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How to Identify a Run Capacitor

Single-phase motors often incorporate start and run capacitors. As the motor starts, a start capacitor shifts the current phase to provide additional magnetic fields that help the motor start. A run capacitor stores electricity as the current phase rises and releases it as the phase falls during its normal, alternating current cycle. This increases the efficiency of the motor. A run capacitor only needs to store a little electricity and release it at the right time, while a start capacitor has much higher storage capability. Determining which capacitor is the run capacitor is not always straightforward.

Things You'll Need

Screwdriver
Capacitance meter

Instructions

- 1
  Turn off power to the motor by unplugging it or turning the disconnect switch off. Locate the capacitor in question and short the two terminals by placing the screwdriver on both capacitor leads at the same time. Do not touch the metal screwdriver shaft.
- 2
  Remove the leads attached to the capacitor by unscrewing the terminals. Turn on the capacitance tester and touch one tester lead to each terminal. Read the display on the tester and note the capacitance measured.
- 3
  Read the motor nameplate to find the horsepower rating. Compare the reading on the meter to the tables found in the Resources link. For example, the capacitor on a 1/2 horsepower motor is approximately 10 microfarads. On the run capacitor table, this motor has a capacitor of about 10 microfarads. The capacitor is run capacitor. As a second example, the capacitor on a 1/2 horsepower motor is between 80 and 110 microfarads. This is a start capacitor.
- 4
  Look at the capacitor for polarity markings such as plus (+) or minus (-). A polarity mark indicates the capacitor is an electrolytic capacitor. You only use electrolytic capacitors as start capacitors and never as run capacitors. This is not a definitive test, but can help rule out a capacitor as a run capacitor.
- 5
  Compare the tester reading to 70 microfarads. Run capacitors are typically less than 70 microfarads and start capacitors on 1/2 horsepower motors or larger are greater than 70 microfarads. This test is definitive for motors greater than 1/2 horsepower, but not definitive for smaller motors.

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