Capacitor Start vs Capacitor Run Motors: What's the Difference?

If you work with single-phase motors, you have likely come across two common types: capacitor start and capacitor run. While they sound similar, they serve very different purposes. Choosing the wrong one can lead to poor performance or motor failure.

This article explains how each motor works, where they are used, and how to tell them apart.

Why Single-Phase Motors Need Capacitors

Single-phase power does not create a rotating magnetic field on its own. A single-phase motor with just one winding will vibrate but not start. It needs something to create a phase shift between two windings. That is where capacitors come in (Herman 45).

Capacitors store and release electrical energy. When placed in a motor circuit, they change the timing of the current in the auxiliary winding. This creates a rotating magnetic field that starts the motor turning.

 Capacitor Start Motors: Built for Heavy Lifting

A capacitor start motor uses a capacitor only during starting. Once the motor reaches about 75% of full speed, a switch disconnects the capacitor.

 How It Works

The motor has a main winding and a start winding. A start capacitor connects in series with the start winding. This capacitor is usually an electrolytic type that provides high capacitance in a small package (Nidec Corporation). The high capacitance creates a strong phase shift, producing high starting torque.

When the motor speeds up, a centrifugal switch or relay cuts the start winding and capacitor out of the circuit. The motor then runs on the main winding alone.

 Typical Capacitance Values

Start capacitors typically range from 70 µF to over 400 µF. For example:

- A 1/3 HP motor might use a 160–190 µF capacitor

- A 1 HP motor often requires 380–440 µF

These capacitors are rated for intermittent duty. They are designed to be energized for only a second or two. If they stay in the circuit too long, they overheat and fail.

Common Applications

Because they provide high starting torque, capacitor start motors are ideal for equipment that starts under load:

- Compressors

- Conveyors

- Pumps

- Air conditioners

Capacitor Run Motors: Smooth and Efficient

A capacitor run motor (also called permanent split capacitor or PSC motor) keeps the capacitor in the circuit at all times. There is no switch.

 How It Works

A capacitor is permanently connected in series with the auxiliary winding. The capacitor value is much lower—typically 2.5 µF to 50 µF (TEMCo Industrial). This lower value provides a continuous phase shift while the motor runs.

Because the auxiliary winding stays energized, the motor runs smoother with less vibration and better efficiency than a capacitor start motor during operation.

 Run Capacitor Characteristics

Run capacitors use different construction. They are oil-filled or polymer-based, designed for continuous duty (EEPower). They can last for tens of thousands of hours. Common voltage ratings are 370 VAC or 440 VAC.

 Common Applications

Capacitor run motors excel where smooth operation matters more than raw starting power:

- Ceiling fans

- Furnace blowers

- Unit heaters

- Equipment that runs continuously

 A Limitation to Know

The trade-off is starting torque. Capacitor run motors have low starting torque. If you try to start a loaded compressor with one, it may struggle or fail to start.

 Capacitor Start Capacitor Run Motors: The Best of Both

There is a third type: the capacitor start capacitor run motor (CSCR) . These motors use two capacitors—one for starting and one for running.

 How It Works

At startup, both capacitors connect. This gives high starting torque. When the motor reaches speed, a switch disconnects the start capacitor, but the run capacitor stays in. The motor then runs with the efficiency of a capacitor run motor (Goodheart-Willcox).

This design produces the highest torque of any single-phase motor, both at startup and during operation.

 Where You Find Them

CSCR motors are used in demanding applications:

- Commercial refrigeration

- Large air conditioning compressors

- Industrial equipment that starts under heavy load and runs continuously

The downside is cost. Two capacitors and more complex windings make these motors more expensive.

Can You Interchange Start and Run Capacitors?

Never use a start capacitor as a run capacitor. Start capacitors are not designed for continuous current. If left energized, they overheat and can fail dangerously (Ivanov et al.).

Can a run capacitor be used as a start capacitor? Rarely. Run capacitors have much lower capacitance, so they rarely provide enough starting torque. Always use the correct type.

 How to Choose the Right Motor

Your choice depends on your load.

If your equipment starts under a heavy load—like a compressor or conveyor—you need a capacitor start motor (or CSCR motor). The high starting torque gets the load moving.

If your equipment starts easily but runs for long periods—like a fan or blower—a capacitor run motor makes more sense. It runs smoother and more efficiently.

For critical applications needing both high starting torque and efficient continuous operation, invest in a CSCR motor. The higher cost pays off in performance.