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Synchronous motor can be run on unity, lagging or leading power factor. it is controlled with the field excitation.
Case1: if lagging power factor is required, the field excitation voltage is decreased.
Case2: for unity power factor, equal the excitation voltage to rated voltage.
Case3: for leading power factor, the field excitation voltage is increased.
Thus synchronous motor is used in over-excitation mode and it contributed to the losses.
To improve the power factor, the inductive power is to be compensated.
The SM is the only electrical machine which - when under-exited - can deliver a capacitive power(= consumes the inductive power) , which is needed to reduce the inductive power, and hence increasing the power factor..
More than90 % Motors on a Usual Industrial Plant are Asynchronous, having affect of Lagging / Decreasing Power Factor back into the Electrical Generation & Distribution System.
The Current drawn from the Source & Distrbution System is used in a Given Asynchronous Motor Magnetising System of Coils & obviously for Rotation / Torque output of the Motor. Current Losses in the Magnetising Systems appears as Power Factor Decrease in system.
Synchronous Motors usually Used Permanent Magnet or a Internally / Externally Controllable Magnetising System for its Rotors Coils Magnetizations to run at constant Synchronous Speed. So, it does not draw current from the Input Electrical Power System for its Magnetizing System. hence it doesnot affect Power Factor losses in the Electrical Distribution System
Rather with no-Load Mechanically connected Operation, It can operate with Leading Power Factor. Hence, may be used as Power Factor improvement in a Given Situation.
