What a capacitor start motor is and why it matters

A capacitor start motor is an electric motor that uses a capacitor — an electrical component that stores and releases energy — to get itself spinning from a standstill. The capacitor gives the motor an extra push at startup, then disconnects once the motor reaches normal speed. You'll find these motors in air conditioning units, refrigerators, well pumps, and many workshop tools because they can start under load (meaning they can begin turning even when something heavy is already attached to them).

Understanding how these motors work helps you recognize problems early, know when a repair is worth the cost, and make sense of what a technician is telling you. A failed capacitor is one of the most common reasons these motors stop working, and it's often a cheap fix if you catch it before the motor itself burns out.

Key Takeaways

  • Capacitor start motors use a temporary electrical boost to begin spinning, then run on standard AC power once moving.
  • The capacitor disconnects automatically after startup through a centrifugal switch, so it only works during the first few seconds.
  • A failed capacitor is the most common reason these motors stop starting, and replacement costs far less than a new motor.
  • Signs of a failing capacitor include humming without spinning, a burning smell, or the motor refusing to start even when power reaches it.

The basic parts and how they work together

A capacitor start motor has four main components: the stator (stationary coils that create a magnetic field), the rotor (the spinning part inside), a capacitor, and a centrifugal switch. When you turn the motor on, electricity flows through both the main winding and the start winding simultaneously. The capacitor sits in series with the start winding and shifts the electrical phase — the timing of the current — so that the magnetic field from the start winding pulls the rotor in a different direction than the main winding alone would.

This phase shift creates a stronger rotating magnetic field, which gives the rotor enough torque to overcome resistance and begin spinning. Once the rotor reaches about 75 percent of its full speed, the centrifugal switch opens automatically. This disconnects the capacitor and the start winding from the circuit, and the motor continues running on the main winding alone. The whole process takes just a few seconds.

The reason this design exists is straightforward: a single winding cannot create a rotating magnetic field by itself — it can only push and pull in one direction. Two windings out of phase with each other create the rotation needed to start. Once the rotor is already moving, it has enough momentum that the single main winding can keep it going.

Why the capacitor fails and what happens when it does

Capacitors are made of two conductive plates separated by an insulating material, all rolled up tightly inside a metal can. Over time, heat, moisture, and electrical stress break down that insulating material. The capacitor loses its ability to store charge, or it develops an internal short circuit. Most capacitors last five to fifteen years depending on how often the motor runs and how hot the environment gets.

When a capacitor fails, the start winding no longer receives the phase-shifted current it needs. The rotor cannot build up enough torque to overcome the load, so the motor hums loudly but does not spin. If you keep trying to start it, current flows through the start winding continuously (since the centrifugal switch never closes), and the start winding overheats and burns out. This is why a motor with a failed capacitor can fail completely if you keep forcing it to start.

A second failure mode is a capacitor short circuit, where the insulation breaks down completely. This can cause a spark inside the motor, a burning smell, or even a small explosion inside the capacitor can itself. If you see smoke or smell burning plastic or electrical odor, stop using the motor when ready.

How to recognize a failing capacitor before the motor dies

The most common sign is a loud hum when you switch the motor on, followed by nothing — no spinning, no movement. The motor is trying to start but cannot generate enough torque. You may also notice the motor is slower than usual once it does start, or it takes longer to reach full speed than it used to.

A second warning sign is a clicking or buzzing sound at startup, which means the centrifugal switch is trying to open and close repeatedly because the rotor is not reaching the speed needed to keep it open. A third sign is a burning smell near the motor, which indicates the start winding is overheating because it is staying energized too long.

If the motor is part of an air conditioning unit, refrigerator, or other appliance, you may notice the appliance is not cooling as well, or it is running constantly without reaching the set temperature. This happens because the motor is not starting reliably or is running at reduced speed.

Replacing a capacitor versus replacing the motor

A replacement capacitor typically costs between $15 and $75 depending on the size and type, plus labor if you hire a technician. A replacement motor can cost $200 to $2,000 or more depending on the process. For this reason, replacing the capacitor is almost always the right choice if the capacitor has failed but the motor itself is still sound.

The catch is that you need to know the capacitor's specifications before you buy a replacement. The capacitor is labeled with its microfarad rating (abbreviated µF or MFD) and its voltage rating (usually 370V or 440V for AC motors). These numbers are printed on the metal can. You must match both numbers exactly — using a capacitor with the wrong rating will not work and may damage the motor further.

If you replace the capacitor and the motor still will not start, the problem is likely in the motor itself — a burned-out winding, a seized bearing, or a broken centrifugal switch. At that point, motor replacement is usually the only option. But if the capacitor was the problem, the motor will start when ready and run normally once the new capacitor is installed.

Capacitor start versus other motor types

A capacitor run motor is different: it keeps the capacitor connected all the time, not just at startup. This makes the motor more efficient and allows it to run at higher speeds, but it requires a larger, more expensive capacitor. Capacitor run motors are common in air conditioning compressors and high-speed applications.

A split-phase motor has a start winding but no capacitor — it relies on the resistance difference between the start and main windings to create the phase shift. Split-phase motors are cheaper to build but weaker at startup, so they are used only for light loads like fans and small pumps.

A permanent split capacitor motor (PSC) is similar to a capacitor run motor but uses a smaller capacitor and is designed for continuous duty. You will see these in furnace blowers and some air handler units.

Safety and when to call a technician

If you see smoke, smell burning, or hear a loud pop or crack from the motor, stop using it when ready and unplug it. Do not try to restart it. A failed capacitor can cause electrical arcing inside the motor, which is a fire hazard.

If you are comfortable working with electrical equipment and you know the capacitor specifications, you can replace the capacitor yourself. The process usually involves unplugging the motor, discharging the capacitor (important — a charged capacitor can shock you), disconnecting the old capacitor, and connecting the new one in the same way. However, if you are unsure about any step, or if the motor is part of a built-in appliance, hire a technician. The cost of a service call is usually less than the cost of a new motor if something goes wrong.

Before you call a technician, write down the motor's model number, the capacitor's microfarad and voltage ratings, and a description of what the motor is doing (or not doing). This information helps the technician diagnose the problem quickly.

Frequently Asked Questions

Can I use a capacitor with a higher microfarad rating than the original?

No. Using a capacitor with a higher microfarad rating will cause the motor to draw too much current at startup, which can damage the start winding or trip a circuit breaker. The microfarad rating must match exactly. The voltage rating can be equal to or higher than the original (for example, 440V instead of 370V), but never lower.

Why does the motor hum but not spin when the capacitor fails?

The hum is the magnetic field trying to push the rotor, but without the phase shift from the capacitor, the force is not strong enough to overcome the load and get the rotor moving. Once the rotor is stationary, it stays stationary because the single main winding cannot create a rotating field on its own.

How long does a capacitor usually last?

Most capacitors last five to fifteen years. Heat shortens this lifespan — a capacitor in a hot attic or near a furnace may fail in three to five years, while one in a cool basement might last twenty years. Running the motor frequently also wears the capacitor faster than occasional use.

What does it mean if the motor starts but then shuts off after a few seconds?

This usually means the centrifugal switch is stuck or the rotor is not reaching full speed. A partially failed capacitor can cause this — the motor gets enough boost to start, but not enough to reach the speed needed to keep the switch open. Have the capacitor tested or replaced.

Is it safe to run a motor with a failed capacitor if I only need it for a short time?

No. Running a motor with a failed capacitor causes the start winding to overheat within seconds. This can damage the winding permanently, turn a $50 repair into a $500 one, or create a fire hazard. Replace the capacitor before using the motor again.