Single-Phase Induction Motors

Single-phase induction motors are very useful in electrical Engineering field and thus it is important to understand its application properly.


  • Single phase induction is used in an excellent way, basically in the field of horse-power.

The electric drive tools which have low power tools require the apparatus with constant speed like agriculture machinery, machine tools and domestic tools.

  • The demand is high in the industry and for the various applications as the ranges vary and it is from fractional horsepower up to 5 HP.


Though the demand is high and it is very important for the different machinery, but it is also important to know about some disadvantages because of its limitations. Basically, they are not used where large power is required. This is the prime reason that three phase machines do not have single-phase induction motors. Some disadvantages are as follows-

  • Only 50 % output is acquired of the three phase motor where temperature rise and a proper frame is divided.
  • Low power factor
  • Low efficiency
  • There is not inherent starting torque
  • This is more expensive as compare to the three phase motors

The output is not perfect for all kinds of machines. So, it is important to understand the capacity and apply according to that.

Construction and working of Single phase induction motor

3-ɸ squirrel-cage induction motor and a single phase induction motor are same. If you go through its features, then it will be clear that the rotor and the stator are very similar in both the motors. Only the shaded pole is different in motors. It is also important to understand that there is an air gap rotor and stator; however, there is no electrical connection is there. There can be wound for only the even number of poles as 2, 4 and 6 are the common poles. There are opposite magnetic properties in adjacent poles have opposite magnetic property. Thus, the synchronous speed equation can be written as –

How the stator winding is different?

This can be different in two ways as follows-

  • Single phase motors are generally provided along with concentric coils.
  • The motors have two different stator windings. The main winding uses heaviest wire while the others are known as auxiliary winding. In case of opening, an auxiliary winding starts or runs. The main winding is placed at the bottom while starting winding is generally at the top and this is shifted 90 degree.


When a single phase stator motor is connected with AC supply of single phase, then there will be an exact development of magnetic field. The axis of this magnetic field is always along the magnetic field of stator coil. The stator coil that produces the magenta field is pulsating and also varying sinusoidal with time. Currents get induced by transforming in the rotor conductors. The main aim of this is opposing the stator e.m.f. Thus, the rotor e.m.f. waves coincide with stator emf and the angle of its torque is zero. In case the rotor gets pushed up by hands or by any other source in any direction, then it picks that particular force as a speed and start rotating in the same direction. So, it is clear that there is not any self starting, but some other means are required to start as this is not inherently a self starting.

Thus, to understand the above working ability, it is important to explain this by anyone of the theories from the below 2 and outcome of any of the two would be same –

  • Double revolving field theory
  • Cross-field theory.

The explanation of Double revolving field theory

The magnetic field that stator coil is produced is pulsating. It is pointed out by Ferrari that two fields are rotating equally, but they are also in opposite direction. In addition, their angular velocity will also be equal. It means the maximum value that is produced by a single component is always half of the whole.

The two component waves = ɸ1 and ɸ2 coincide along the Y –axis.

Now, what is the outcome of this two? This is ɸmax.

Waves intersect at a point A and this travel along Y. So, the exact resultant can be written as 2OA

ɸ1 = OA =-ɸ 1 max cos (wt – θ) — (i)

ɸ2 = OA =-ɸ 2 max cos (wt – θ) — (ii)


ɸ1max =  ɸ 2max

By expanding and adding both the equation,

ɸ1 + ɸ 2 = 1max cos θ  cos wt

Now, we have 2OA = ɸmax  cos θ  cos wt

This proves the Ferrari’s statement, so there is no self- starting facility.

The two different fluxes work in just opposite way and thus a single phase induction rated voltage is there to supply a fractional horsepower. The motor needs to get a proper start just by moving the shaft in a clockwise direction. It directly indicates that the rotor conductors work properly in the clockwise direction. In case of the stopping of the motor without having a switch off, then rotor comes at rest. However, if the shaft in this condition is moved in anticlockwise direction, then the motor start working like that. It means the other condition of the motors.

Now, there are two different features of the motor works. This behavior of the motors is due to opposite behavior of torques.

If the N is the speed of the rotor which runs in the direction of the forward field, then,

sf = (Ns – N)/ Ns = s——-1

sb = (Ns – (- N))/ Ns = (2Ns – ( Ns –N ))/Ns = (2-s)

Now, there are some important points that you should know about and these are as follows –

  • The result would be (2-s) >> s , this is because the rotor of backward field current is larger than that of the those which have comparatively low power factor. The weakness of backward field and the strength of the forward field take place only due to the speed of the rotor or the slip with respect to the increase in rotor towards the forward field or backward field. The torque T1 exceeds its opposite torque changes into T2 and then this motor began to accelerate.
  • If you go through the single phase induction motor, then it will be clear that the increase in the resistance of the rotor field is responsible in increasing in the effectiveness of the backward field.
  • In the motor with single phase induction shows the performance characterestics, which is inferior to a 3-phase induction motor and it happens only in the presence of backward rotating field.

The performance charaters are –

  • An induction motor of single phase has a torque of lower breakdown at higher slip and increased loss of power.
  • The Power input is excellent
  • The poor speed regulation is availed that of a polyphase motor.
  • The power factor must be lower.
  • A single phase motor has a larger frame size as compared to the 3 D phase.
  • A single phase is noiser than that of 3 phase motors.


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