Wednesday, 17 July 2013

Split – Phase Induction Motor

  • Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90o electrical from the main winding and operates for a short time when the motor starts up.
  • The two windings are so designed that the starting winding has a high resistance and relatively small reactance while the main winding has relatively low resistance and large reactance. This can be done by having main winding of large diameter and number of turns while starting winding of a fine wire of a small number of turns. The currents taken by these two windings are out of phase (25o to 30o). Schematic diagram and phasor diagram of split phase induction motor are shown in figure
  • Operation: When the stator windings are energised from a single phase supply, the main winding carries current Im and the starting winding carries current Is. These two currents are out of phase (25o to 30o) and produce a weak rotating field which starts the motor. The starting torque is given as,

                                    Ts = K Im Is Sin Ф
            K = constant whose magnitude depends upon the design of the motor
When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed is below the synchronous speed and depends upon the load on the motor.

Characteristics of Split-Phase I / M:

  1. The starting torque is 1.5 to 2 times the full load torque and the starting current is 6 to 8 times the full-load current.
  2. The power ratings of such motors are from 60 Watts to 250 Watts.
  3. They have efficiency of 50 – 70 % and overload of 50%.
  4. As they have moderate starting torque and low starting current, so they are commonly used to drive fans, blowers, centrifugal pumps, domestic refrigerators, washing machines, oil burners, small machine tools, etc.
  5. These motors are essentially constant-speed motors. The speed variation is 2 to 5 % from no-load to full-load.
  6. The speed range of such motors is from 2875 to 700 r.p.m.
  7. Such a motor can be operated with a power factor of 0.55 – 0.65.
  8. The percent slip for such motors is about 4 – 6 percent.


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    AC Motor India
    AC / DC Motors

  2. Hi,

    Great post! Thanks you so much for the share. It is indeed a helpful one. I am looking forward of reading more article with the similar topic as this one. Good luck and More Power.

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  6. Hi can I know why centrifugal switch need to be open at 75-80% of synchronous speed?