Capacitor Start Induction Motor

Last Updated on February 7, 2023 by Electricalvolt

This article describes circuit construction, characteristics, phasor diagram, working, and applications of the capacitor-start induction motor.

Working of Capacitor-Start Induction Motor

Based on the starting method used, we have several types of single-phase induction motors such as split-phase induction motors, capacitor-start induction motors, capacitor-start capacitor-run induction motors, etc.

A Capacitor Start Induction Motor is a type of single-phase induction motor which has a capacitor that is primarily used to produce the starting torque in the machine. Therefore, the capacitor start single phase induction motor has a starting capacitor connected in series of its starting winding or auxiliary winding. The circuit construction of the capacitor start induction motor is shown in figure-1.

It consists of two windings namely, starting winding (also called auxiliary winding) and main winding (also known as running winding). It has a rotor having squirrel-cage construction. From the circuit diagram, it is clear that the starting capacitor (C) is connected in series with the starting winding.

The function of this starting capacitor is to create a phase difference between the currents of starting winding and running winding. By creating this phase difference, the motor can start on its own when a single phase supply is connected to its terminals.

Actually, the capacitor in series of the starting winding converts the single-phase input supply into a two-phase supply by introducing a phase shift of almost 90° between currents of two windings of the motor.

Because the capacitor in this motor functions only to start the motor, hence the motor is named a capacitor start induction motor.

Now, by technically analyzing the motor circuit, we found that the value of starting capacitor (C) is chosen in such a way that it makes the starting winding current (IS) that leads the main winding current (Im) by an angle nearly equal to 80°.

This high angle of phase shift gives a very high starting torque produced by the motor. However, once the capacitor start motor reaches a speed of about 75% of its normal speed, the starting winding and capacitor are isolated from the circuit with the help of a centrifugal switch (S). After that, the motor will work as a normal single-phase induction motor and remains accelerating till it attains its normal speed.

As we commented on its relatively high starting torque, it is because the starting torque of the capacitor start induction motor is directly proportional to the sine of the angle of phase shift, i.e.

Or

formula for torque of the capacitor start induction motor

Where, K is a constant, and ϕ is the angle of the phase difference between starting winding current and the main winding current.

Phasor Diagram of Capacitor Start Induction Motor

The phasor diagram of the capacitor start motor showing the phase relationship between its starting winding and running winding currents and supply voltage is shown in figure-2.

Phasor Diagram of Capacitor Start Induction Motor

From this phasor diagram of the capacitor start motor, we can conclude that the starting winding of the motor leads the main winding current by an angle of about 80° (ideally 90°) electrical.

Therefore, this shows that the two windings of the motor are displaced by 80° in space which resembles a two-phase supply system and produces a rotating magnetic field in the machine making it self-starting.

Now, let us discuss the important characteristics of the capacitor start induction motor.

Characteristics of Capacitor Start Induction Motor

The important characteristics of the capacitor start induction motor are listed below

  • Capacitor start induction motor has better starting performance.
  • Capacitor start induction motor developed high starting torque, about 3 to 4.5 times its full load torque. The motor produces high starting torque when the capacitor value is large and starting winding resistance is low. The speed torque characteristics of the motor are given below.
speed torque characteristics of Capacitor Start Induction Motor
  • Since the phase difference between starting current and the running current is high, it draws less starting current.
  • These motors are relatively expensive due to the presence of starting capacitor and centrifugal switch.
  • These motors are available in 120 watts to 750 Watts power ratings.

Applications of Capacitor Start Induction Motor

The capacitor start induction motors are mainly used in applications that involve frequent or prolonged starting durations. Some important applications of capacitor start induction motors are listed below:

  • These motors are used to run compressors.
  • Wide applications in pumps
  • Used to drive pumps
  • The motor is most suitable for driving high intertie load that needs frequent starting.

Hence, this is all about capacitor start single phase induction motors, their circuit diagram, phasor diagram, characteristics, and applications.

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About Satyadeo Vyas

Satyadeo Vyas, M.Tech,M.B.A. is an electrical engineer and has more than 36 years of industrial experience in the operation, maintenance, and commissioning of electrical and instrumentation projects. He has good knowledge of electrical, electronics, and instrumentation engineering.

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