Question

If the field of a synchronous motor is under excited. The power factor will be :

• A. zero
• B. unity
• C. lagging
• D. leading

Hint:

Synchronous motor excitation and power factor: \[ \text{Under excited} \rightarrow \text{Lagging PF} \] \[ \text{Normally excited} \rightarrow \text{Unity PF} \] \[ \text{Over excited} \rightarrow \text{Leading PF} \]

Answer 1

The Correct Option is C

Concept: The power factor of a synchronous motor depends upon field excitation.
• Under excitation \(\rightarrow\) lagging power factor
• Normal excitation \(\rightarrow\) unity power factor
• Over excitation \(\rightarrow\) leading power factor This property makes synchronous motors useful for power factor correction.

Step 1: Understanding under excitation. Under excitation means: \[ E_f<V \] where:
• \(E_f\) = internal generated EMF,
• \(V\) = supply voltage. In this condition the motor draws reactive power from the supply.

Step 2: Effect on current phase angle. Because reactive power is absorbed: \[ \text{Current lags voltage} \] Hence the motor behaves like an inductive load.

Step 3: Understanding power factor behavior. Therefore: \[ \text{Under excited synchronous motor} \rightarrow \text{Lagging power factor} \]

Step 4: Checking other options.
• Zero power factor \(\rightarrow\) Incorrect
• Unity power factor \(\rightarrow\) Occurs at normal excitation
• Lagging power factor \(\rightarrow\) Correct
• Leading power factor \(\rightarrow\) Occurs under over excitation

Step 5: Selecting the correct answer. Hence the correct option is: \[ \boxed{(3)\; \text{lagging}} \]