What is Capacitor Motor : Working & Its Applications

A capacitor motor is a split-phase induction motor where the starting winding of this motor has a capacitor that is connected in series with it. This is an improved form of a split-phase motor. The main benefit of capacitor motors as compared to split-phase types motors is; that they have running torque as well as higher starting. So this article gives brief information on a capacitor motor – working with applications.

What is Capacitor Motor?

Capacitor motor definition is; a type of capacitor which is mainly designed for operating the ac motors otherwise compressors. This capacitor changes the flow of current to single or multiple windings of a single-phase AC induction motor to form a rotating magnetic field.

Capacitor Motor
Capacitor Motor

A single-phase ac induction motor includes two windings like main winding and auxiliary winding. The main winding directly gets energy from the power line whereas an auxiliary winding gets energy throughout a capacitor, so the energies within the two windings of the motor differ within phase & generating torque.

This is most frequently used in water pumps, fans, oxygen concentrators, freeze, etc. The capacitor motor working is that the capacitor is used to store electrical energy for the operation of the motor. If the capacitance of the capacitor is high then it stores more energy. A burnt-out or damaged capacitor may hold simply a portion of the energy required for the electric motor if its capacitance is small.

Capacitor Motor Connection Diagram & Working

The circuit diagram of the single-phase capacitor start motor is shown below. The physical construction of a capacitor-motor can be done by connecting a capacitor unit near the motor. The shape of the capacitor-motor is a cylindrical hump. In the below circuit, both the L1 & L2 are the two connection points where the electricity supplies throughout these points to both the start & the run coil windings with the start capacitor.

Single Phase Capacitor Start Motor Circuit
Single Phase Capacitor Start Motor Circuit

Once the motor gets sufficient rotating velocity, then both the start capacitor & start coil will be turned OFF by opening the centrifugal switch contact which is activated mechanically by the centrifugal force which is formed from the rotary velocity of the rotating shaft of the motor, thus placing the motor on its run condition through simply the run coil performing on the revolution of the rotor.


The centrifugal switch function in the above circuit is to provide the electrical switching function with a spring action that activates the mechanical contacts which activate and deactivate from the driving force of the motor’s rotating shaft.

Once the motor is deactivated by detaching the power supply from the two connection points like L1 and L2, the centrifugal switch also comes to its normal close state because the motor gets slow until its rotary speed slowly gets down, and finally, the motor will be stopped. Once the centrifugal switch is normally closed then once again start capacitor can be connected to the circuit of a motor to arrange it for another start series.

Capacitor Motor Types

There are three types of capacitor motor which include the following.

Start Capacitors

Start capacitors are very helpful in enhancing the starting torque of a motor & allow a motor to be On & OFF quickly. These capacitors stay within the circuit for a long time to bring the motor rapidly to a fixed speed, which is generally about 75% of the complete speed, and after that taken out from the circuit through a centrifugal switch frequently that releases at that speed. After that, the motor performs more proficiently through a run capacitor.

Start Capacitor
Start Capacitor

The ratings of start capacitors have above 70 µF including four main voltages like 125 V, 165 V, 250 V & 330 V. Start capacitors above 20 µF are non-polarized aluminum electrolytic capacitors always including nonsolid electrolyte. So these are simply applicable for the short motor starting time. Once the centrifugal switch is damaged then the motor will not work correctly. If the switch is open always, the motor does not work. Once the switch is closed always, then the start capacitor will be in the circuit always, as a result, the windings of the motor will be exhausted.

Run Capacitors

Run capacitors are used in some single-phase AC motors to strengthen the auxiliary coil so that a rotating magnetic field can be formed when the motor is running. Run capacitors work continuously once the motor is activated.

Run Type
Run Type

These capacitors are generally polypropylene film capacitors. The rating of run capacitors ranges from 1.5 to 100 µF including different voltages like 250V, 370V & 440 V. If an incorrect capacitance value is used then an uneven magnetic field can be formed around the rotor. So the rotor results in an uneven rotation, particularly under load. As a result, the motor becomes noisy, energy consumption can be increased and its performance is reduced.

Dual Run Capacitors

These capacitors are used in two types of motors a compressor motor and a fan motor. This dual-run capacitor saves space by merging two capacitors into one. So this capacitor includes three terminals like labeled C for common, FAN & HERM for a tightly sealed compressor. These capacitors are available in a round cylinder shape and are used in ACs to assist at the beginning of the compressor & the condenser fan motor.

Dual Run
Dual Run

Advantages & Disadvantages

The advantages of a capacitor motor include the following.

  • The run capacitor is used in the motor to enhance its performance.
  • They have high efficiency.
  • When the capacitor is permanently connected to the circuit, then the power factor is maximum.
  • It includes a high pullout torque.
  • Capacitors can operate approximately for 10 years without maintenance.
  • Capacitors are available at less cost as compared to other energy storage devices.

The disadvantages of capacitor motor include the following.

  • The cost of the paper capacitor used in the motor is expensive and also its size is larger as compared to an electrolytic capacitor with the same ratings.
  • It has less starting torque.


The applications of capacitor motors include the following.

  • Capacitor induction motors are broadly used in heavy-duty applications which need high starting torque like compressors, refrigerators, conveyors & pumps.
  • Capacitor motors are used with ACs, powered gates, forced-air heat furnaces, large fans, hot tubs,s or jacuzzi spa pumps.
  • In some AC compressor units, a dual run capacitor is used to enhance both the fan as well as compressor motors.
  • These are used for higher inertia loads wherever repeated starting is necessary.
  • These are used in machine tools, compressors, conveyors, and pumps.

Thus, this is all about the capacitor motor working and its applications. They develop high starting torque which is 3 to 4 times high than the full load torque. But this high starting torque can be obtained by two essential conditions the value of starting capacitor should be high and the starting winding resistance valve should be less. Here is a question for you, what is the function of a motor?