How to Test a Refrigerator Compressor Start Capacitor

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Hermetically sealed compressors use motor start capacitors to help them develop the extra torque needed to start under load. Motor starting capacitors can fail for many reasons, with age and heat being the two biggest causes. The refrigerator will not start in either case. The thermostat closes and the hermetically sealed compressor/motor starts to hum loudly and continues to hum until the motors "Inrush Current," which can be as high as 30 Amperes, trips out the branch circuits circuit breaker or blows the branch circuits fuse. Testing and replacing a motor starting capacitor is easy.

Things You'll Need

  • Screwdrivers
  • two alligator clip test leads
  • 150,000 Ohm, 20 Watt power resistor
  • Needle nose pliers
  • Nut drivers
  • DMM (Digital Multimeter) with "Capacitance" function or a DCM (digital capacitance meter)
  • Locate the motor starting capacitor. The motor starting capacitor, attached to the compressor/motor or secured to the refrigerator frame, looks like a black plastic can with two or more wires attached to it.

  • Carefully remove the end cap or any other enclosure, exposing the electrical connection. Do not touch these connections with your fingers because a capacitor stores a line voltage charge even after the refrigerator is unplugged.

  • Discharge the capacitor by connecting the power resistor between two screwdrivers using the alligator clip leads and touching one screwdriver to each of the capacitors terminals. The charge will drain off slowly because of the resistance so keep the screwdrivers in place for several seconds to make sure the capacitor is completely discharged. repeat this process again after a couple of seconds to discharge any charge the may remain.

  • Remove the push-on connectors using the needle nose pliers and remove the capacitor from the refrigerator. The capacitor's voltage and capacitance will be marked on the case. A typical rating might be 110-Volts, 185 to 225 mfd. The mfd indicates the capacitor's capacitance in Microfarads.

  • Set up your DMM to read capacitance. Attach the test leads to the capacitor's terminals and wait a few seconds for the reading to stabilize. A good capacitor should read within 5 percent of the capacitor's specified range. Anything above 175.75 mfd would indicate a good capacitor for one rated at 185 to 225 mfd.

  • A simple resistance check will tell you if a capacitor is capable of taking a charge but it won't tell you how much of a charge it will accept and hold. All the resistance test really tells you is whether the capacitor's plates are shorted out or if it's open and unable to take a charge

References

  • Photo Credit Jupiterimages/Goodshoot/Getty Images
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