Which one of the following statements is NOT true for synchronous motor used for power factor improv

Question

Which one of the following statements is NOT true for synchronous motor used for power factor improvement in comparison with static condenser

Examoogle · Accepted Answer

It requires D. C. supply

Explanation: The statement 'It operates at zero leading power factor only' is false because a synchronous condenser can be operated at a variable power factor (lagging, unity, or leading) by adjusting the field excitation. Option C is technically true, as a synchronous motor requires a separate DC supply for its rotor field winding.

Working Principle: A synchronous condenser acts as a variable reactive power source. When over-excited, it behaves like a capacitor (supplying reactive power), and when under-excited, it behaves like an inductor (absorbing reactive power). The degree of leading or lagging VARs is controlled solely by varying the DC excitation current in the rotor winding.

Key Formulas: $Q = V I_r \sin(\phi)$ — Reactive power control based on excitation; $E_b = \frac{4.44 f \Phi N K_w}{1}$ — Induced EMF depends on flux $\Phi$ adjusted via DC current

Key Points: Synchronous condensers are large synchronous motors running at no load.; They provide smooth, continuous control of reactive power.; Over-excitation provides leading VARs for power factor correction.; Unlike static capacitors, they can compensate for both lagging and leading reactive power.

Advantages: Smooth and stepless reactive power control; Voltage regulation capabilities; Higher thermal stability

Disadvantages: High maintenance due to rotating parts; Requires DC source and starter; Generates acoustic noise

Applications: Large scale industrial power factor correction; Voltage stability in high voltage transmission lines

Additional Info: The phrase 'zero leading power factor' implies an ideal capacitor, whereas a synchronous motor has resistive losses, meaning it can never reach a purely theoretical zero power factor angle.. Static condensers (banks of capacitors) are maintenance-free but provide fixed reactive power.

Answer: B is the correct choice to identify as 'NOT true' because synchronous motors provide variable, not just zero-leading, power factor.

Exam Tip: Always remember that static capacitors are passive elements, while synchronous motors are active rotating machines requiring a DC excitation system.

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