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A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power similarly. As a switch, it has exceptionally long life, considering the number of circuit makes and breaks that occur in normal operation.

A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor's armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit provides unidirectional — direct — current to the external circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832, based on a suggestion by André-Marie Ampère.

Contents

Principle of Operation

Commutator.png


In the simplified image to the right, DC current is supplied to the circuit by the battery (bottom of diagram). The "commutator" (shown as red and blue semi-circular segments and typically made of a conductive copper alloy) are terminals of the motor winding (shown as a single violet line but representing multiple loops of wire) and are intended to slide or rotate under the brushes. The "brushes" (shown as black blocks, typically made of carbon), are stationary electrical contacts which permit current to flow in to one commutator segment, through the motor winding(s) and out through the other commutator segment. The windings and the commutator segments comprise a rigid assembly fixed about a shaft turning in a bearing, called the "rotor".

As the rotor turns, the current in the winding reverses every time the commutator makes half a turn. This reversal of the winding current compensates for the fact that the winding has also rotated half a turn relative to the fixed magnetic field (not shown). The current in the winding causes the fixed magnetic field to exert a rotational force (a torque) on the winding, making it turn. As the rotor's field comes close to aligning itself with that of the stator, the commutator switches the rotor's polarity, so the motor is perpetually trying to settle, so to speak.

Note that no practical, real-world motor or generator uses the commutators shown in these two examples. However, this two-segment simplification does explain the basic principles. All practical commutators have at least three segments, and in some instances (such as the N.Y. City transit system's old rotary AC-to-DC converters), up to several hundred. In these elementary diagrams, there is a dead position where the motor will not start.

For the image to the right, when the brushes make contact across both commutator segments, the commutator is short-circuited and current passes directly from one brush to the other across the commutator, doing no work in the rotor windings, and drawing a destructive fault current from the power source. As well, practical rotors have more turns in their windings. For the image to the left, there is a dead spot when the brushes cross the insulation between the two segments and no current flows. In either case, in a motor, the rotor cannot begin to spin if it is stopped in this position.

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Simplest Practical commutator

This has three segments, and the rotor has three poles. The left image shows the three rotor poles with their windings. The commutator is near the end of the shaft, as it points up and to the left. It is a metal cylinder (note the yellowish reflection) with three equally-spaced cuts parallel to the shaft, and has white plastic discs on both ends. Each segment connects to the nearest junction between two of the three rotor coils.

In the middle illustration, the brushes (in this instance, flat metal springs -- carbon brushes are not needed at the low voltages used by such motors as these) are the two straight horizontal pieces; when assembled, the brushes are under tension, slightly away from each other, to stay in contact with the commutator. Power connects to two solder terminals on the outside of the end disc shown in this image. Those terminals are likely to be the same pieces of metal as the brushes themselves.

Inside the exterior metal cylinder (right image — the complete motor) is a hollow cylindrical permanent magnet with its south pole opposite its north pole. Interaction between the rotor and that magnet's field is what makes the motor spin. This motor's diameter is greater than its length, something less common than typical motors of this sort. Otherwise, it is very typical. Considering that it was used to spin the disc in a CD drive, short length was quite important.

This type of motor is widely used in small toys, models, and electromechanical/electronic devices.

Simplest Possible Commutator - Rotor View.JPG Simplest Possible Commutator - Brushes.JPG Simplest Possible Commutator - Motor Body.JPG


Although the rotor can potentially stop in a position where two commutator segments touch one brush, this only de-energizes one of the three rotor arms while the other two are correctly powered. The motor produces sufficient torque with the two powered rotor arms to begin spinning the rotor, and no direct shorting can occur between the commutator brushes.

Although, so far, this explanation has assumed a permanent-magnet field, (or a wound field — electromagnet fed by DC), so-called universal motors in appliances such as vacuum cleaners have wound fields, and operate well on AC. Power goes to both the field and the brushes, so the magnetic fields of both rotor and stator reverse together. These motors also operate on DC, hence the term "universal".

Ring/Segment Construction

Cross-section of a commutator that can be disassembled for repair.[1]

A commutator typically consists of a set of copper segments, fixed around part of the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded brushes fixed to the stationary frame of the machine. The external source of current (for a motor) or electrical load (for a generator) is connected to the brushes. For small equipment the commutator segments can be stamped from sheet metal. For very large equipment the segments are made from a copper casting that is then machined into the final shape.

Each conducting segment on the armature of the commutator is insulated from adjacent segments. Initially when the technology was first developed, mica was used as an insulator between commutation segments. Later materials research into polymers brought the development of plastic spacers which are more durable and less prone to cracking, and have a higher and more uniform breakdown voltage than mica.

The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment, using insulating wedges around the perimeter of each commutation segment. Due to the high cost of repairs, for small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed; when the motor fails it is simply discarded and replaced. On very large industrial motors it is economical to be able to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced.

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments.

