3. Why do single-phase motors have two capacitors?

This is then an operating capacitor and a starting capacitor.

About the AC motor function

As the at least two phase-shifted voltages required to generate a rotating field are not available with 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 second winding, which is arranged at 90 degrees to this. This causes the phase shift of the voltage on the second winding. Although the rotating field generated in this way is sufficient to move the rotor, it is also load-dependent and results in a low starting torque. For this reason, capacitor motors should start up to the rated operating speed with as little load as possible. The starting torque can be significantly improved if an additional starting capacitor, approximately 2-3 times the size of the existing one, is connected in parallel for the duration of the start-up. The higher starting current requirement, which can be several times the operating current at nominal speed, must be taken into account. Strictly speaking, the size of the capacitor can only be optimised for a single load case. With optimised capacitor dimensioning, around 65% of the mechanical power can be achieved in comparison to an approximately identical asynchronous motor with three-phase supply. High-quality capacitors are usually used for the phase-shifting or starting capacitor. Typical capacitance values are approx. 20µF per kW motor power. With heavy starting, the capacitance value can be up to approx. 50µF/kW.