Welcome to ELECTRICAL ENCYCLOPEDIA. This article will cover about the principle of Single Phase Induction Motor.
A single phase induction motor consists of single phase winding on stator and a cage winding on rotor. When a single phase supply is given to the stator winding, a pulsating magnetic field is produced. It means that magnetic field builds up in one direction, then falls to zero and again builds up in opposite direction. Under these conditions, rotor does not rotate due to inertia.
This means that single phase induction motor is inherently not self-starting . If however, the stator winding is exicted and the rotor is rotated by some auxiliary means, then motor will continue to rotate in the direction in which it started even if starting device is removed.
There is Double field revolving theory which helps us in analysing the performance of single phase induction motor.
A single phase induction motor consists of single phase winding on stator and a cage winding on rotor. When a single phase supply is given to the stator winding, a pulsating magnetic field is produced. It means that magnetic field builds up in one direction, then falls to zero and again builds up in opposite direction. Under these conditions, rotor does not rotate due to inertia.
This means that single phase induction motor is inherently not self-starting . If however, the stator winding is exicted and the rotor is rotated by some auxiliary means, then motor will continue to rotate in the direction in which it started even if starting device is removed.
There is Double field revolving theory which helps us in analysing the performance of single phase induction motor.
DOUBLE FIELD REVOLVING THEORY
The double field revolving theory basically states that a "a stationary pulsating magnetic field can be resolved into two rotating magnetic field, each of equal magnitude rotating in opposite direction with same speed". Induction Motor responds to each of the magnetic field separately and the net torque produced in the motor is equal to the sum of torque produced due to each of the two magnetic field.
Equating for alternating magnetic field is given by:
y = Y * sin⍵t * cos𝛼 ....................1
where Y = Maximum value of Air gap Flux Density.
⍵ = Frequency of Alternating Current
𝛼 = Space-displacement angle measured from Stator axis.
By trigonometry, Eq.1 can be written as :
y = (Y*sin(⍵t+𝛼))/2 + (Y*sin(⍵t-𝛼))/2
So, (Y*sin(⍵t+𝛼))/2 represents the forward rotating field.
and, (Y*sin(⍵t-𝛼))/2 represents the backward rotating field.
Both field has same magnitude of Y/2 and moving with speed (⍵ = 2*π*f) in opposite directions.
Thus, the theory based on resolution of an alternating magnetic field into two counter rotating fields is called double- field revolving theory.
When the rotor is at rest, emf induced due to both rotating magnetic field are same in magnitude and opposite in direction. Due to this, two torques will also be equal and opposite . Thats why, at standstill net torque becomes zero. In simple words, Single-phase induction motor with single stator winding inherently has no starting torque.
However, if rotor is given an initial rotation by external means in either direction then torque in that direction will become more than the other torque. Hence, a net positive torque will develop in the same direction as the initial direction and motor will keep running in that direcion.
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