Friction between the segments and the brushes eventually causes wear to both surfaces. Carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. Older copper brushes caused more wear to the commutator, causing deep grooving and notching of the surface over time. The commutator on small motors (say, less than a kilowatt rating) is not designed to be repaired through the life of the device. On large industrial equipment, the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter. The largest of equipment can include a lathe turning attachment directly over the commutator.

Brush Construction

Various types of copper and carbon brushes.[2]

Early in the development of dynamos and motors, copper brushes were used to contact the surface of the commutator. However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator. As the copper brushes wear away, the dust and pieces of the brush could wedge between commutator segments, shorting them and reducing the efficiency of the device. Fine copper wire mesh or gauze provided better surface contact with less segment wear, but gauze brushes were more expensive than strip or wire copper brushes. The copper brush was eventually replaced by the carbon brush.

Carbon brushes tend to wear more evenly than copper brushes, and the soft carbon causes far less damage to the commutator segments. There is less sparking with carbon as compared to copper, and as the carbon wears away, the higher resistance of carbon results in fewer problems from the dust collecting on the commutator segments.

Copper and carbon are each better suited for a particular purpose. Copper brushes perform better with very low voltages and high amperage, while carbon brushes are better for high voltage and low amperage. Copper brushes typically carry 150 to 200 amperes per square inch of contact surface, while carbon only carries 40 to 70 amperes per square inch. The higher resistance of carbon also results in a greater voltage drop of 0.8 to 1.0 volts per contact, or 1.6 to 2.0 volts across the commutator.[3]

Modern rotating machines with commutators now use carbon brushes, which may have copper powder mixed in to improve conductivity. Metallic copper brushes would only be found in toy or very small motors, such as the one illustrated above.

Brush Holders

Compound carbon brush holder, with individual clamps and tension adjustments for each block of carbon.[4]

A spring is typically used with the brush, to maintain constant contact with the commutator. As the brush and commutator wear down, the spring steadily pushes the brush downwards towards the commutator. Eventually the brush wears small and thin enough that steady contact is no longer possible or it is no longer securely held in the brush holder, and so the brush must be replaced.

It is common for a flexible power cable to be directly attached to the brush, because current flowing through the support spring causes heating, which may lead to a loss of metal temper and a loss of the spring tension.

When a commutated motor or generator uses more power than a single brush is capable of conducting, an assembly of several brush holders is mounted in parallel across the surface of the very large commutator.

This parallel holder distributes current evenly across all the brushes, and permits a careful operator to remove a bad brush and replace it with a new one, even as the machine continues to spin fully powered and under load.

High power, high current commutated equipment is now uncommon, due to the less complex design of alternating current generators that permits a low current, high voltage spinning field coil to energize high current fixed-position stator coils. This permits the use of very small singular brushes in the alternator design. In this instance, the rotating contacts are continuous rings, called slip rings, and, of course, no switching happens.

Modern devices using carbon brushes usually have a maintenance-free design that requires no adjustment throughout the life of the device, using a fixed-position brush holder slot and a combined brush-spring-cable assembly that fits into the slot. Replacement simply involves pulling out the old brush and inserting a new one.

Older commutator motors sometimes had all brushes mounted on movable frames so that the position of the brushes in relation to the magnetic fields of the stator poles could be adjusted manually.

AmplidyneTM rotary electrical amplifier-generators have adjustable brush position; this is usually set at manufacture and checked at overhaul.

Brush Contact Angle

Brush angle definitions.[5]

The different brush types make contact with the commutator in different ways. Because copper brushes have the same hardness as the commutator segments, the rotor cannot be spun backwards against the ends of copper brushes without the copper digging into the segments and causing severe damage. Consequently strip/laminate copper brushes only make tangential contact with the commutator, while copper mesh and wire brushes use an inclined contact angle touching their edge across the segments of a commutator that can spin in only one direction.

The softness of carbon brushes permits direct radial end-contact with the commutator without damage to the segments, permitting easy reversal of rotor direction, without the need to reorient the brush holders for operation in the opposite direction. Although never reversed, common appliance motors that use wound rotors, commutators and brushes have radial-contact brushes. In the case of a reaction-type carbon brush holder, carbon brushes may be reversely inclined with the commutator so that the commutator tends to push against the carbon for firm contact.

The Commutating Plane

Commutating plane definitions.[6]

The contact point where a brush touches the commutator is referred to as the commutating plane. In order to conduct sufficient current to or from the commutator, the brush contact area is not a thin line but instead a rectangular patch across the segments. Typically the brush is wide enough to span 2.5 commutator segments. This means that two adjacent segments are electrically connected by the brush when it contacts both.

Compensation for stator field distortion

Centered position of the commutating plane if there were no field distortion effects.[7]

Most introductions to motor and generator design start with a simple two-pole device with the brushes arranged at a perfect 90-degree angle from the field. This ideal is useful as a starting point for understanding how the fields interact but it is not how a motor or generator functions in actual practice.

