Dear Readers, Welcome to Single Phase Induction Motor Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Single Phase Induction Motor. These Single Phase Induction Motor Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
(i) split-phase type
(ii) capacitor type
(iii) shaded-pole type
(i) Repulsion-start induction-run motor
(ii) Repulsion-induction motor
(i) Reluctance motor
(ii) Hysteresis motor
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has
(i) a squirrel-cage rotor identical to a 3-phase motor
(ii) a single-phase winding on the stator
Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means.
The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after
each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage
If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation.
As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at
this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is
generally not convenient for large motors. Nor can it be employed fur a motor located at some inaccessible spot.
The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. This theory is based on
the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions.
Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. Note that the resultant vector of two revolving flux vectors is a stationary vector that oscillates in length with time along X-axis.
Consider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. The alternating flux produced by the
stator winding can be presented as the sum of two rotating fluxes 1 and 2, each equal to one half of the maximum value of alternating flux and
each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.
At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase induction motor is not
Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip for one field
corresponds to 200% slip for the other . The value of 100% slip (standstill condition) is the same for both the fields.
The single-phase induction motor is not self- starting and it is undesirable to resort to mechanical spinning of the shaft or pulling a
belt to start it. To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be
achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.
(i) *Split-phase motors*-started by two phase motor action through the use of an auxiliary or starting winding.
(ii) *Capacitor motors*-started by two-phase motor action through the use of an auxiliary winding and a capacitor.
(iii) *Shaded-pole motors*-started by the motion of the magnetic field produced by means of a shading coil around a portion of the pole structure.
As with a 3-phase supply, a 2-phase balanced supply also produces a rotating magnetic field of constant magnitude. With the exception of the
shaded-pole motor, all single-phase induction motors are started as 2-phase machine. Once so started, the motor will continue to run on
If the two windings arc displaced 90° electrical but produce fields that are not equal and that are not 90° apart in time, the resultant field is
still rotating but is not constant in magnitude. One effect of this non uniform rotating field is the production of a torque that is non-uniform
and that, therefore, causes noisy operation of the motor. Since 2-phase operation ceases once the motor is started, the operation of the motor
then becomes smooth.
The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current.
Due to their low cost, split-phase induction motors are most popular single- phase motors in the market.
Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5
seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent.
An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full-
These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive:
(b) washing machines
(c) oil burners
(d) small machine tools etc.
The power rating of such motors generally lies between 60 W and 250 W.