1-phase IM not self-starting; two methods

Why single-phase induction motors are not self-starting

A single-phase supply on a single stator winding produces a pulsating field along a fixed axis, not a rotating one. By the double revolving field theory, this is the sum of two equal fields, forward and backward, each of half the magnitude, turning at synchronous speed in opposite directions.

At standstill both fields have slip 1 and induce equal rotor currents. They produce equal and opposite torques, T_f = T_b, so the net starting torque is zero. The rotor does not start by itself, but if it is pushed in either direction, it continues to rotate in that direction.

To start the motor, a rotating field must be created at starting. This is done with a second winding (the auxiliary or starting winding), displaced 90° in space, whose current is out of phase with the main winding current.

Method 1: Split-phase (resistance-start) motor

          +--- Main winding (low R, high L) ---+
 Supply --|                                     |-- Supply
          +--- Aux winding (high R) -- Centrifugal switch --+

• The stator has two windings displaced 90° in space, connected in parallel across the supply.
• The main winding has low resistance and high inductance, so its current I_m lags V by a large angle (about 60° to 80°).
• The auxiliary winding is of thin wire, with a high R/X ratio, so its current I_a lags V by a smaller angle (about 20° to 30°).
• The two currents are about 30° to 40° apart in time and 90° apart in space, producing a weak rotating field and a starting torque (1.5 to 2 times full-load torque).
• At about 75 to 80 % of synchronous speed, a centrifugal switch cuts out the auxiliary winding, and the motor runs on the main winding.
• Uses: fans, blowers, washing machines, grinders (up to about 0.5 kW).

Method 2: Capacitor-start motor

          +--- Main winding ---------------+
 Supply --|                                 |-- Supply
          +--- Aux winding -- C -- Switch -+

• A capacitor (electrolytic, 50 to 400 μF) is placed in series with the auxiliary winding and the centrifugal switch.
• The capacitor makes the auxiliary current I_a lead the voltage, while I_m lags V. The phase angle between them is close to 90°, so the rotating field is nearly circular.
• This gives a high starting torque (3 to 4.5 times full-load torque) with a lower starting current. When the speed reaches about 75 %, the switch opens and the auxiliary winding and capacitor are cut out.
• Uses: compressors, refrigerators, pumps, conveyors.

Other methods are the shaded-pole motor, the permanent-split (capacitor-run) motor and the capacitor-start capacitor-run motor.