Single-phase Motors: Types and Operation

Why it matters

Single-phase supplies power many small motors. Starting arrangements differ between induction motors and universal motors.

Key ideas

• A single main winding produces a pulsating field. In double-revolving-field theory this resolves into equal forward and backward rotating fields. At standstill their induction torques cancel, so the ideal single-winding induction motor has zero starting torque.
• Split-phase motors use an auxiliary winding with a different impedance to create a starting phase difference.
• Capacitor-start motors place a capacitor in the auxiliary circuit for greater starting phase displacement. The starting circuit is disconnected by its designed switching arrangement.
• Capacitor-run and capacitor-start/capacitor-run designs retain a suitably rated running capacitor. Starting and running capacitor duties are different.
• Shaded-pole motors use a shading coil and provide relatively low starting torque.
• A universal motor is a commutator series motor designed for AC or DC. It is self-starting; the induction-motor zero-starting-torque argument does not apply to it.

Formulas and assumptions

For a p-pole induction motor, synchronous speed Ns = 120f/p rpm. Forward slip s = (Ns − N)/Ns; backward-field slip is 2 − s when the rotor runs forward at N rpm. These are fractional slips. In a simplified two-winding starting model, starting torque is proportional to Im Ia sin(α), where α is the phase difference between main and auxiliary currents. A numerical torque requires the machine-dependent coefficient; it cannot be obtained from a guessed universal constant.

Worked example

A four-pole, 50 Hz single-phase induction motor runs forward at 1440 rpm.

1. Ns = 120 × 50 / 4 = 1500 rpm.
2. Forward slip = (1500 − 1440)/1500 = 0.04 = 4%.
3. Backward slip = 2 − 0.04 = 1.96. The forward and backward field torques oppose each other; their difference is the net electromagnetic torque. Determining that torque requires the equivalent-circuit data.

Common mistakes

• Applying the no-starting-torque statement to universal motors.
• Treating a capacitor as an independent source of sustained energy.
• Using cos(α) as a general starting-torque formula for two windings.
• Confusing fractional slip with percentage slip.

For GATE EE

Explain double-revolving-field theory, compare starting arrangements and calculate forward and backward slips.

Quick check

1. What are the two slips at standstill? Both are 1.
2. Why use a starting capacitor? To increase the phase displacement between winding currents and obtain starting torque.
3. Is a universal motor self-starting? Yes, when operated as designed.