Stators and Rotors
Many rotary electrical machines require current to be conveyed to (or extracted from) a moving rotor, usually by means of brushgear. This brushgear is often the most complex and least reliable part of such a machine. It may also represent a limit on the maximum current that the machine can handle. For this reason, when machines must use two sets of windings, the windings carrying the least current are usually placed on the rotor and those with the highest current on the stator.
The field coils can be mounted on either the rotor or the stator, depending on whichever method is the most cost-effective for the device design.
For electric motors, the field is mounted on the stator. In a brushed DC motor the field is static but the armature current must be commutated, so as to continually rotate. This is done by supplying the armature windings on the rotor through a commutator, a combination of rotating slip ring and switches. AC induction motors also use field coils on the stator, the current on the rotor being supplied by induction in a squirrel cage.
For generators, the field current is smaller than the output current. Accordingly the field is mounted on the rotor and suppplied through slip rings. The output current is taken from the stator, avoiding the need for high-current sliprings. In DC generators, which are now generally obsolete in favour of AC generators with rectifiers, the need for commutation meant that brushgear and commutators could still be required. For the high-current, low-voltage generators used in electroplating, this could require particularly large and complex brushgear.
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