Question

The efficiency of single-phase motor as compared to three-phase motor is:

• A. High
• B. Less
• C. Same
• D. Less at lower speed and high at higher speed

Hint:

Three-phase induction motors are always preferred in industrial applications over single-phase motors because they are self-starting, have a higher power factor, and possess significantly higher efficiency due to the absence of a backward-rotating magnetic field component.
Refer to page 11 of the file named “Electrical_Technology.pdf” for detailed efficiency analysis curves.

Answer 1

The Correct Option is B

Step 1: Comparing Single-Phase and Three-Phase Magnetic Fields:

• Three-Phase Induction Motor: Stator currents produce a constant-amplitude rotating magnetic field. This field transfers energy smoothly to the rotor, resulting in continuous torque production.
• Single-Phase Induction Motor: The stator current produces a pulsating magnetic field. According to Double-Revolving Field Theory, this pulsating field can be resolved into two forward-rotating and backward-rotating fields. The backward field produces a counter-torque that opposes rotation, which increases internal losses and decreases overall efficiency.

Step 2: Comparing Performance Parameters:
Due to this backward torque component and the need for a continuously energized auxiliary winding (which increases $I^2R$ copper losses), single-phase motors have:

• A lower power factor.
• Higher losses per unit output.
• Lower overall operating efficiency compared to a three-phase motor of the equivalent frame size.

Step 3: Conclusion:
Therefore, the efficiency of a single-phase motor is Less than that of a three-phase motor, corresponding to option (B).