4 Ways to Test a Start Capacitor

How To Test A Start Capacitor
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Start capacitors are essential components of many electrical devices, such as air conditioners, refrigerators, and washing machines. They provide the extra boost of power needed to start the motor. Over time, start capacitors can fail, which can prevent the device from starting. There are a few simple tests you can perform to check if a start capacitor is bad.

The first step is to visually inspect the capacitor. Look for any signs of damage, such as cracks, bulges, or leaks. If you see any damage, the capacitor is likely bad and should be replaced. If the capacitor looks fine, you can proceed to the next step.

The next step is to use a multimeter to test the capacitance of the capacitor. A multimeter is a device that can measure electrical properties, such as voltage, current, and capacitance. To test the capacitance of a capacitor, set the multimeter to the capacitance setting and connect the probes to the terminals of the capacitor. The multimeter will display the capacitance value in farads (F). The capacitance value of a start capacitor should be within a certain range, which is typically specified on the capacitor itself. If the capacitance value is outside of this range, the capacitor is likely bad and should be replaced.

Identifying the Start Capacitor

Start capacitors play a crucial role in starting single-phase AC motors and are essential for maintaining optimal motor performance. To effectively test a start capacitor, it’s imperative to first identify it accurately. Here are some helpful steps to guide you through the identification process:

1. Visual Examination:

Inspect the motor and locate the capacitor. Typically, start capacitors are cylindrical or oval-shaped, with two terminals and a marking indicating their capacitance. They are often positioned near the motor’s terminal box or mounted directly on it. Look for any visible signs of damage, such as bulges, cracks, or leaks. These indications may suggest a faulty capacitor.

Capacitor Type	Shape	Terminals	Location
Cylindrical	Round, cylindrical	2	Near terminal box
Oval	Elongated, oval	2	Near terminal box
Box	Rectangular	2	Mounted on motor

2. Terminal Labeling:

Examine the capacitor’s terminals. In most cases, start capacitors will have two terminals labeled “S” or “ST” for start.

3. Capacitance Measurement:

Using a capacitance meter, measure the capacitance of the capacitor. The measured value should be close to the value printed on the capacitor’s label. A significant deviation may indicate a faulty capacitor.

Safety Precautions

1. Discharge the Capacitor

Before testing the start capacitor, it is extremely important to discharge it first. A charged capacitor can store a significant amount of electricity, which can be dangerous if released suddenly. To discharge the capacitor, turn off the power to the appliance it is connected to and use an insulated screwdriver to short the terminals of the capacitor together. This will discharge the capacitor and make it safe to test.

2. Test the Capacitor with a Multimeter

Once the capacitor is discharged, you can test it using a multimeter. Follow the steps below to test the capacitor:

Set your multimeter to the capacitance range.
Connect the positive lead of the multimeter to one terminal of the capacitor.
Connect the negative lead of the multimeter to the other terminal of the capacitor.
The multimeter will display the capacitance of the capacitor in microfarads (µF).

Compare the measured capacitance to the rated capacitance of the capacitor. If the measured capacitance is significantly lower than the rated capacitance, the capacitor is likely faulty and should be replaced.

3. Check for Leakage Current

In addition to testing the capacitance of the capacitor, you should also check for leakage current. Leakage current is a small amount of current that flows through the capacitor even when it is not charged. Excessive leakage current can indicate a problem with the capacitor.

To check for leakage current, set your multimeter to the milliampere (mA) range. Connect the positive lead of the multimeter to one terminal of the capacitor and the negative lead of the multimeter to the other terminal of the capacitor. The multimeter will display the leakage current in milliamps.

If the leakage current is greater than 1 mA, the capacitor is likely faulty and should be replaced.

Capacitor Discharge

Before performing any capacitor testing, it is crucial to discharge any residual electrical charge stored in the capacitor. This step ensures both safety and accurate testing results. Follow these steps to safely discharge a capacitor:

Short the capacitor terminals: Using insulated pliers or a screwdriver, temporarily connect the positive and negative terminals of the capacitor together. This will create a short circuit and discharge the stored charge.
Wait at least 5 minutes: Allow sufficient time for the capacitor to discharge completely. The discharge process may take longer for larger capacitors with higher capacitance values.
Test the voltage with a multimeter: Set your multimeter to the voltage measurement mode and connect the probes to the capacitor terminals. The voltage reading should be near zero, indicating a fully discharged capacitor.

Caution: Always wear appropriate safety gear, including insulated gloves and safety glasses, when handling electrical components. Ensure the area is well-ventilated, as discharging a capacitor may release a small amount of smoke or gas.

