how to test a motor start capacitor

how to test a motor start capacitor

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How to Test a Motor Start Capacitor

1. Safety First

• Disconnect Power: Before starting any testing, ensure that the motor is completely disconnected from the power source. This could be unplugging it from the wall outlet or turning off the circuit breaker that supplies power to the motor. Working on live electrical components can lead to severe electric shock and injury.


• Wear Protective Gear: Put on safety glasses to protect your eyes from any potential sparks or debris. If possible, wear insulated gloves to further reduce the risk of electric shock.

2. Gather Tools

• Multimeter: A digital multimeter is the most common tool for testing capacitors. Make sure your multimeter has a capacitance measurement function. The capacitance symbol on the multimeter dial is usually represented as "F" (for farads) or a set of two vertical lines like "-||-". Some multimeters also have a continuity testing function, which is useful for preliminary checks.


• Insulated Pliers: You'll need these to safely remove the capacitor from the motor and to handle any wires or connections.


• Discharge Resistor (Optional but Recommended): A discharge resistor (around 15 - 20 kΩ) can be used to safely discharge the capacitor before testing. Capacitors can store electrical charge even when the power is off, and discharging them is crucial to prevent shock.

3. Locate the Capacitor

• The start capacitor is typically a cylindrical or box - shaped component located near the motor. In some appliances like washing machines, air conditioners, or refrigerators, it may be inside an electrical enclosure. You may need to remove covers or panels using a screwdriver to access the capacitor.

4. Visual Inspection

• Physical Damage: Look for any signs of physical damage on the capacitor. Check for bulges, cracks, or leaks. A bulging capacitor is often a sign of internal failure. If the capacitor has a metal casing, look for corrosion or rust.


• Label Information: Check the label on the capacitor. It should indicate the capacitance value (measured in microfarads, μF) and the voltage rating. Note down these values as you will need them for further testing.

5. Discharge the Capacitor

• Using a Resistor: If you have a discharge resistor, connect one end of the resistor to one terminal of the capacitor and the other end to the other terminal. Hold it in place for a few seconds. You may see a small spark as the capacitor discharges. This is normal.


• Using a Screwdriver (with Caution): Some electricians use an insulated screwdriver to discharge a capacitor. Touch the metal shaft of the screwdriver to both terminals of the capacitor simultaneously. However, this method is not recommended for those without electrical experience, as it can be dangerous if not done properly.

6. Testing for Continuity

• Set the Multimeter: Set your multimeter to the continuity testing mode. This is usually indicated by a sound waves icon or the word "continuity" on the dial. Touch the two probes of the multimeter together to ensure it is working correctly. You should hear a beeping sound.


• Test the Capacitor: Place one probe on each terminal of the capacitor. If the capacitor is working correctly, you should not hear a beeping sound. A beeping sound indicates that there is continuity (a short circuit) within the capacitor, which means it is faulty and needs to be replaced.

7. Testing for Shorts to the Case (if applicable)

• For Metal - Cased Capacitors: If the capacitor has a metal casing, you can test for shorts between the capacitor's internal components and the case. Set the multimeter to the continuity mode again. Touch one probe to one of the capacitor's terminals and the other probe to the metal case. Repeat this for the other terminal. If you hear a beeping sound in either case, it means there is a short between the capacitor and the case, and the capacitor is bad. Plastic - cased capacitors do not require this test as they are inherently insulated.

8. Measuring Capacitance

• Set the Multimeter: Set the multimeter to the capacitance measurement mode. Make sure the red and black probes are inserted into the appropriate ports on the multimeter for capacitance measurement. Usually, the red probe goes into the port labeled with the capacitance symbol.


• Take the Reading: Place one probe on each terminal of the capacitor. The multimeter will take a few seconds to display a stable reading. Compare this reading with the capacitance value indicated on the capacitor's label.


• Acceptable Tolerance: For start capacitors, the acceptable tolerance is typically ±20% of the rated capacitance value. For example, if the capacitor is rated at 50 μF, a reading between 40 μF and 60 μF would generally be considered acceptable. If the measured capacitance is outside this range, the capacitor may be faulty and should be replaced.

9. Re - installation and Testing

• If the capacitor passes all the tests, you can reinstall it in the motor. Make sure to reconnect the wires correctly using the insulated pliers. Once reinstalled, reconnect the motor to the power source and test if the motor starts smoothly. If the motor still has starting problems, the issue may lie with other components in the motor system, such as the motor windings or the centrifugal switch.