Question

A synchronous motor can be used for power factor correction when it is:

• A. Under-excited only
• B. Over-excited only
• C. Operating at no load only
• D. Both Over-excited and Operating at no load

Hint:

Always remember the excitation rules for a synchronous motor:
1. Under-excited $\implies$ draws lagging current (behaves as an inductor).
2. Over-excited $\implies$ draws leading current (behaves as a capacitor).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks about the specific operating condition under which a synchronous motor can improve the power factor of an electrical system.

Step 2: Key Formula or Approach:
The excitation level determines the relationship between the back EMF ($E_b$) and the terminal voltage ($V$).
An over-excited synchronous motor ($E_b \cos\delta > V$) draws a leading current, which is equivalent to acting as a source of reactive power.

Step 3: Detailed Explanation:

• Synchronous motors have the unique ability to run at a wide range of power factors by adjusting their DC field excitation.

• When the DC field excitation is low (under-excited), the motor draws a lagging current from the AC supply to help establish the necessary air-gap flux.

• When the field current is increased beyond a certain limit, the motor becomes over-excited.

• In the over-excited state, the rotor poles produce more flux than is required by the terminal voltage.

• Consequently, the motor draws a leading current from the supply to counteract this excess flux via demagnetizing armature reaction.

• A leading current behaves exactly like the current drawn by a capacitor bank.

• Connecting an over-excited synchronous motor to a system with lagging inductive loads helps neutralize the lagging reactive current, thereby correcting and improving the overall system power factor.

• Note that while a synchronous motor operating at no load with over-excitation is specifically called a "synchronous condenser", any over-excited synchronous motor (even under load) draws leading current and corrects the power factor. Therefore, "Over-excited only" is the fundamental condition.

Step 4: Final Answer
Thus, the correct option is (B).