Using a Multimeter

Testing a start capacitor with a multimeter is a straightforward process that involves measuring its capacitance and resistance. Here’s a step-by-step guide:

Set the multimeter to capacitance mode. This mode is typically indicated by the symbol "F" or "uF." If your multimeter doesn’t have a capacitance mode, you can approximate it using the resistance mode (see step 3).
Disconnect the capacitor from the circuit. This is to prevent any residual voltage from affecting the measurement.
Connect the multimeter’s leads to the capacitor terminals. The positive lead goes to the positive terminal and the negative lead goes to the negative terminal.
Read the capacitance value on the multimeter display. Note: If your multimeter doesn’t have a capacitance mode, you can approximate the capacitance using the following formula:

C = 2πfRC

C = Capacitance in farads

f = Frequency in hertz

R = Resistance in ohms

First, set the multimeter to resistance mode and measure the resistance of the capacitor (see step 5). Then, calculate the capacitance using the formula above.

C =	2πfRC
C =	Capacitance in farads
f =	Frequency in hertz
R =	Resistance in ohms

Testing with a Capacitance Meter

1. Gather Necessary Materials

To test a start capacitor using a capacitance meter, you will need: A multimeter with a capacitance range, a start capacitor to be tested, and insulated gloves for safety.

2. Set the Meter

Select the capacitance range on your multimeter that is appropriate for the value of the capacitor you are testing. Refer to the capacitor’s label for its capacitance value.

3. Discharge the Capacitor

Before testing, discharge the capacitor using an appropriate method, such as connecting a resistor across its terminals. Do not touch the capacitor terminals directly.

4. Connect the Meter

Connect the test leads of the capacitance meter to the terminals of the capacitor. The polarity does not matter for this test.

5. Take the Measurement

Read the capacitance value displayed on the meter. Compare the measured value with the rated capacitance of the capacitor as indicated on its label. A tolerance of 5-10% is typically acceptable.

Table: Interpretation of Capacitance Test Results

Measured Capacitance	Interpretation
Close to Rated Capacitance (within 5-10%)	Capacitor is likely functional
Significantly Lower than Rated Capacitance	Capacitor may have lost capacitance and needs to be replaced
Infinite or Open Circuit	Capacitor is open and no longer functional

Inspecting for Physical Damage

Visual inspection is the first step in testing a start capacitor. Examine the capacitor for any signs of physical damage that could indicate a malfunction. Here are some specific areas to inspect:

1. Housing

Inspect the capacitor’s housing for any cracks, dents, or other damage. Check for any loose wires or terminals, which could also indicate a problem.

2. Terminals

Check the terminals for any corrosion or discoloration. Loose or damaged terminals can lead to poor electrical connections and can affect the capacitor’s performance.

3. Leads

Examine the capacitor’s leads for any breaks or damage. Broken leads can prevent the capacitor from functioning properly.

4. Capacitor Body

Inspect the capacitor’s body for any bulges or swelling. A swollen capacitor can indicate internal damage and should be replaced immediately.

5. Case

Check the capacitor’s case for any leaks or seepage. A leaking capacitor can be a safety hazard and should be replaced promptly.

6. Oil Leaks, Bulges, or Scorching

Start capacitors typically contain an oily substance as a dielectric. If the capacitor is damaged, it may leak oil or exhibit bulges or scorching around its terminals. These signs indicate a serious problem that requires the capacitor to be replaced.

Leakage Signs	Severity
Minor oil seepage	Monitor and replace if leakage worsens
Significant oil leakage	Replace capacitor immediately
Scorching or bulges	Replace capacitor immediately

Parallel Connection Test

Connect the capacitor in parallel with a known-good capacitor of the same capacitance value. Connect a voltmeter across the terminals of both capacitors. Charge the capacitors by connecting them to a DC power source for a few seconds. Disconnect the power source and observe the voltmeter reading. If the voltmeter reading is equal to the voltage of the power source, then the start capacitor is likely good.

Additional Details

Ensure that the known-good capacitor has a capacitance value within 10% of the start capacitor.
Use a voltmeter with a range that is suitable for the voltage of the power source.
Charge the capacitors for a sufficient amount of time to allow them to reach their full charge.
Disconnect the power source quickly to prevent any discharge current from flowing through the voltmeter.
If the voltmeter reading is significantly lower than the voltage of the power source, it indicates that either the start capacitor or the known-good capacitor is faulty.
Repeat the test with a different known-good capacitor to confirm the diagnosis.
If the test results are inconclusive, refer to the manufacturer’s specifications for further troubleshooting.

Running Capacitor Test

8. Safety Precautions

Always discharge the capacitor before handling it. To discharge a capacitor, connect a jumper wire between the terminals for a few seconds. This will allow the capacitor to discharge safely.

Wear gloves and safety glasses when working with capacitors. Capacitors can store a lot of energy and can release it suddenly, causing injury.

Do not attempt to repair a capacitor. If a capacitor is damaged, it should be replaced.

If you are not comfortable working with capacitors, consult with a qualified electrician.

Safety Precautions
Discharge the capacitor before handling it.
Wear gloves and safety glasses when working with capacitors.
Do not attempt to repair a capacitor.
If you are not comfortable working with capacitors, consult with a qualified electrician.

