The most common application for capacitors in electric motors is creating the rotating magnetic field that single-phase motors need to start and run. Single-phase power, the kind supplied to most homes and small businesses, cannot produce a rotating magnetic field on its own. Without that rotating field, the motor has no starting torque and simply sits there humming. A capacitor solves this by shifting the timing of electrical current in a second winding, effectively simulating a second phase of power and giving the motor the push it needs to spin.
Why Single-Phase Motors Need Capacitors
Three-phase industrial power naturally creates a rotating magnetic field inside a motor, but single-phase power does not. A single phase of current produces a magnetic field that just pulses back and forth rather than rotating. That pulsing field can keep a motor spinning once it’s already moving, but it cannot generate the torque to start from a standstill.
To get around this, single-phase motors use a second winding alongside the main winding. The capacitor is wired in series with that second winding and shifts the timing of the current flowing through it so that it leads the current in the main winding. When the two currents peak at different moments, the combined effect is a magnetic field that rotates rather than pulsing in place. The starting torque the motor produces is proportional to the strength of both currents and the sine of the phase angle between them, so the closer that timing difference gets to 90 degrees, the stronger the startup torque.
Start Capacitors vs. Run Capacitors
Motors use two distinct types of capacitors depending on the job. Start capacitors provide a large burst of phase-shifted current during startup, then disconnect once the motor reaches speed. They typically range from 70 to over 500 microfarads and carry voltage ratings of 125 V, 165 V, 250 V, or 330 V. Their high capacitance delivers strong starting torque for loads that are hard to get moving, like compressors and pumps.
Run capacitors stay connected the entire time the motor operates. They have much lower capacitance, usually between 1.5 and 100 microfarads, with higher voltage ratings of 240 V or 440 V since they must handle continuous duty. A run capacitor keeps the phase shift going at a lower level, which improves efficiency, reduces vibration, and keeps the motor running smoothly. A quality run capacitor typically lasts 30,000 to 60,000 running hours.
Some motors use both. A capacitor-start, capacitor-run motor uses a large start capacitor to get spinning, then switches to a smaller run capacitor for steady operation. Others, called permanent split capacitor (PSC) motors, use a single run capacitor that stays in the circuit at all times. PSC motors trade some starting torque for simplicity, quiet operation, and long life.
Where You’ll Find Capacitor Motors
PSC motors are everywhere in HVAC systems, powering the fans in air conditioners, furnaces, and heat pumps. Their quiet, efficient operation makes them a natural fit for blowers and ventilation equipment. You’ll also find them in water pumps, oil pumps, and office equipment like copiers. Capacitor-start motors handle heavier startup loads, so they show up in workshop tools, well pumps, and refrigeration compressors where the motor needs to overcome significant resistance before reaching operating speed.
Power Factor Correction
Beyond starting and running motors, capacitors also serve a second important role in larger industrial settings: correcting power factor. Induction motors draw more current than they technically need to do useful work because some of that current goes toward building the magnetic fields inside the motor rather than producing torque. This extra current “lags” behind the voltage, which utilities penalize industrial customers for because it wastes capacity on the electrical grid.
Power factor correction capacitors are installed alongside motors to offset that lagging current. A properly sized correction capacitor can raise a motor’s power factor to about 95%, meaning nearly all the current drawn is doing productive work. This reduces electricity costs, lowers the load on wiring and transformers, and can extend equipment life by reducing the total current flowing through the system.
Signs a Motor Capacitor Is Failing
Because capacitors are so central to motor operation, a failing one produces noticeable symptoms. If a start capacitor fails, the motor simply won’t turn on at all. You might hear a hum or buzz from the motor straining against a magnetic field that isn’t rotating. A failing run capacitor is subtler: the motor may start slowly, overheat during operation, vibrate more than usual, or produce a persistent buzzing sound.
You can test a suspect capacitor with a multimeter set to measure capacitance in microfarads. Every capacitor has its rated value printed on the case, and if the measured value falls outside the manufacturer’s tolerance (run capacitors are commonly rated at plus or minus 6%), the capacitor needs replacing. When selecting a replacement, match the microfarad rating exactly and choose a voltage rating at least 1.5 times higher than the circuit’s maximum voltage. Using the wrong capacitance changes the phase shift and can damage the motor windings, while an undersized voltage rating risks the capacitor failing prematurely or rupturing.