A run capacitor keeps a single-phase electric motor running smoothly and efficiently after it starts up. It stays connected to the motor’s circuit the entire time the motor operates, storing and releasing small amounts of energy to maintain steady torque under load. You’ll find run capacitors in air conditioners, refrigerators, heat pumps, fans, washing machines, and water pumps, essentially any appliance powered by a single-phase AC motor.
How a Run Capacitor Works
Single-phase motors have a fundamental problem: unlike three-phase industrial motors, they can’t naturally create the rotating magnetic field needed to spin a rotor. They solve this by using two sets of copper windings, a main winding and an auxiliary winding, that receive current at slightly different times. The run capacitor is wired in series with the auxiliary winding, and its job is to shift the timing of the electrical current flowing through that winding so it’s roughly 90 degrees out of phase with the current in the main winding.
That 90-degree phase shift is the key. It makes the motor behave like a smoother, quieter two-phase machine. Without the capacitor, the phase difference between the two windings falls well short of 90 degrees, which means weaker torque, more vibration, and higher energy consumption. With it, the motor runs closer to its designed efficiency and produces less noise.
Run Capacitor vs. Start Capacitor
These two components serve different roles and aren’t interchangeable. A start capacitor delivers a large burst of energy to get a motor spinning, then disconnects from the circuit once the motor reaches about 75% of full speed. Start capacitors are rated at 70 microfarads (µF) or higher, reflecting the heavy energy demand of getting a compressor or pump moving from a dead stop. They’re designed for momentary use only.
A run capacitor, by contrast, is rated between 3 and 70 µF and stays energized for as long as the motor runs. It’s built for continuous duty. Where a start capacitor is about raw power at startup, a run capacitor is about sustained efficiency and smooth operation during normal use. A typical example in a residential HVAC system might be a 35 µF run capacitor rated at 370 volts, paired with an 88–108 µF start capacitor rated at 250 volts.
Dual Run Capacitors in HVAC Systems
Many air conditioners and heat pumps use a dual run capacitor, which combines two capacitors into a single canister. It has three terminals instead of two: one marked “C” (common), one marked “HERM” (for the hermetic compressor motor), and one marked “FAN” (for the condenser fan motor). This design saves space inside the unit’s electrical compartment while powering both motors from a single component.
Dual capacitors carry two ratings, written as something like 35/5 µF. The larger number always corresponds to the compressor, which needs more energy support, and the smaller number goes to the fan. If a dual capacitor fails, you can replace it with two separate single run capacitors of matching ratings, though the dual unit is the more compact solution.
Typical Ratings and Lifespan
Run capacitors for residential HVAC and appliance motors typically range from 2 µF to 80 µF, with voltage ratings of either 370 or 440 volts AC. The capacitance tolerance for HVAC applications is usually plus or minus 6%, meaning a 35 µF capacitor should measure somewhere between about 32.9 and 37.1 µF to be considered within spec.
Under normal conditions, a run capacitor lasts 10 to 20 years. Heat is the biggest enemy. Capacitors mounted inside outdoor condenser units bake in summer temperatures and endure thousands of on-off cycles per cooling season, which gradually degrades their internal components. Units in extremely hot climates or those that run frequently tend to land on the shorter end of that range.
Signs of a Failing Run Capacitor
A bad run capacitor usually announces itself through changes in how your equipment sounds or performs. The most common symptoms include:
- Humming without starting: The motor makes a loud hum but won’t spin, or takes a long time to get going.
- Clicking sounds: A repetitive clicking noise coming from the outdoor unit every few seconds.
- Fan not spinning: The unit turns on but the fan blades stay still, even though you can hear the system trying to run.
- Burning smell: A failing capacitor can overheat and produce a noticeable odor near the outdoor unit, though this can also indicate a failing motor or compressor.
- Visible damage: A healthy run capacitor has a flat top and smooth sides. Bulging, cracking, or any liquid oozing from the casing means it has failed.
When a run capacitor weakens but hasn’t fully failed, the motor it supports draws more electricity to compensate. Your energy bills may creep up before any obvious symptoms appear. In an air conditioner, a weak capacitor can also cause the compressor to overheat and shut off on its safety switch, leading to short cycling where the system turns on and off repeatedly without cooling your home.
Testing a Run Capacitor
You can check a run capacitor’s health with a digital multimeter that has a capacitance measurement mode. The critical safety step comes first: capacitors store electrical charge even after the power is off. Before touching anything, turn off all power to the unit, verify with your multimeter that there’s no voltage present, and discharge the capacitor by placing a resistor across its terminals for about five seconds.
Once the capacitor is safely discharged and disconnected from the circuit, set your multimeter to capacitance mode, connect the leads to the capacitor’s terminals, and wait a few seconds for the reading to stabilize. The displayed value should fall within 6% of the number printed on the capacitor’s label. If the reading is significantly lower, the capacitor has weakened and is no longer supporting the motor properly. If the multimeter displays “OL” (overload), the capacitor has either shorted internally or its value exceeds the meter’s range, either way pointing to a bad component.
Replacement run capacitors need to match both the microfarad rating and the voltage rating of the original. You can safely use a higher voltage rating (a 440V capacitor in place of a 370V one), but the microfarad value should match exactly. Using the wrong capacitance forces the motor to work outside its design parameters, shortening the life of both the motor and the new capacitor.