Consider the following statements regarding Synchronous Motor:1.A synchronous motor is not self-starting and requires auxiliary means for starting.2.It can operate at unity, lagging, or leading power factor depending on excitation.3.Over-excited synchronous motors are used as synchronous condensers for power factor correction.4.The speed of a synchronous motor varies significantly with load torque.Which of the above statements is/are correct?

Correct Answer: A - 1, 2 and 3 only

Quick Concept Explanation

A conventional synchronous motor is not inherently self-starting. By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support. Key relation: N_s=120f/P.

Exam focus: Excitation controls power factor; frequency and pole count set speed. Common trap: Load changes torque angle, not steady synchronous speed.

Statement-wise Verification

• Statement 1 - Correct.
  A synchronous motor is not self-starting and requires auxiliary means for starting.
  A conventional synchronous motor is not inherently self-starting.
• Statement 2 - Correct.
  It can operate at unity, lagging, or leading power factor depending on excitation.
  By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support.
• Statement 3 - Correct.
  Over-excited synchronous motors are used as synchronous condensers for power factor correction.
  By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support.
• Statement 4 - Incorrect.
  The speed of a synchronous motor varies significantly with load torque.
  Once synchronized, speed remains fixed by f and pole count over the stable load range

Core Concept

A conventional synchronous motor is not inherently self-starting. Once synchronized, it runs at synchronous speed set by supply frequency and pole count, essentially independent of load until pull-out. By adjusting field excitation it can operate at lagging, unity or leading power factor; over-excited operation is used for reactive-power support.

Formula / Key Relationship

N_s=120f/P.

Why the Other Options Are Wrong

Option B is incorrect because it includes incorrect statement(s) 4 and omits correct statement(s) 2, 3. Option C is incorrect because it omits correct statement(s) 1. Option D is incorrect because it includes incorrect statement(s) 4.

Exam Shortcut / Approach

Excitation controls power factor; frequency and pole count set speed.

Common Mistake

Load changes torque angle, not steady synchronous speed.

Quick Revision

Excitation and Power Factor Control is linked with Synchronous Motor, Synchronous Machines and Electrical Machines. Excitation controls power factor; frequency and pole count set speed.