Short note: Capacitor starting 1-phase IM

A capacitor-start motor is a split-phase single-phase induction motor in which a capacitor is connected in series with the auxiliary (starting) winding, to give a larger phase difference between the main and auxiliary currents.

 Supply ---+--- Main winding -------------+
           |                               |
           +--- Aux winding --- C --- S ---+
                        S = centrifugal switch

Working

• Main winding: high inductance, current I_m lags V.
• Auxiliary winding in series with an electrolytic capacitor (50 to 400 μF, short-time rating): its current I_a leads V when the capacitor is selected correctly.
• The phase difference between I_a and I_m becomes nearly 90°, with the windings 90° apart in space, so the starting field is almost a circular rotating field.
• At about 75 to 80 % of N_s the centrifugal switch opens and removes the auxiliary winding and capacitor. The motor then runs as a single-phase motor on the main winding.

Characteristics

• High starting torque: 3 to 4.5 times full-load torque, with a lower starting current than a split-phase motor.
• Torque ∝ I_m I_a sin α, where α is nearly 90°, which gives maximum starting torque.
• Direction of rotation is reversed by interchanging the auxiliary winding terminals.

Uses: compressors, refrigerators, air conditioners, pumps, conveyors, and loads that need good starting torque (up to about 6 kW).