Which motor use as power factor improvement?

Question

Examoogle · Accepted Answer

Synchronous motor

Explanation: A synchronous motor is uniquely capable of operating at leading, lagging, or unity power factor depending on its field excitation. When over-excited, it acts as a synchronous condenser, drawing leading current from the supply to compensate for lagging loads, thereby improving the overall power factor of the system.

Working Principle: The power factor of a synchronous motor is controlled by its DC excitation. In an over-excited state, the induced back EMF ($E_b$) exceeds the terminal voltage ($V$), causing the motor to act like a capacitor. This generates reactive power ($Q$) which balances the inductive reactive power of other loads, bringing the system power factor closer to unity.

Key Formulas: $Q = V I \sin(\phi)$ — Reactive power calculation; $E_b > V$ — Condition for leading power factor (Over-excitation)

Key Points: Over-excited synchronous motors supply lagging reactive power to the system.; A synchronous motor running at no load for power factor correction is called a 'Synchronous Condenser'.; Induction motors always consume lagging reactive power, hence they degrade power factor.; Power factor improvement reduces transmission line losses and improves voltage regulation.

Advantages: Continuous control of power factor by adjusting field excitation; Can operate at a leading power factor unlike induction motors; Provides voltage regulation for the local bus; High efficiency in large capacity ratings

Disadvantages: Requires a separate DC excitation source; More complex starting mechanism than induction motors; Costly for small industrial installations compared to capacitor banks

Applications: Large industrial power plants; Transmission substations; Heavy load factories with many induction motors

Additional Info: DC motors and Universal motors are not used for power factor improvement as they are not connected to AC supply or operate on different commutation principles.. Capacitor banks are also used for PF improvement but are static devices, whereas synchronous motors are rotating machines.

Answer: B is correct — Synchronous motors, when over-excited, provide leading reactive power to the network, effectively improving the system power factor.

Exam Tip: Always remember: Over-excited Synchronous Motor = Capacitor (Leading PF), Under-excited Synchronous Motor = Inductor (Lagging PF).

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