, control speed and fine-tune efficiency. Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run briefly backwards, adding even more complexity to the startup sequence.

The controller unit is often referred to as an "ESC", meaning Electronic Speed Controller.

Variations on construction

The poles on the stator of a two-phase BLDC motor. This is part of a computer cooling fan; the rotor has been removed.
Schematic for delta and wye winding styles. (This image does not illustrate a BLDC motor's inductive and generator-like properties)

BLDC motors can be constructed in several different physical configurations: In the 'conventional' (also known as 'inrunner') configuration, the permanent magnets are mounted on the spinning armature (rotor). Three stator windings surround the rotor. In the 'outrunner' configuration, the radial-relationship between the coils and magnets is reversed; the stator coils form the center (core) of the motor, while the permanent magnets spin on an overhanging rotor which surrounds the core. The flat type, used where there are space or shape limitations, uses stator and rotor plates, mounted face to face. Outrunners typically have more poles, set up in triplets to maintain the three groups of windings, and have a higher torque at low RPMs. In all BLDC motors, the stator-coils are stationary.

There are also two electrical configurations having to do with how the wires from the windings are connected to each other (not their physical shape or location). The delta configuration connects the three windings to each other (series circuits) in a triangle-like circuit, and power is applied at each of the connections. The wye ("Y"-shaped) configuration, sometimes called a star winding, connects all of the windings to a central point (parallel circuits) and power is applied to the remaining end of each winding.

A motor with windings in delta configuration gives low torque at low rpm, but can give higher top rpm. Wye configuration gives high torque at low rpm, but not as high top rpm. [1]

Although efficiency is greatly affected by the motor's construction, the wye winding is normally more efficient. In delta-connected windings, half voltage is applied across the windings adjacent to the undriven lead (compared to the winding directly between the driven leads), increasing resistive losses. In addition, windings can allow high-frequency parasitic electrical currents to circulate entirely within the motor. A wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses.

From a controller standpoint, the two styles of windings are treated exactly the same, although some less expensive controllers are intended to read voltage from the common center of the wye winding.

Spindle motor from a 3.5" floppy disk drive.

Applications

BLDC motors can potentially be deployed in any area currently fulfilled by brushed DC motors. Cost and control complexity prevents BLDC motors from replacing brushed motors in most common areas of use. Nevertheless, BLDC motors have come to dominate many applications: Consumer devices such as computer hard drives, CD/DVD players, and PC cooling fans use BLDC motors exclusively. Low speed, low power brushless DC motors are used in direct-drive turntables. High power BLDC motors are found in electric vehicles, hybrid vehicles and some industrial machinery. These motors are essentially AC synchronous motors with permanent magnet rotors.

The Segway Scooter and Vectrix Maxi-Scooter also use BLDC technology.

A number of electric bicycles use BLDC motors that are sometimes built right into the wheel hub itself, with the stator fixed solidly to the axle and the magnets attached to and rotating with the wheel. The bicycle wheel hub is the motor. This type of electric bicycle also has a standard bicycle transmission with pedals, gears and chain that can be pedaled along with, or without, the use of the motor as need arises. [1]

Certain HVAC systems, especially those featuring variable-speed and/or load modulation, use ECM motors (electronically-commutated BLDC). In addition to the BLDC's higher efficiency, the motor's built-in microprocessor allows for programmability, better control over airflow, and serial communication.

AC and DC power supplies

  • Direct current motor: DC in both the stator and the rotor
  • Synchronous motor: AC in one, DC in the other (i.e., rotor or stator)
  • Induction motor: AC in both stator and rotor

Although BLDC motors are practically identical to permanent magnet AC motors, the controller implementation is what makes them DC. While AC motors feed sinusoidal current simultaneously to each of the legs (with an equal phase distribution), DC controllers only approximate this by feeding full positive and negative current to two of the legs at a time. The major advantage of this is that both the logic controllers and battery power sources also operate on DC, such as in computers and electric cars. In addition, the approximated sine wave leaves one leg undriven at all times, allowing for back-EMF-based sensorless feedback.

Vector drives are DC controllers that take the extra step of converting back to AC for the motor. The DC-to-AC conversion circuitry is usually expensive and less efficient, but they have the advantage of being able to run smoothly at very low speeds or completely stop in a position not directly aligned with a pole. Motors used with a vector drive are typically called AC motors.

A motor can be optimized for AC (i.e. vector control) or it can be optimized for DC (i.e. block commutation). A motor which is optimized for block commutation will typically generate trapezoidal EMF. One can easily observe this by connecting the motor wires (at least two of them) to a scope and then hand-cranking/spinning the shaft.

Another very important issue, at least for some applications like automotive vehicles, is the constant power speed ratio of a motor. The CPSR has direct impact on needed size of the inverter. Example: A motor with a high CPSR in a vehicle can deliver the desired power (e.g. 40 kW) from 3,000 rpm to 12,000 rpm, while using a 100 A inverter. A motor with low CPSR would need a 400 A inverter in order to do the same.

Kv rating

The Kv rating of a design of brushless motor is the constant relating the motors unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the "back-EMF"). For example, a 5,700 Kv motor, supplied with 11.1 V, will run at a nominal 63,270 rpm. By Lenz's law, a running motor will create a back-EMF proportional to the RPM. Most ESCs do not boost the battery voltage.

Once a motor is spinning so fast that the back-EMF is at the battery voltage (also called DC line voltage), then the motor has reached its "base speed". It is impossible for the ESCs to "speed up" that motor, even with no load, beyond the base speed without resorting to "field weakening". For some applications (e.g. automotive traction and high speed spindle motors) it is normal to exceed the base speed with a factor of 200 to 600%.

Kv is the voltage constant (capital-K, subscript v), not to be confused with the kilovolt, whose symbol is kV (lower-case k, capital V).[2][3]

Model aircraft, cars and helicopters

BLDC motors (generally referred to simply as Brushless (BL) motors) are the most popular motor choice now in the model aircraft industry. With their superior power to weight ratios, a large range of sizes, from under 5 grams to large motors rated at thousands of watts, they have revolutionized the market for electric-powered model flight.

Their introduction has redefined performance in electric model aircraft and helicopters, displacing most brushed and internal combustion power sources. The large power to weight ratio of modern batteries and brushless motors allows models to ascend vertically, rather than climb gradually. The silence and lack of mess compared to small glow fuel internal combustion engines that were used is another reason for their popularity.

Their popularity has also risen in retrospect to the Radio Controlled Car, Buggy, and Truck scene, where Sensored-type (motors with an extra Six wires, connected to Hall Effect Sensors which allow the position of the magnet to be detected). Brushless motors have become legal in RC Car Racing in accordance to ROAR (the American governing body for RC Car Racing), since 2006. Several Modern RTR (Ready-To-Run) cars such as the Traxxas Rustler VXL, exceed rc dynamite and the HPI Sprint 2 Flux come out standard with Stepper-type (or sensorless) Brushless motors. Several RC Car Brushless motors, such as the LRP Electronic/Associated Electrics Vector X12 Motor, feature replaceable and upgradeable parts, such as Sintered Neodymium (Rare Earth Magnets), Ceramic Bearings, and Replaceable Motor Timing inserts. these motors as a result are quickly rising to be the preferred motor type of Electric On and Off-Road RC Racers and Recreational Drivers alike, for their low maintenance, high running reliability and power efficiency (most Sensored motors have an efficiency rating of 80% or greater).

See also

References

  1. ^ www.E-BikeKit.com
  2. ^ RC Toys: "Brushless Motors"
  3. ^ United Hobbies: "Kv rating explained"

External links








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