Capacitor Start Motor

A capacitor motor is also a split-phase induction motor. In this motor, starting winding has a capacitor in series with it. This is improved form of split phase motor. This type of motor was developed at the later stage. These motors have higher starting as well as running torque. This is the main advantage of capacitor motors over split-phase types.
In the beginning, the split phase motor was designed by keeping the difference of resistance to reactance (inductive) ratio in the main winding and starting winding. Thus the use of the term split phase therefore has become established for the designation of the inductively split phase machine, and to avoid confusion it is not used to designate the capacitor induction motors.
The use of capacitor has many advantages. The fluxes in the two windings, main winding as well as starting winding can be made to have a difference of 90o so that the motor becomes two phase motor. Since the starting torque is proportional to the sine of the angle between the two currents produced by the main winding as well as starting winding. The starting torque is much higher than the usual split phase motor.
Starting line current reduced because of the capacitor connected in series with the starting winding. The power factor of the motor gets improved. It can be made very near to unity in the capacitor motor where the capacitor is permanently fixed in the winding and it does not get disconnected. There are three types of capacitor motors:

  1. Capacitor start motor.
  2. Capacitor run motor.
  3. Capacitor start capacitor run motor.

Capacitor Start Motor

In the capacitor start motor capacitor C is of large value such that the motor will give high starting torque. Capacitor employed is short time duty rating. The capacitor is of electrolytic type. Electrolytic capacitor C is connected in series with the starting winding along with centrifugal switch S as shown in the diagram.

capacitor start motor

When the motor attains the speed of about 75% of synchronous speed starting winding is cut-off. The construction of the motor and winding is similar to usual split phase motor.
The capacitor start motor is used where high starting torque is required like refrigerators.

Characteristics of Capacitor Start Motor

  • Speed is constant within 5% slip.
  • Capacitor start motor develops high starting torque about 4 to 5 times the full load torque and reduces the starting current.
  • The direction of rotation can be changed by interchanging the connection of supply to the either of the winding.

Capacitor Run Motor

Connection scheme for the capacitor run motor is the same as the capacitor start motor except for the absence of centrifugal switch S.
The capacitor is of paper type. The capacitor is permanently connected to the starting winding. In case of the paper capacitor, the value of the capacitance is small since it is difficult and becomes uneconomical to manufacture paper capacitor of higher value.
The electrolytic capacitor cannot be used since this type of capacitor is used for only short time rating and hence cannot be permanently connected to the winding. Both main as well as starting winding is of equal rating

Characteristics of Capacitor Run Motor

capacitor start capacitor run motor
  • Starting torque is lower about 50% of full load torque. Power factor is improved. It may be about unity. Efficiency is improved to about 75%.
  • The direction of rotation can be reversed as written in case of capacitor start motor.
  • The capacitor run motor is used in fans, room coolers, portable tools and other domestic and commercial electrical appliances.

Capacitor Start Capacitor Run Motor

capacitor start induction motor, capacitor start and capacitor run motor

 Two capacitors are used in capacitor start capacitor run motor or two value capacitor motor, one for starting purpose and other for running purpose. Starting purpose capacitor is of electrolytic type and is disconnected from the supply when the motor attains 75% of synchronous speed with the help of centrifugal switch S, connected in series with Cs. The value of the two capacitors is different. Starting capacitor Cs, which is electrolytic type, is of high value.

Characteristics of Capacitor Start Capacitor Run Motor

capacitor start capacitor run motor
  • The capacitor start capacitor run motor gives the best running as well as starting conditions. Such motors operate as two-phase motors giving the best performance.
  • Starting torque is high, starting current is reduced and gives better efficiency, better p.f. The only disadvantage is high cost.
  • Direction can be reversed by interchanging the connection of supply to either of the main winding or starting winding.

 Thanks for reading about capacitor start motor.

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