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

As a capacitor draws leading current , therefore, I_S leads V, this increases the phase between the two currents and therefore Sin α  is increased and therefore starting torque ,  T_st = K I_SI_M 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 N_S it follows main winding alone characteristics. The complete characteristics is shown in bold line.

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.

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

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  I_S 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

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

Related :-

(1) split phase-induction motor