Motor Capacitor 101: Types, Working and Applications

A motor capacitor is a power device connected in series with the auxiliary winding to change the phase of the AC power source, create a rotating magnetic field, and set the motor in motion.

The main purpose of a capacitor is to create a multi-phase power supply from a single-phase power source. This helps to create the phase difference needed to create a rotatory magnetic field.

Types Of Motor Capacitors

Majorly there are two types of capacitors, namely:

Start Capacitor: A Start Capacitor is used to store and provide energy that can be used to generate the initial torque sufficient to start the engine. It is then removed using a centrifugal switch after a predetermined speed has been reached, which is normally 75% of full speed. After that, it works more efficiently with a run capacitor.

They generally have a higher capacitance than run capacitors, largely in the 30uF to 120uF range. The start capacitor provides an immediate electrical push to get the motor rotation started. Without a start capacitor when the voltage is applied, the motor would just hum.

Run Capacitor: Motor running capacitors are used in single-phase motors to energize the secondary winding by phase-shifting the power supplied. This setup enables the motor to operate efficiently with sufficient torque.

These capacitors use a dielectric charge to increase the performance of the motor. They are also used in conjunction with starting capacitors when a single-phase power supply is more practical than a three-phase power supply. Almost all single-phase electric motors have run capacitors, with notable exceptions involving small motors such as fan motors.

How Motor Capacitors Works

A Motor Capacitor draws energy from the power supply and stores it on metal conductors by a dielectric medium such as glass, ceramic, plastic film, air, paper, mica, etc. The given energy is stored in the form of an electrostatic field. The difference between the capacitor and battery is that a capacitor can release the entire energy stored in a fraction of seconds. Whereas a battery would take minutes to discharge completely.

When you connect a capacitor to a battery, the following happens:

  • The metal conductor, which is attached to the negative terminal of the battery, receives the electrons produced by the battery.
  • The metal conductor, which is attached to the positive terminal of the battery, releases electrons to the battery.

For example, here you have a battery, a light bulb, and a capacitor. When you connect the battery, the light bulb will light up as current flows from the battery to the capacitor to charge it.

The light bulb keeps getting dimmer and eventually goes out once the capacitor is fully charged. A fully charged capacitor has the same voltage as the battery. Now if you remove the battery and replace it with a conductive wire, current will flow from one plate of the capacitor to the other. This will make the lightbulb glow initially and then dim, only to eventually go out when the capacitor discharges it completely.

Application Of Motor Capacitors


Any type of fan, be it a cooler, ceiling fan, or table fan, requires a capacitor to start the motor by creating a rotating magnetic field that an AC motor cannot create by itself. Otherwise, they will need an external force to produce a net non-zero torque. Before buying a capacitor, you need to know the power rating, which depends on the type of fan used.


Industrial motors have been the driving force behind the industrial revolution for centuries. They must perform at their best for many industries to remain economically viable. For this, Pressfit motor capacitors are the perfect energy solution, their performance does not deteriorate over time. They deliver the power you need to keep your factory moving as swift as possible.


Submersible pumps have been widely used for agricultural and domestic purposes. This necessitates that pumps are powered by durable capacitors that are ready to operate even in damp, humid, and harsh environments. Pressfit Submersible Motor Capacitors and Cables meet precisely this need to provide a reliable energy solution for consumers.


Start and run capacitors are mainly used in an air conditioning system. Being a capacitor, their fundamental use is to store energy, but the difference is in their application. A starting capacitor is used to provide initial energy as Air Conditioners require a lot of torque to start. While a running capacitor is used to turn the fan motor to blow the condensed air produced by the system.

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