Split phase induction motor

A split-phase induction motor is a single-phase induction motor with two stator windings, the main (running) winding and the auxiliary (starting) winding, placed 90° apart in space, connected in parallel to the supply and designed to give a phase difference between their currents so that a rotating field is created at starting.

Construction

   Supply ----+---------------+
              |               |
          Main winding   Aux winding
          (thick wire,   (thin wire,
          many turns,    fewer turns,
          low R, high X) high R)
              |               |
              |        Centrifugal switch S
              +---------------+

• Stator: laminated core with the two windings. The main winding is of thick wire, occupying the bottom of the slots, and has high inductance. The auxiliary winding is thin wire with high R/X, kept at the top of the slots.
• Rotor: squirrel cage.
• Centrifugal switch in series with the auxiliary winding.

Operating principle

1. At starting both windings are connected. The main winding current I_m lags V by about 60° to 80° because of its high reactance. The auxiliary winding current I_a lags V by only about 20° to 30°, because of its high resistance.
2. The phase difference between I_m and I_a is about 30° to 40°, and the windings are 90° apart in space, so a weak elliptical rotating field results. The starting torque is T_st ∝ I_m I_a sin α (α = angle between currents).
3. The rotor accelerates in the direction of the rotation of the field. At about 75 to 80 % of synchronous speed, the centrifugal switch opens and the auxiliary winding is disconnected; the motor runs as a single-phase induction motor on the main winding alone.

 Phasors (V as reference):
   Ia lags V by about 25°
   Im lags V by about 65°
   angle between Ia and Im = about 40° (not 90°),
   so the starting field is elliptical and weak

Characteristics

• Starting torque: 1.5 to 2 times the full-load torque; starting current: 6 to 8 times full-load current (high).
• Torque-speed: torque rises to the pull-out value at about 80 % of N_s; after the switch operates, the curve follows the main-winding curve.
• Speed regulation is good: nearly constant speed, with 3 to 5 % speed drop.
• Power factor is about 0.6 to 0.7 lagging; efficiency is low to medium.
• Direction of rotation can be reversed by interchanging the terminals of either the main winding or the auxiliary winding (not both).
• Rating: 1/20 to 1/2 kW (fractional kW).
• Uses: fans, blowers, washing machines, grinders, small lathes, centrifugal pumps, office equipment.
• Limitation: low starting torque and high starting current, so it is not used where heavy loads start. The capacitor-start motor is better.