How to work the commutator

 Commutator (electric)

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
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  • 1 Principle of Operation
    • 1.1 Simplest practical commutator
  • 2 Ring/Segment Construction
  • 3 Brush Construction
    • 3.1 Brush Holders
    • 3.2 Brush Contact Angle
  • 4 The Commutating Plane
    • 4.1 Compensation for stator field distortion
    • 4.2 Further Compensation for Self-Induction
  • 5 Limitations and alternatives
  • 6 Repulsion induction motors
  • 7 Laboratory commutators
    • 7.1 Ruhmkorff commutator
    • 7.2 Pohl commutator
  • 8 See also
  • 9 Patents
  • 10 References
  • 11 External links

 
[edit] Principle of Operation
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 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.
[edit] 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 (see image at right for 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 uncommon in motors of this sort. In other sorts of motors, it is 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.
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 with the 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".
[edit] 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.