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Motor Start and Run Capacitors

Picture of Motor Run and Start Capacitors.

What are Motor Capacitors?

A motor capacitor is special type of capacitor that works in conjunction with AC induction motors, these capacitors are responsible for starting up AC motors or powering them up to keep them running. Motor capacitors are available in three diffrent types, a Start capacitor, Run capacitor, and a Dual Run capacitor. With each type having its own specific application that it's used for.

A start capacitor attached to an AC motor sends a jolt to the motor to start it. Then a run capacitor attached to an AC motor sends a regular series of jolts that keep the motor running. Meanwhile a dual run capacitor is responsible for powering up two separate motors. The most common application of motor capacitors is air conditioners; these capacitors work in conjunction with three different motors the compressor motor, the blower motor, and the fan motor.

Popular manufacturers include:

  • Genteq
  • Aerovox
  • CDE
  • Barker Microfarads Inc. (BMI)
circuit diagram

Motor Capacitor Circuit Diagram

Start Capacitors

Start capacitors are responsible for increasing the starting torque of a AC motor, which in return cycles the AC motor on and off rapidly. Start capacitors stay in the circuit long enough for the motor to reach a determined speed (typically 75% of full power), and then it’s taken out of the circuit by a centrifugal switch. After the starting process AC motors work more efficiently with run capacitors.

Start capacitors are electrochemical devices consisting of compactly wound aluminum foil separated by layers of paper, which are impregnated with a conducting electrolyte. Etching of the foil prior to formation and winding increases both the effective foil surface area and the capacitance per unit volume of the finished capacitor. The entire assembly is housed in a case of moisture and oil resistant molded plastic. Start capacitors are rated for operation in ambient temperatures from -40°C to +65°C and at a frequency of 50Hz to 60Hz (applications at higher frequencies are not recommended).

Start Capacitors have a fixed capacitance and voltage. They typically have a capacitance range rated above 70uF.
The most common voltages are:

Note: Any start capacitor that is rated above 20uF is a non polarized aluminum electrolytic capacitor with a electrolyte that is not solid. Meaning that it's only applicable for mommentary use.

Run Capacitors

In order to run many single phase AC motors need a rotating magnetic field. A run capacitor is responsible for powering up the second phase winding (auxiliary coil) in an AC motor, which in return creates a rotating magnetic field that keeps the motor running.

Run capacitors are constructed to be used continuously while the AC motor is running, unlike start capacitors which are only in the circuit for a short period of time just to start the motor. This is why low loss polymer capacitors are used as run capacitors because of a longer life time and lower loss of current, opposed to electrolytic capacitors which are ideal for momentary use.

Run capacitors come in two different styles a wet style and a dry style. The wet style run capacitor is filled with a liquid that prevents the capacitor from overheating. The dry style comes with the same dielectric but it's not filled with a liquid which makes this style weigh considerably less than the wet style. Currently most run capacitors come with a film polypropylene or polyester dielectric.

Run capacitors have a fixed capacitance and voltage. The capacitance ranges from 1.5uF to 100uF.
The most common voltages are:

Dual Run Capacitors

Dual run capacitors are run capacitors that are able to power up two electric motors instead of one. This capacitor basically saves you space when you utilize it because it combines two capacitors in one case. Dual run capacitors typically have at least three leads or terminals, which are labeled “C”, “FAN”, and “HERM”.

  • Common
  • FAN
  • HERMetically sealed compressor

They are rated with two capacitance values, allowing the capacitor to be used for two different applications at the same time. An example would be 20uF + 5uF at 370VAC. Dual run capacitors are often found in air conditioners. They are used to supply power to both the fan motor and the compressor motor.


Motor Start/Run/Dual Run Capacitors can be found in large fans, forced-air heat furnaces, air conditioners, powered gates, and hot tub/jacuzzi water pumps.