Short note: Capacitor start and run motor

Capacitor-start motor

It has two stator windings placed $90^\circ$ apart in space: a main (running) winding of thick wire and an auxiliary (starting) winding of thinner wire in series with a capacitor ($C$ about 200 to 600 $\mu$F, electrolytic) and a centrifugal switch.

• The capacitor makes the auxiliary current lead the main current by nearly $90^\circ$, so the two windings together give a good two-phase rotating field.
• The starting torque is high (3 to 4.5 times the full-load torque) with a low starting current.
• At about 75 to 80 % of synchronous speed the centrifugal switch opens, removing the auxiliary winding and capacitor. The motor then runs as a single-phase motor on the main winding alone.
• Applications: compressors, refrigerators, pumps, washing machines, air-conditioners, conveyors.

Capacitor-run motor

A capacitor-run motor keeps the auxiliary winding and a capacitor (oil-filled paper, 2 to 20 $\mu$F) permanently in the circuit; there is no centrifugal switch.

• The motor behaves like an unbalanced two-phase motor, so it runs smoothly and quietly, with higher power factor and efficiency.
• The starting torque is low (50 to 100 % of full load), and the cost is low.
• Applications: ceiling fans, blowers, fan heaters, small pumps, and office equipment.

Capacitor-start, capacitor-run motor

This motor uses both capacitors: a large electrolytic starting capacitor that is cut out by the centrifugal switch at about 75 % speed, and a small oil-filled running capacitor that stays permanently in series with the auxiliary winding. It gives high starting torque as well as good running performance (high power factor and efficiency, quiet operation), and is used for compressors, refrigerators and air-conditioners.

 Capacitor start:   Main ---------+
                    Aux--[C]--[S]-+-- supply
 Capacitor run:     Aux--[C]------+ (no switch)