This quick electrical check for compressors that fail to start does not cover those compressors that operate with capacitors. If you are checking a compressor with a capacitor(s), follow the same procedure after checking the capacitor(s) as outlined below. If the capacitor is bad, it must be replaced with one of the same microfarad (mfd or mf) rating.
IMPORTANT: After disconnecting power from a unit, discharge the capacitor first by shunting between its two poles with well-insulated heavy wire before handling it. If there is no exact capacitor replacement (or a capacitor tester) available, you can make a compressor test cord that can test capacitors too. See figure 34a and follow these instructions:
1. Disconnect the unit from the power supply.
2. Discharge the capacitor and remove it from the unit. (See step b in “Testing Capacitors Using an Ohmmeter.”)
3. Put a 60-watt light bulb in socket A (fig. 34a).
4. Insulate the alligator clip marked S and connect the alligator clips marked C and R to the two capacitor poles.
5. Connect plug B to power.
The lamp will,
a. glow dimly if the capacitor is good,
b. not glow if the capacitor is open (replace the capacitor),
c. glow brightly if the capacitor is shorted (replace the capacitor).
6. If the capacitor checks good, replace the 60-watt bulb with a 200-watt bulb and leave the capacitor as it is for no longer than five seconds.
7. Disconnect the cord from the power. Using a heavy insulated wire short across the two capacitor terminals. A spark is the indication of a good capacitor (meaning that the capacitor can load and discharge).
Bigger compressor motors that require more starting or running torque due to heavier loads use capacitors to increase their torque. This is especially true in commercial refrigeration and air-conditioning. They are referred to as start or run capacitors. Some compressors use only run capacitors, and some use both.
Capacitor testers can be purchased at refrigeration supply houses for very affordable prices.