Among three-phase power supply the synchronous machines are widely used and they called so because the speed of the rotor of these machines are the same as the rotating magnetic field. They are commonly used as generators especially for large power systems, such as turbine and hydroelectric generators in the grid power supply. Because the rotor speed is proportional to the frequency of excitation,synchronous motors can be used in situations where constant speed drive is required. Since the reactive power generated by a synchronous machine can be adjusted by controlling the magnitude of the rotor field current, unloaded synchronous machines are also often installed in power systems solely for power factor correction or for control of reactive kVA flow. Their construction is almost similar to that a three-phase induction motor except the fact that the rotor is given DC supply.
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