Capacitor-Start Induction Motor | Capacitor-Start Capacitor-Run Induction Motor



The capacitor start induction motor is identical to a split phase-induction motor except that

(1) starting winding and main winding have equal number of turns, and
(2) a capacitor is connected in series with the starting winding as shown in diagram below.




Starting Torque

Capacitor-Start Induction Motor phasor diagram


As a capacitor draws leading current , therefore, IS leads V, this increases the phase between the two currents and therefore Sin α  is increased and therefore starting torque ,  Tst = K ISIM Sin α   is increased.


Therefore, these motors are used in applications where high starting torque is required  e.g compressors, large fans, water pumps, drilling machines etc.

When the motor attains about 75% of synchronous speed, the centrifugal switch opens and the capacitor and the starting winding are disconnected. The motor continues to run as a single phase induction motor.


Torque-speed Characteristics

Motor follows main+auxiliary winding characteristics till N0 . From N0  to NS it follows main winding alone characteristics. The complete characteristics is shown in bold line.

Capacitor-Start Induction Motor characteristics



Capacitor-Start Capacitor-Run Induction Motor

It is identical to a capacitor-start induction motor except that  the starting winding is not disconnected even when the motor is running.
It uses two capacitors :- a low value capacitor C1 for optimum running performance and a much larger capacitor C2 for optimum starting, as shown in below diagram.

Capacitor-Start Capacitor-Run Induction Motor  diagram



At the time of starting, both capacitors C1 and C2 are connected,  therefore, equivalent capacitance ( C1+C2 ) is more, IS leads V by a larger angle and therefore the angle between  IM  and  IS   i.e α  is larger , Sin α is larger and thus the starting torque    , Tst = K ISIM Sin α      is more.


Capacitor-Start Capacitor-Run Induction Motor  phasor diagram


Once the motor reaches somewhere around 75% of synchronous speed, centrifugal switch which is sensitive to rotor speed, opens disconnecting C2 .

Now, only low value capacitor C1 is in the circuit and therefore, the angle of lead of  IS from V is decreased and angle between V and I is decreased and thus motor power factor  ( cosine of angle between V and I ) is improved and motor keeps running in this configuration.

Therefore, C2  ( along with C1 ) provides larger starting torque and C1 alone provides improved pf and hence better efficiency of the motor.


These motors are the most commonly used single phase induction motors due to their high starting torque and better power factor (advantages) . 

Capacitor start capacitor run induction motors are used in applications like compressors, conveyors, refrigerators, air conditioners, ceiling fans etc. 
  

Torque-speed Characteristics



Capacitor-Start Capacitor-Run Induction Motor characteristics


Motor follows characteristics with both C1 and C2 connected till N0 . From N0  to NS it follows characteristics with C1 connected alone. The complete characteristics is shown in bold line.