7.2 Why do some single-phase motors have two capacitors?

When a single-phase motor has two capacitors, one is the running capacitor and the other one is the starting capacitor

The AC motor function 

Since the minimum two phase-shifted voltages required to generate a rotating field are not available with a single-phase alternating current, a so-called ‘auxiliary phase’ must be generated. One of the two stator windings is supplied directly from the AC mains, while a capacitor is connected in series to supply the secondary winding, which is arranged at 90 degrees for this purpose. This results in the phase shift of the voltage at the second winding.  The rotating field generated in this way is sufficient to move the rotor, but it is also load-dependent and results in a low starting torque. Therefore, capacitor motors should start up to the rated operating speed with as little load as possible. The starting torque can be noticeably improved by briefly connecting an additional so-called starting capacitor of about 2-3 times the size in parallel with the existing one for the duration of the start-up. In this case, the higher starting current requirement must be considered, which can be several times the operating current at the rated speed. Strictly speaking, the size of the capacitor can only be optimised for a single load case. With optimum capacitor sizing, approx. 65% of the mechanical power can be achieved compared to an approx. identical asynchronous motor with a three-phase supply. High-quality capacitors are usually used for the phase-shifting or starting capacitor. Typical capacitance values are about 20µF per kW of motor power. For heavy starting, the capacitance value can be up to approx. 50µF/kW.



Single phase AC motor