What a motor start capacitor does

A motor start capacitor is an electrical component that gives an AC motor the extra push it needs to begin turning. When you flip the switch on a motor — in an air conditioner, refrigerator, well pump, or compressor — the motor needs more power at that first moment than it does while running. The start capacitor stores electrical charge and releases it in a burst, giving the motor's windings enough force to overcome static friction and begin rotation.

Once the motor reaches about 75 percent of its full speed, a device called a centrifugal switch or relay disconnects the capacitor from the circuit. From that point on, the motor runs on its normal operating power. The capacitor sits idle until the motor stops and starts again.

Start capacitors are different from run capacitors, which stay connected to the motor circuit the entire time the motor is running. A motor may have both types, or just one, depending on its design and power requirements.

Key Takeaways

  • Motor start capacitors provide a temporary power boost when a motor first turns on, then disconnect once the motor reaches operating speed.
  • A failed start capacitor usually shows up as a motor that hums but does not turn, or one that turns only after you give it a manual push.
  • Start capacitors are rated by microfarads (μF) and voltage, and you must match both numbers exactly when replacing one.
  • Capacitors fail because of age, heat exposure, electrical surges, or manufacturing defects, and replacement typically costs between $15 and $100 for the part alone.

Signs that a start capacitor is failing

The most common sign of a bad start capacitor is a motor that hums loudly but does not turn. You may hear a buzzing or humming sound from the motor, feel it vibrating, but see the shaft stay still. This happens because the capacitor cannot deliver the starting torque the motor needs, so the motor's windings energize but the rotor cannot overcome the resistance to begin spinning.

Another pattern is a motor that starts only after you manually turn the shaft or give it a physical push. Once it is moving, it runs normally. This indicates the capacitor is weak — it provides some starting power, but not enough to start the motor on its own.

You may also notice the motor takes longer than usual to start, or it starts and stops repeatedly before settling into normal operation. In some cases, the motor straightforward will not start at all, and the circuit breaker may trip if the motor draws too much current while trying to overcome the lack of starting power.

Occasionally a failed capacitor will bulge, leak, or smell burnt. These are signs of internal breakdown and mean the capacitor should be replaced when ready.

Why start capacitors fail

Capacitors are electrolytic devices with a limited lifespan. The internal chemical compounds that store and release charge degrade over time, especially in hot environments. A motor in an attic, garage, or outdoor equipment enclosure will age faster than one in a climate-controlled space.

Electrical surges — from lightning, power line faults, or sudden load changes — can damage a capacitor's internal structure. A single large surge can rupture the dielectric material inside, causing when ready failure. Repeated smaller surges wear the capacitor down gradually.

Manufacturing defects are less common but do occur. A capacitor may fail within months of installation if there was a flaw in the materials or assembly process. This is why some manufacturers offer warranties on new capacitors.

Heat is the primary enemy of capacitor life. For every 10 degrees Celsius above the capacitor's rated temperature, the expected lifespan is cut roughly in half. A capacitor rated for 70°C in a 100°C environment will fail much sooner than one in a cooler location.

How to identify the correct replacement capacitor

Before you buy a replacement, you need two pieces of information from the old capacitor: the microfarad rating (μF) and the voltage rating (V). Both are printed on the capacitor's label. A typical start capacitor might be rated 88 μF at 250 volts, or 145 μF at 250 volts. You must match both numbers exactly — using a capacitor with the wrong microfarad rating will prevent the motor from starting properly, and using one with too low a voltage rating risks failure and fire.

If the old capacitor is too damaged to read, check the motor's nameplate or the equipment's manual. The motor nameplate often lists the required capacitor specifications. If you cannot find that information, a motor repair shop can identify the correct capacitor by examining the motor itself.

Start capacitors come in standard sizes: 88, 110, 125, 145, 160, 200, and 250 microfarads are common. Voltage ratings are usually 250V or 370V (sometimes called 440V). Buying the wrong size will not work, and it may damage the motor or create a safety hazard.

Replacing a motor start capacitor

Replacing a start capacitor requires disconnecting power to the motor and discharging the capacitor before you touch it. Even when power is off, a capacitor can hold an electrical charge that is strong enough to cause injury. Use an insulated screwdriver to short the capacitor terminals together, or use a discharge tool designed for this purpose.

Once the capacitor is safely discharged, note where each wire connects — take a photo if you are unsure. Remove the wires from the old capacitor terminals (usually by loosening small screws), unbolts or unclip the capacitor from its mounting bracket, and install the new one in the same position. Connect the wires to the new capacitor's terminals in the same order, making sure each connection is tight.

After installation, restore power and test the motor. It should start smoothly without humming or hesitation. If it does not, turn off power when ready and check that the wires are connected correctly and the capacitor rating matches the motor's requirements.

If you are not comfortable working with electrical components, a motor repair technician or HVAC service company can replace the capacitor for you. Labor costs vary, but the total job — part and labor — usually runs between $150 and $300 for most household motors.

Start capacitors versus run capacitors

A run capacitor stays in the circuit while the motor is operating, whereas a start capacitor disconnects after the motor reaches speed. Run capacitors are typically smaller in microfarad rating but are designed to handle continuous operation. They improve the motor's efficiency and power factor.

Some motors have both a start capacitor and a run capacitor. The start capacitor provides the initial torque, and the run capacitor keeps the motor running smoothly. Other motors use only a run capacitor, or only a start capacitor, depending on the motor's design and the load it must move.

If a motor has both types and only one fails, the motor may still run but with reduced power or efficiency. A failed run capacitor often causes the motor to run hot, draw more current, and wear out faster. A failed start capacitor prevents the motor from starting at all.

Frequently Asked Questions

Can I run a motor without a start capacitor?

Not reliably. Some single-phase motors are designed to start without a capacitor using a special winding configuration, but most household and commercial motors need a start capacitor to begin rotation. Trying to run a motor that requires a start capacitor without one will result in humming and no motion, or very slow starting that strains the motor.

What happens if I use a capacitor with the wrong microfarad rating?

The motor will not start properly. Too low a rating means insufficient starting torque; too high a rating can cause excessive current draw and overheat the motor windings. Always match the microfarad rating exactly.

How long does a motor start capacitor usually last?

In a cool, stable environment, a quality start capacitor may last 10 to 20 years. In hot or electrically noisy environments, the lifespan may be only 3 to 5 years. There is no way to predict failure in advance, so replacement is done only when the capacitor stops working.

Is it safe to touch a capacitor after the motor is turned off?

No. Even with power off, a capacitor holds an electrical charge. Always discharge it using an insulated tool or a discharge device before handling. If you are unsure how to do this safely, have a technician handle the replacement.

Can a bad start capacitor damage the motor?

A failed start capacitor itself does not usually damage the motor, but the motor's repeated attempts to start without proper capacitor support can cause overheating and wear. Replacing a failed capacitor promptly prevents this secondary damage.