Guide to 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. I

If your 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. The most common failure mode in these capacitors is open circuit.  

Three Criteria used to select motor run capacitors

  • Capacitance:  This is most commonly measured in microfarads, abbreviated MFD or uF.  For run capacitors this is usually a single number ranging from 5 to 100 followed by either MFD or uF.  The replacement capacitor should match the original capacitance as closely as possible.
  • Voltage Rating:  A replacement capacitor must have a voltage rating that is at least as much as the original capacitor.  It is acceptable to be rated higher, and will actually last longer if it is higher rated.  The downside to scaling up in voltage is the physical size will probably be larger than the original.
  • Size:  Finally, the capacitor size should be such that it will fit in the original housing of the motor.  Typically increasing either capacitance or voltage will result in a larger size. 

Our product filters below can be used to help select the ideal capacitor using the criteria above.