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Commutatorless D.C. Motors can perform continuous (DC) or alternating (AC) rotation with little loss of power, often with no brush. The motor is similar to the squirrel-cage, but the winding bars are not directly connected. Wire coil windings, rather than bars, have a primary winding and a secondary winding, both connected at the stator to produce the polarity of rotation. To reverse direction, simply connect the stator at the midpoint of the two primary windings. This construction greatly increases starting torque, as the windings can be connected to form a synchronized motor with the extra winding. Typically, D.C motors are found in high speed units like fans and blowers, and in small low-voltage units, like fans, which can be operated without electrical connection to the power source.
Fans can be controlled or started at low speeds, and stopped without electromagnetic braking when the proper voltage is applied. As their design allows the fan to speed automatically, they are used in applications where the line frequency varies. They can also be operated backwards with little or no braking, and acceleration and deceleration can be accomplished by changing the voltage supplied to the fan, or by changing the speed at which it is run. For example, for the same voltage, a fan will run slow when less current is drawn.
AC motors are driven at 60 Hz or 50 Hz (single line) frequency by this arrangement of stator, rotor, and iron laminations. The AC induction motor generally has fewer external parts than other electric motors, and the simplicity of its construction can make it attractive for use in small electric devices. In common AC induction motors, the iron laminations are installed in slots in the iron core. Rotor wire wound around the rotor may be connected to the external terminals to rotate the rotor, and may also be connected to the stator's windings. It is common to use a three-phase rotor, with field coils connected to the stator windings and run in phase with the number of poles in the rotor. Quite often the magnetic flux in the field can be increased by forming the winding with multiple turns, acting somewhat like more field windings. d2c66b5586