There are many kinds of motors. Among them 98% cases ac induction motors are used. There are two further kinds of induction motors. First and the most commonly used one is squirrel cage induction motor and the second one is off course wound rotor or slip ring induction motor. The squirrel cage induction motor is the most used on in industry from a fraction of Kw to a few Mw power rating. The other type wound rotor or slip ring induction motor is used to drive very high torque load and have ability to start very slowly under very high torque load and controls its current by resistance addition in the rotor circuit. The rotor circuit impedance is controlling the stater supplied current by transformer action. So the way to control the stater current is to insert impedance in the rotor circuit. For this purpose slip rings are used to bring the rotor power out of the motor on carbon brushes to outside circuit where impedance is introduced to limit the current in the motor starting condition. Latter on such impedance is reduced to zero or short circuit is created on the rotor circuit to insure maximum current flow through the rotor circuit hence maximum torque from the motor. Such motors are used to drive high inertia loads. Such loads examples are mills, gearboxes and huge fans.
There is another kind of ac motor used rarely in industry is synchronous motor. Synchronous motor is rarely used due to its complex construction and difficult starting procedure. Such motor have its special benefits which makes it unique for special purpose use. Those benefits are very high torque and this motor can run any power factor value. It acts like a synchronous capacitor. Meaning to say it is correcting the system power factor also if installed in industry. Also the other benefit is of-course running some mechanical load along with power factor correction.
There are DC motors used too in industry and in house hold use. In industry in older days such dc motors were very commonly used due their powerful torque and due to there speed control characteristics. As in older days frequency inversters or vfd were not invented to vary the induction motor, so DC motor was the only solution to achieve variable speed control.
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