Electric Motors
Electric Motors
Posted By MMASBA

Working principle of single phase induction motor and its construction

Working principle of single phase induction motor

Previously we talked about 3 phase induction motor construction, in this article, we will explain Working principle of single phase induction motor and construction of single phase induction motor … it’s clear that you are interested in single phase motors. That’s very well, look around you in your home, office, factory, workshop, business establishment and everywhere, what do you find? Undoubtedly there is a single phase motor in:

  •  air conditioner.
  • refrigeration systems.
  • ceiling fan.
  • blowers.
  • hair dryers.
  • clocks.
  • pump drive.
  • washing machines.
  • vacuum cleaners.
  • drills.
  • compressors.
  • hair drier.
  • food mixer and so forth of machines which supplied by single phase supply and produce around 1 HP output power.

Of course, I don’t need to tell you why you can’t dispense those motors, yes you are right they are:

  • Simple in construction.
  • Cheap in cost and maintenance.
  • Reliable.
  • Lightweight and compact in size.
  • High efficiency.
  • Require very little maintenance.
  • Rarely require repairs, and can easily repair.
  • They will last for years of operation with very little problems.

construction of single phase induction motor


The figure shows a section of the single phase motor.

Don’t worry I’ll tell you the more accurate details about each part.

Of course, it’s not what we need to know about single phase motor we need to deepen in the motor .so we’ll start with the construction of the motor, let’s go.

Construction of single phase motors:

As we know single phase motors are very simple in everything particularly in construction as they construct of:

A stator:

It’s stationary and consists of lamination.

 it’s made of stamping and this stamping consists of slots carrying the stator windings.

I were explained this part in detail before in this article >>> ac motor parts and functions

The stator windings (main windings) are excited with a single phase AC supply and create a set of N.S pols. And the stator also carries small auxiliary winding operates only during the brief period when the motor starts up and it has the same number of poles at the main winding.


working principle of single phase induction motor

A rotor

It’s a squirrel cage (as at 3 phase squirrel cage induction motor) consists of insulated aluminum or copper bars placed in the slots, the rotor bars are shorted at both ended by end rings.

the rotor of single phase induction motor


A terminal box:

We use it to protect and provide safety to the electric connections as it made of metal or plastic which isolate the inner connections from any threating.

A drive shaft:

It’s a mechanical component which we use to transmit torque and rotation.

 We deeply care to be strong enough to bear the stress.


I think we now have a pretty background about the construction of single phase induction motor but it isn’t enough it’s very slight we should understand more and the more will start with how does the motor work!

Let’s see

Working principle of single phase induction motor

We should concentrate well in this part because it’s the most important.

As we will work on only one coil in the main and auxiliary winding to help to understand very well.

When we apply AC current to the main winding a fluctuating magnetic field will be produced, and when we put an already rotating rotor inside this field the rotor will keep on rotating in the same direction; that’s because of the fluctuating field equal the sum of two oppositely rotating magnetic field.

If the two rotating magnetic fields are opposite, an equal and opposite torque will be produced and the total torque on the rotor will be zero and the rotor won’t start.

So, single phase induction motor not self-starting

But if one torque is greater than the other the equivalent torque will be in the same direction of initial rotation and the rotor will keep on rotation and work, but it can’t achieve without using an initial rotation. So we need to know how to add the initial rotation and why it is importance .let’s see.

Initial rotation provided to single phase motor:

We use an auxiliary winding capacitor arrangement to cancel any of the rotating fields; why? Because the auxiliary winding will produce two oppositely revolving magnetic fields, hoe I benefit from it!.

Well, one of this fields will cancel the RMF of the main winding and the other will added up which will produce a single magnetic field revolving under a specific speed. That’s very well and so on?

The final single magnetic field will give the rotor starting torque (the rotor will be self-started), and when the rotor attached its specific speed it will keep on rotating even we dispense the auxiliary winding, we can stop working only with a centrifugal switch.

Now we cover two things about single phase induction motor:

1- construction of single-phase induction motor.

2- working principle of single phase induction motor.

It’s very simple and interesting but it’s not all we will get more by knowing the types of a single phase induction motor, their usage, and importance.



Electrical Engineer