Troubleshooting Capacitor Issues

1. Visual Inspection

Examine the capacitor for any physical damage, such as bulges, cracks, or leaking electrolyte.

2. Capacitance Measurement

Use a multimeter to measure the capacitance of the capacitor. Compare the measured value to the rated capacitance specified on the capacitor.

3. Resistance Measurement

Measure the resistance across the capacitor terminals. A good capacitor should have a very high resistance (usually several megaohms).

4. Leakage Current Test

Apply a voltage across the capacitor and measure the current flowing through it. A high leakage current indicates a faulty capacitor.

5. Dissipation Factor Test

This test measures the energy lost in a capacitor as heat. A high dissipation factor indicates a capacitor that is not performing at its optimum.

6. Inductance Measurement

For electrolytic capacitors, measure the inductance to identify any internal shorts or other defects.

7. High-Voltage Test

Apply a higher voltage across the capacitor than its rated voltage to check for breakdowns or other problems.

8. Temperature Test

Subject the capacitor to extreme temperatures to test its stability and reliability.

9. In-Circuit Testing

Disconnect the capacitor from the circuit and connect it to a capacitor tester or an LCR meter to measure its capacitance, resistance, and other parameters while the capacitor is still in the circuit.

Test	Purpose
Visual Inspection	Identify physical damage
Capacitance Measurement	Verify correct capacitance value
Resistance Measurement	Detect high resistance or shorts
Leakage Current Test	Identify leaky capacitors
Dissipation Factor Test	Measure energy loss
Inductance Measurement	Identify internal shorts in electrolytic capacitors
High-Voltage Test	Check for breakdowns
Temperature Test	Verify stability at extreme temperatures
In-Circuit Testing	Test capacitor in-situ

Interpreting Test Results

Once you’ve tested your start capacitor, you’ll need to interpret the results to determine if it’s faulty. Here’s a guide to help you:

1. Resistance Measurement:

A good start capacitor should have a very high resistance, typically in the megaohm range. If you measure a resistance below 1 megohm, it indicates a shorted capacitor.

2. Capacitance Measurement:

The capacitance of a start capacitor should be within the manufacturer’s specified range. If the capacitance is significantly lower or higher than expected, it suggests a faulty capacitor.

3. Breakdown Voltage:

The breakdown voltage measures the capacitor’s ability to withstand high voltage without breaking down. A faulty capacitor may have a lower breakdown voltage than the specified value.

4. Leakage Current:

A small amount of leakage current is normal, but excessive leakage indicates a faulty capacitor. Leakage current can be detected by connecting a multimeter in series with the capacitor and measuring the current flowing through it.

5. Dissipation Factor (DF):

The DF measures the capacitor’s energy loss due to resistance and other factors. A high DF indicates a faulty capacitor. DF is typically expressed as a percentage.

6. Power Factor (PF):

The PF measures the capacitor’s ability to store and release energy efficiently. A low PF indicates a faulty capacitor. PF is typically expressed as a percentage.

7. ESR (Equivalent Series Resistance):

ESR is a measure of the capacitor’s internal resistance. A high ESR can limit the capacitor’s performance and cause overheating.

8. Voltage Polarity:

Start capacitors have a specific voltage polarity (+ and -). Connecting the capacitor with the wrong polarity can damage it. Ensure you connect the capacitor correctly during testing.

9. Physical Inspection:

Inspect the capacitor for any physical damage, such as bulges, cracks, or leaks. A damaged capacitor should be replaced immediately.

10. Additional Tests:

Dielectric Absorption (DAR): Measures the capacitor’s ability to store energy after being charged and discharged.
Insulation Resistance (IR): Measures the capacitor’s insulation resistance to prevent electric shock.
Admittance: Measures the capacitor’s ability to pass current at different frequencies.

How to Test a Start Capacitor

A start capacitor is a type of capacitor that is used to provide a boost of power to an electric motor when it is starting up. Start capacitors are typically used in single-phase motors, and they can help to improve the motor’s starting torque and reduce its starting current.

If you suspect that your start capacitor is faulty, there are a few simple tests that you can perform to confirm your suspicions.

Visual Inspection: The first step is to visually inspect the capacitor. Look for any signs of damage, such as bulging or leaking. If you see any damage, the capacitor should be replaced.
Ohmmeter Test: An ohmmeter can be used to test the capacitance of a start capacitor. To do this, set the ohmmeter to the lowest resistance setting and then touch the probes to the terminals of the capacitor.
The ohmmeter should read a very high resistance, and it may take several seconds for the reading to stabilize. If the ohmmeter reads a low resistance, the capacitor is shorted and should be replaced.
Capacitance Meter Test: A capacitance meter can be used to measure the capacitance of a start capacitor. To do this, set the capacitance meter to the appropriate range and then touch the probes to the terminals of the capacitor. The capacitance meter should read the capacitance of the capacitor in microfarads (µF).
If the capacitance meter reads a capacitance that is significantly different from the value that is printed on the capacitor, the capacitor should be replaced.