PSC and capacitor start capacitor run motors

Both motors are capacitor motors in which a capacitor in series with the auxiliary winding makes its current lead the main winding current, so a rotating field is produced.

Permanent split capacitor (PSC) motor

 AC o----+-------------+
         |             |
      Main wdg      Aux wdg
         |             |
         |           C (oil/paper,
         |             permanently in)
 o-------+-------------+
     no centrifugal switch

Principle:

1. One small capacitor (a few µF, continuous rating) stays in series with the auxiliary winding at all times; there is no centrifugal switch.
2. The capacitor makes $I_a$ lead $I_m$ by nearly $90^\circ$, producing a rotating field and starting torque.
3. The capacitor value is chosen for good running performance, so it is small; starting torque is therefore low (about 50-100 % of full-load torque).
4. While running, it behaves like a two-phase motor: quiet, smooth, with good power factor and efficiency. Speed can be varied by tapping the main winding or changing voltage, and direction is easily reversed.

Applications: ceiling and table fans, blowers, air-circulators, room coolers, office machines, and loads with low starting torque.

Capacitor-start capacitor-run (CSCR) motor

 AC o----+-----------------------+
         |                       |
      Main wdg                Aux wdg
         |                   +---+---+
         |                  Cs       Cr
         |                   |       |
         |                switch S   |
 o-------+-------------------+-------+

Principle:

1. Two capacitors: a large starting capacitor $C_s$ (electrolytic) with a centrifugal switch, and a small running capacitor $C_r$ permanently connected.
2. At start, $C_s \parallel C_r$ gives large capacitance and phase difference near $90^\circ$ with high auxiliary current, so starting torque is high (about 3-4 times full load).
3. At about 75 % of $N_s$ the switch removes $C_s$; $C_r$ keeps the motor working as a balanced two-phase motor for good running performance.

Applications: refrigerators, air-conditioners, compressors, pumps, conveyors.

Comparison