WHAT IS A CAPACITOR?
A capacitor is a device the stores electrical energy. A capacitor can convert current into voltage (Charge) and voltage into current (Discharge). When it is in the voltage form, the energy is being stored not unlike a battery. The conversion back and forth is instantaneous. How much current to voltage or voltage to current conversion happens depends on how fast the applied voltage to the capacitor terminals changes on the capacitor terminals and how big the capacitor is. To learn more about capacitor basics, go here: Capacitance
WHY A MOTOR CAPACITOR?
The capacitor is part of larger electrical filter circuit that is use to control the electrical noise being generated by the motor. This motor noise contains very high frequencies that are the same frequencies used in in the wireless radio world of portable TV's and AM/FM radios. When those motor frequencies are present and get out in the air via the motor wires they can cause TV and AM/FM radio devices to not work properly. The random motor noise creates random static noise which can INTERFERING with the radio device like the noise you often hear on a AM radio in the background. These interfering frequencies are collectively called Radio Frequency Interference (RFI). The electrical filter that capacitor is part of is called a RFI filter who job is to prevent the RFI noise from escaping the motor.
HOW DOES THIS WORK?
The capacitor job is to short out the noise so to speak. Keep it contained right at the motor and not allow the RFI noise to get into the wires which act as a transmitting radio antenna. The capacitor impedance is very low at the high radio frequencies which allows the capacitor to appear to be a short circuit between it two terminals to radio frequencies but not with DC voltage/current which has no frequency (0 hertz). To learn more go here:Capacitor Impedance
RFI (EMI) COMPLIANCE
Limiting RFI is very important so much so the companies selling electrical based products must comply with country rules regarding RFI. A companies sale of a product that does not comply comes with strict penalties. This is vary important to companies that sell products world wide.
To learn more go here. RFI & Governments