Motor Run Capacitors

Motor run capacitors are used in single-phase motors to engage the secondary winding by phase-shifting the incoming single-phase power. This in effect creates a second phase and allows the motor to run efficiently and with more torque. Nearly all single-phase electric motors have run capacitors, with notable exceptions being very small motors, such as fan motors. 

If a single phase motor is running but drawing high-current, does not have much torque, or is blowing fuses after running for a short while, then the run capacitor could possibly be bad (usually open). 

Single phase motors will often also have start capacitors.  A description of starting capacitors and products can be found on our Start Capacitors web page.

Caldwell Electric can diagnose electric motor problems and offer solutions for repair or replacement. Run capacitors can also be purchased directly from our web site on this page.

Selecting a Run Capacitor

The two most important values when replacing a run 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.