While induction motors rotate with a slip, i.e., rpm is less than the synchronous speed, the synchronous motor rotate with no slip, i.e., the RPM is same as the synchronous speed governed by supply frequency and number of poles. The speed of the rotor is a function of the supply frequency and the number of magnetic poles in the stator. The rotor magnetic field locks onto the stator rotating magnetic field and rotates at the same speed. The rotor is fed by DC from a separate source. Synchronous Motors AC power is fed to the stator of the synchronous motor. Direct current motors are used in special applications-where high torque starting or where smooth acceleration over a broad speed range is required. Direct-Current Motors Direct-Current motors, as the name implies, use direct-unidirectional, current. The 3-phase induction motor has three windings each connected to a separate phase of the power supply. These motors drive pumps, blowers and fans, compressors, conveyers and production lines. The 3-phase squirrel cage motor is the workhorse of industry it is rugged and reliable, and is by far the most common motor type used in industry. This induced rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate. In induction motors, the induced magnetic field of the stator winding induces a current in the rotor. 2.2 Motor Types Induction Motors Induction motors are the most commonly used prime mover for various equipments in industrial applications. All motor types have the same four operating components: stator (stationary windings), rotor (rotating windings), bearings, and frame (enclosure). Industrial electric motors can be broadly classified as induction motors, direct current motors or synchronous motors. Motors convert electrical energy into mechanical energy by the interaction between the magnetic fields set up in the stator and rotor windings.
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