How To Test Start And Run Capacitors
Start and run capacitors are found in many single-phase motor circuits and is one of the biggest causes of motor failure. They are found in your refrigerator, deep freezer, air conditioner, furnace, and many of your motor driven power tools. A starting capacitor that has failed or a starting capacitor that is about to fail is enough to keep a capacitor start motor from starting under load.
A run capacitor that has failed or a run capacitor that is about to fail will keep a motor from running at peak performance, can cause the motor to over heat, or can keep it from running at all. Capacitors are uncomplicated devices and are easy to test using an analog multimeter. This is the one time when an analog multimeter will serve you better than a digital multimeter and you need to purchase one just for testing capacitors. An inexpensive but very accurate analog multimeter can be had for under $20 from Radio Shack.
Degree of difficulty: 2 on a scale of 1 to 10.
Tools and test equipment that you will need.
Needle nose pliers
Capacitors are a great deal like a battery in that they store electrical energy. Capacitors can store a lethal charge even after the motor is disconnected from the power source so use caution when working with capacitors. Always discharge the capacitor before touching the capacitor's terminals or the wires connected to their terminals. The quickest way to discharge a capacitor is to short its terminals together using a screwdriver with a well insulated handle.
Touch the blade to one terminal and then slowly bring the shaft into contact with the second terminal. If the capacitor is good and has a charge, the current will arc over to the screwdriver's shaft before it makes contact with the second terminal.
There will be a loud crack when the arcing discharge takes place, so be prepared for it. Many people have been injured when they yanked their hand back and struck it against a sharp metal object. Also, you should wear safety glasses when discharging a capacitor.
It does not happen often but a capacitor with a defective case can explode when discharged suddenly. It is not a powerful explosion, but it does not take much of an explosion to send a small piece of the case flying and that tiny piece can blind you if it strikes you in the eye. Always keep one hand in your pocket, so you don't become a gound for the electricity.
Testing a run or start capacitor.
- Discharge the capacitor as described above. The worse case scenario is the failure to discharge the capacitor will cause you to receive a lethal electric shock. The best case scenario is the failure to discharge the capacitor will cause your analog VOM to be destroyed.
- Remove the leads attached to the capacitor's terminals. The leads may be connected to the capacitor with push-on terminals or attached with screws. In the case of push-on terminals, the most common type, pull them off using needle nose pliers.
- Set the function switch on your multimeter to the Ohms function which may be indicated with an “Omega” (Ω) sign. Set the range switch to R X 1,000 or higher. Most VOMs have a 10,000 and 100,000 range and I prefer using the R X 10,000 range for testing motor capacitors.
- Touch the meters probes to the capacitor terminals. If the capacitor is a good capacitor, the needle will swing towards “0” and then slowly drift upwards toward the high side of the scale. If the meter does not react in this manner, the capacitor is bad and should be replaced.