Capacitor testing
Capacitors are used in most refrigeration systems and are regularly seen next to, or attached to the compressor or large motors.
Start Capacitor
Start capacitors are usually housed in a black plastic housing and have a mfd range.
Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors are designed for momentary use. They stay energized long enough to rapidly bring the motor to 3/4 of full speed and are then taken out of the circuit
Run Capacitor
Run capacitors are rated in a range of 3-70 microfarad (mfd). They are also rated by voltage classification.
The voltage classifications are 370V and 440V.
Run capacitors are designed for continuous duty, and are energized the entire time the motor is running. Single phase electric motors need a capacitor to energize a second phase winding. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat
Capacitor Discharge
If you suspect that the motor is not at fault by checking the windings and resistance levels it may be a defective capacitor that may be problematic.
For your safety and to avoid damaging the component or test equipment capacitors must be fully discharged before tests are made. This also applies to soldering or before any of the motor circuitry is touched in any way.
Some of the capacitors commonly found in refrigeration system store a potentially lethal charge therefore full discharge is a real necessety.
To discharge a capacitor this procedure is often followed.
- Do this by touching the blade of an insulated-handled screw driver to one terminal, then slide it toward the other terminal until it makes contact and hold it there for a few seconds. You may get a flash or pop which shouldn't be a problem.
- Repeat this procedure to get a short between each capacitor terminal and chassis ground.
- If the capacitor has three terminals, use the same method to create a short circuit between each terminal and then from each terminal to ground.
To test to find out if the capacitor is faulty do the following
- Disconnect the power supply by unplugging.- Dishcharge the capacitor (as above).
- Carefully record which wires connect to the terminals and carefully remove all leads from the capacitor.
- Set you multimeter / ohmmeter (I prefer to use an anlogue rather than digital for this test) to its highest resistance zero or infinity.
- Measure from one terminal to the other for a normal reading of infinity.
- Place one probe of your multimeter on one terminal, then place the other probe on the other terminal. The meter should deflect quickly and then gradually move back to infinity.
- Reverse the leads placing the probes in the opposite direction. This should produce the same meter deflection and act in the same way.
- If the test does the above, the compressor is usueable. If nothing happens, or the reading is different to above its a faulty capacitor
- Visual inspection will also reveal certain defects, such as:
- Evidence of arcing or burning at the insulators.
- Evidence of an oil or smell suggests a leak
- A bulging case indicates dielectric breakdown
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