A capacitor motor is also a split-phase induction motor. The 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.
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 figure.

capacitor start capacitor run 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


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 Working

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


Characteristics of 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


Two capacitors are used in capacitor start capacitor run 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


Characteristics of 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 capacitor run motor.

Single Phase Motors – 2 | Objective Type Question Answers


#1 Which of the following motors will operate at high power factor?

Capacitor run motor

#2 In a two value capacitor motor, the capacitor used for running purposes is

paper spaced oil-filled type

#3 In A.C. series motor compensating winding is employed to

reduce sparking at the brushes

#4 Which of the following motors has highest starting torque ?

Capacitor-start motor

#5 The repulsion start induction-run motor is used because of

high starting torque

#6 In case of shaded pole motor the direction of rotation of the motor is

from main pole to shaded pole

#7 In case of high speed universal motor which of the following needs more attention?

Balancing of rotor

#8 The wattage rating for a ceiling fan motor will be in the range

50 to 150 W

#9 Which of the following single-phase motors has relatively poor starting torque ?

Capacitor motor

#10 Which type of load is offered by cranes and hoists ?

Reversing, heavy start

#11 The speed of a universal motor is generally reduced by using

gear trains

#12 Which of the following motors can be used for unity power factor ?

Schrage motor

#13 When a D.C. series motor is connected to A.C. supply, the power factor will be low because of

high inductance of field and armature circuits

#14 The direction of rotation of universal motor can reversed the by reversing the flow of current through

either armature winding or field winding

#15 Which motor is normally free from mechanical and magnetic vibrations?

Hysteresis motor

#16 As hysteresis motors are free from mechanical and magnetic vibrations therefore these are considered as suitable for

sound equipment

#17 A reluctance motor

all of the above

#18 In a hysteresis motor, the rotor must have


#19 The rotor of a hysteresis motor is made of

chrome steel

#20 Which of the following applications always have some load whenever switched on ?

Pistol drills

#21 In repulsion motor, maximum torque is developed when

brush axis is at 45 degrees electrical to the field axis



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