Not self starting and permanent split capacitor motor

Why a single-phase induction motor is not self-starting

A single-phase stator winding produces a pulsating (alternating) field along one fixed axis, not a rotating field. By double field revolving theory, this field equals two fields of half amplitude, $\phi_m/2$, rotating at synchronous speed in opposite directions.

• At standstill the rotor slip with respect to both fields is $s = 1$.
• Both fields induce equal rotor currents and produce equal and opposite torques, $T_f = T_b$.
• Net starting torque $T = T_f - T_b = 0$, so the rotor only hums and does not start.

(Cross-field view: rotor currents at standstill produce a field in line with the stator field, so there is no torque-producing displacement.)

If the rotor is pushed in either direction, $T_f \ne T_b$ and it accelerates in that direction. So an auxiliary means is needed to create a rotating field at start.

Permanent split capacitor (PSC) motor

Construction:

• Main winding and auxiliary winding displaced $90^\circ$ electrical in space.
• One oil-filled paper capacitor of small value (e.g. 2–20 μF) is permanently connected in series with the auxiliary winding. There is no centrifugal switch.
• Both windings are usually identical (same copper), so the motor works like a two-phase motor.

  o-------+---------------+
          |               |
        Main          Auxiliary
       winding         winding
  1-ph    |               |
  supply  |              [C]  (permanent)
          |               |
  o-------+---------------+

Working principle:

1. The capacitor makes $I_a$ lead the supply voltage, while $I_m$ lags it. The angle between them is large (about $90^\circ$ at the design load).
2. Two space-displaced windings carrying time-displaced currents produce a rotating magnetic field (nearly uniform at rated load), and the cage rotor starts and runs like a two-phase induction motor.
3. Because the capacitor is chosen for good running, the starting torque is only moderate (about 50–100% of full-load torque).
4. Direction can be reversed easily by switching the capacitor from one winding to the other.

        I_a
         ^   alpha ~ 90 deg
         |
  -------+--------> V
          \
           v I_m

Features: no switch to fail, high power factor, good efficiency, quiet and smooth running, speed control by tapped windings or voltage.

Applications:

• Ceiling fans, table fans, exhaust fans and blowers
• Air-conditioner and refrigerator fan motors, room coolers
• Oil burners, office machines, motor-operated valves and reversible drives