Construction and Characteristics with Double Field Revolving Theory.
Construction and Characteristics:
Single phase induction motor or fractional kilowatt motor is a motor built in a frame having a continuous rating of smaller than 1KW.It is constructionally more or less similar to a threephase induction motor with the difference that its stator is provided with single phase winding.When single phase AC voltage is supplied to the single phase stator winding,it will not produce a rotating magnetic field like in three phase induction motor,rather it will produce an alternating magnetic field which magnitude varies with respect to time and its direction changes by 180° at a particular instant at a particular time interval.
With the connection of single phase AC supply to the single phase induction motor,alternating current will flow in its stator winding and the polarity of stator pole would alternatively be N and S.The field so produced will be pulsating i.e. polarities will be alternating with flux rising and falling in strength.The current induced in the rotor will tend to rotate in both directions alternatively and thus, the rotor will be at a standstill due to inertia.If a rotor is given a push by hand or by another means in any direction,it will pick up speed and continue to rotate in same direction developing operating torque,Thus,the single phase induction motor is not selfstarting and requires special starting means.
Fig 1.a shows the basic construction of single phase induction motor and Fig 1.b shows the nature of air gap flux produced by single phase winding.
Double Field Revolving Theory:
Double field revolving theory states that the pulsating magnetic field produced by the singlephase winding is equivalent to the phasor sum of two oppositely rotating magnetic fields each having a magnitude of 0.5φ_{m} with a synchronous speed of
N_{s}= (120f/p)
Fig 2 shows two rotating magnetic fields,namely OA and OB,each having a magnitude of 0.5φ_{m.}OA is rotating in forward
When the two rotating phasors reach the positions such as shown in fig 2,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=0 of the pulsating field shown in fig 1.Let us assume this position as a reference point.
After 30° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to 0.5φ_{m }and the direction is upward. This condition is equivalent to ωt=30° of the pulsating field shown in fig 1.
After 60° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to 0.866φ_{m }and the direction is upward. This condition is equivalent to ωt=60° of the pulsating field shown in fig 1.
After 90° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to φ_{m }and the direction is upward. This condition is equivalent to ωt=90° of the pulsating field shown in fig 1.
After 120° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to 0.866φ_{m }and the direction is upward. This condition is equivalent to ωt=120° of the pulsating field shown in fig 1.
After 180° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=180° of the pulsating field shown in fig 1.
After 210° rotation from the reference position,the positions of two phasors will be as shown in fig 2.Here,the net resultant of two rotating phasors is equal to 0.5φ_{m }and the direction are downward. This condition is equivalent to ωt=210° of the pulsating field shown in fig 1 and so on.
Based on double field revolving theory,the torquespeed characteristics of singlephase induction motor can be drawn as shown in fig 2.a,where torquespeed characteristics are shown,one due to forward rotating magnetic field and other due to the backward rotating magnetic field.
Here, OA=Forward starting torque
OB=Backward starting torque
These two torques are equal and opposite. Hence,the net starting torque of single phase induction motor is zero.so,the single phase induction motor is not selfstarting.
 Rating of single phase induction motor must be smaller than 1KW.

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