Collection: Guide to Motor Start Capacitors

Motor Start Capacitors are used to boost the starting torque of single-phase electric motors by increasing the current through the start windings during startup. They typically only stay in the circuit for a few seconds before being switched off by a centrifugal or electronic switch inside the motor. If your single-phase motor will not start, very often, the start capacitor (if it has one), could be bad. It is a common failure mode in single phase motors.

A decent sized single phase motor will usually have both starting capacitors and run capacitors. Small fan motors may only have a run capacitor, or even no capacitor at ll. Run capacitors have a smaller capacitance rating than a starting capacitor and are designed for continuous duty operation because they stay in the circuit all the time. It is important to never use a start capacitor as a replacement for a run capacitor because start capacitors are not designed for continuous duty.

Discharge Resistors

Some starting capacitors have a discharge resistor across the terminals. These resistors are typically around 15K-ohm and 1 to 2 Watts. They are not strictly necessary for the motor to operate. Their purpose is to discharge the capacitor after the internal centrifugal switch has turned off the capacitor. This serves two purposes:

Prolongs the life of the internal switch by minimizing sparking across the contacts.
Adds a bit of safety to help prevent an electrical shock from a charged capacitor if someone handles the wires shortly after removing power.

Selecting a Start Capacitor

The two most important values when replacing a capacitor are capacitance and voltage rating. Physical size is the third criteria.

Capacitance: For electric motors this is measured in microfarads. This is usually printed on the capacitor as a number, or number-range, followed by the letters MFD, or uF. The replacement capacitor should match the original capacitance as close as possible.
Voltage Rating: A replacement capacitor must have a voltage rating that is at least as much as the original capacitor. It is ok and actually better if the replacement capacitor has a higher voltage rating than the original. However, higher voltage ratings usually result in an overall larger capacitor. So size should also be considered.
Size: They physical size of the replacement capacitor should be such that it can fit in the motor capacitor housing. Typically increasing either the capacitance or the voltage will result in a larger capacitor.