Dynamo - exaggerated rotating field distortion.png Dynamo - iron filings show distored field.png
On the left is an exaggerated example of how the field is distorted by the rotor.[8]On the right, iron filings show the distorted field across the rotor.[9]

In a real motor or generator, the field around the rotor is never perfectly uniform. Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

Actual position of the commutating plane to compensate for field distortion.[10]

The faster the rotor spins, the further this degree of field distortion. Because a motor or generator operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane.

The effect can be considered to be analogous to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.[11]

Further Compensation for Self-Induction

Brush advance for Self-Induction.[12]

In a coil of wire, the magnetic field of each wire compounds together to form a magnetic field that tends to resist changes in current flow, as if the current had inertia. This is known as self-induction.

In the coils of the rotor, there is a tendency for current to continue to flow for a brief moment after the brush has been reached. This energy is wasted as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments.

Spurious resistance is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current.

In order to minimize sparking at the brushes due to this short-circuiting, the brushes are advanced a few degrees further yet, beyond the advance for field distortions. This moves the rotor winding undergoing commutation slightly forward into the stator field which has magnetic lines in the opposite direction and which oppose the field in the stator. This opposing field helps to reverse the lagging self-inducting current flow in the stator.

So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect 90-degree angle as taught in so many beginners textbooks, in order to compensate for self-induction.

Limitations and alternatives

While commutators are widely applied in direct current machines, up to several thousand kilowatts in rating, they have limitations.

Brushes and copper segments wear. On small machines the brushes may last as long as the product (small power tools, appliances, etc.) but larger machines will require regular replacement of brushes and occasional resurfacing of the commutator. Brush-type motors may not be suitable for long service on aerospace equipment where maintenance is not possible.

The efficiency of direct current machines is limited by the "brush drop" due to the resistance of the sliding contact. This may be several volts, making low-voltage direct-current machines very inefficient. The friction of the brush on the commutator also absorbs some of the energy of the machine.

Lastly, the current density in the brush is limited and the maximum voltage on each segment of the commutator is also limited. Very large direct current machines, say, more than several megawatts rating, cannot be built with commutators. The largest motors and generators, of hundreds of megawatt ratings, are all alternating-current machines.

With the widespread availability of power semiconductors, it is now economic to provide electronic switching of the current in the motor windings. These "brushless direct current" motors eliminate the commutator; these can be likened to AC machines with a built-in DC to AC inverter. In these motors, rotor position determines when the stator windings switch polarity. Operating life is limited only by bearing wear, if other factors are not adverse.

Repulsion induction motors

These are single-phase AC-only motors with higher starting torque than can be obtained with split-phase starting windings, and before high-capacitance (non-polar, relatively high-current electrolytic) starting capacitors became practical. They have a conventional wound stator as with any induction motor, but the wire-wound rotor is much like that with a conventional commutator. Brushes opposite each other are connected to each other (not to an external circuit), and transformer action induces currents into the rotor that develop torque by repulsion.

One variety, notable for having an adjustable speed, runs continuously with brushes in contact, while another uses repulsion only for high starting torque and in some cases lifts the brushes once the motor is running fast enough. In the latter case, all commutator segments are connected together as well, before the motor attains running speed.

Once at speed, the rotor windings become functionally equivalent to the squirrel-cage structure of a conventional induction motor, and the motor runs as such.

Web ref. [1] gives a nice, concise description

Laboratory commutators

Commutators were used as simple forward-off-reverse switches for electrical experiments in physics laboratories. There are two well-known historical types [13]:

Ruhmkorff commutator

This is similar in design to the commutators used in motors and dynamos. It was usually constructed of brass and ivory (later ebonite) [14].

Pohl commutator

This consisted of a block of wood or ebonite with four wells, containing mercury, which were cross-connected by copper wires. The output was taken from a pair of curved copper wires which were moved to dip into one or other pair of mercury wells [15].

See also

Patents

References

  1. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 21: Brushes and the Brush Gear, p. 300, fig. 327
  2. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 21: Brushes and the Brush Gear, p. 304, fig. 329-332
  3. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 21: Brushes and the Brush Gear, p. 313
  4. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 21: Brushes and the Brush Gear, p. 307, fig. 335
  5. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 21: Brushes and the Brush Gear, p. 312, fig. 339
  6. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 284, fig. 300
  7. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 285, fig. 301
  8. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 264, fig. 286
  9. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 265, fig. 287
  10. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 286, fig. 302
  11. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 285-287
  12. ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed. 1917, vol. 1, ch. 20: Commutation and the Commutator, p. 287, fig. 303
  13. ^ Hadley, H. E., Magnetism and Electricity for Students, MacMillan, London, 1905, pp 245-247
  14. ^ http://www.fstfirenze.it/collezioni/scientifico_en/isin.asp?Id=0556
  15. ^ http://www.fstfirenze.it/collezioni/scientifico_en/isin.asp?Id=0559

External links


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