AC and DC motors also differ in their construction. AC motors do not use brushes. DC motors use brushes. The use of brushes tends to increase maintenance, shorten the life expectancy and limits the speed of the motor. AC motors can withstand wear and tear and have a longer life span. DC and AC motors have differing means of speed control. While DC motors are controlled by varying the armature winding's current, AC motors simply vary the frequency.
An induction motor and a synchronous motor are two types of AC motors. An induction motor uses a magnetic field on a rotor that is created by an induced current. A synchronous motor is powered by a current through slip rings or a permanent magnet. Synchronous motors usually run much faster than induction motors because of the presence of slip rings. DC electric motor repair is much more expensive than AC electric motor repair. Paper producing machines and steel mill rolling machines are a few examples of a DC electric motor.
