15/03/2018

Split Phase Motor : Construction & Working

Construction
  • The single phase induction motor is not self starting. 
  • There is one extra winding placed in the single phase induction motor in order to make the single phase induction motor self starting therefore the single phase induction motor consists of two winding.

( 1 ) Starting winding ( Auxiliary winding ) and
( 2 ) Running winding ( Main winding )
  • The starting winding has high resistance and low reactance in order to achieve good starting torque whereas the running winding has high reactance and low resistance. 
  • Both winding are connected in parallel with the alternating supply. 
  • The resistance of the starting winding is increased by inserting extra resistance in series with it. 
  • The two winding are placed at 90 degree to each other. These two winding currents are produced revolving flux in the winding.

rotor pf the split phase motor

Working
  • The vector diagram for the split phase induction motor is shown in the Figure B. 
  • As the winding resistance of starting winding is high, the angle between voltage and current Is is small. 
  • Similarly the inductance of the running or main winding is high therefore the angle between Im and V is very large but less than 900. 
  • The starting torque in the split phase motor is directly proportional to angle between starting winding current and running winding current.

                                 T a Sin ( a )


line diagram and vector diagram of the split phase motor





Function of centrifugal switch
  • A centrifugal switch is connected in series with the starting winding. When the speed of the motor is increased up to 70 to 80% of the full load speed, the contact of the centrifugal switch open and starting winding is disconnected from the supply.
Torque – slip characteristic
  • The starting torque is approximately 1.5 to 2 times that of full load torque and starting current is 5 to 8 times that of full load current.




torque speed characteristic of the split phase motor


Application
  • Fans
  • Blowers
  • Washing machine
  • Small machine tools
  • Duplicating machine

Power Rating
  • The wattage rating of such motor is in the range of 40 W to 250 W.

Speed regulation
  • The speed drops from no load to full load is in the range of 2 to 5%.
  • The speed regulation of the split phase motor is nearly same as that the three phase induction motor therefore it is called as constant speed motor.
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