Three Phase Induction Motor(What You Should Know)

Three phase induction motor is the most popular of all motor type because, it has high power factor ,which means high efficiency, and they are self started no capacitor is needed.

This motor is every where around me in the work, The organization that I work for has many electric motors.

While induction motor is the most used, and of course, 3 phase is the most of them.

A 3 phase induction motor is an electric motor which is designed to work on three phase power supply, and need no starting capacitor like 1 phase induction motor.

Induction Motor Parts

Its important to understand the construction of a 3 phase induction motor to get a good understanding of its principle of working

This motor consists of two main parts

  • Motor stator
  • Motor rotor

3 Phase Induction Motor Stator

Stator is made of slots to construct winding circuit which is connected with three phase electricity source.

The three phase winding is arranged to produce rotating magnetic field when connected to power source.

Stator of 3 phase induction motor
Stator of 3 phase induction motor

Rotor of induction motor

Rotor is the rotating part of the motor, that is connected to the mechanical load.

It’s a cylindrical laminated core with parallel slots that can carry conductors.

The conductors are short-circuited by end rings and are made of copper or aluminum fitted in the slots.

Induction Motor Working Principle

Rotating magnetic field production

Stator of 3 phase motor, consists of overlapping winding offset by an electrical angle of 120.

When we connect the stator to a three phase AC source, it establishes rotating magnetic field which rotates at the synchronous speed.

The rotor winding in an induction motor are shorted by end ring, and cut the rotating magnetic field of the stator.

Current flows through the rotor conductor because, an emf is produced in the rotor copper.

The cause of current generation, is the relative speed between the rotating flux and static rotor conductor. “As per Lenz’s law” The rotor rotates in the same direction to reduce the relative velocity .

The rotor speed should not reach the synchronous speed produced by the stator.

Induction Motor Slip

Motor rotor cannot reach the synchronous speed, The difference between the stator speed (synchronous speed) and rotor speeds is called the slip.

3 Phase Induction Motor Advantages

  • One of the important advantages of this motor is that it is self starting. i.e needs no starting capacitor like single phase induction motor.
  • Good speed regulation.
  • High full load efficiency (up to 97%).
  • Not like the single phase motor, a 3 phase motor power factor can reach up to 0.9
  • Easy rotation direction reversing, unlike 1 phase motor, 3 phase motor rotation direction cab be controlled just by reversing two phases of power source.


Although this motor type is the most common and widely used in almost all industries and applications, it has the following disadvantages:

  • High starting current, that causes voltage drop and affect other loads on the same power supply, so this motor requires a starting method to overcome this problem.

I highly recommend Reading My Articles, Star Delta Starter of Motor.

  • Difficult speed control of induction motor is one of the disadvantages because, it is a constant speed motor.
  • Poor starting torque, is another disadvantage. This makes it not suitable for applications that require high starting torque.


As mentioned above, this motor is a constant speed motor, so it is suitable for industrial and commercial applications, that need no speed control or high starting torque. These applications are like:

  • Centrifugal pumps.
  • Constant speed conveyors.
  • Air compressors.
  • Submersible pumps.
  • Air blowers.
  • Grinding machines.

Motor Speed Control and Formula

The speed formula of this motor is N = Ns(1-S),

while Ns = (120 f)/P

Where :

  • Ns is the synchronous speed.
  • f is the frequency.
  • P is number of poles of the motor.

controlling the motor speed is can be done by one of the following methods

  • Frequency control or (V / f) control.
  • Number of poles Changing
  • Voltage Controlling.

Read My detailed article: Electric Motor Speed Formula, Calculator and Control

Motor Current Calculator

3 phase current calculator android app
3 phase current calculator android app

You can use this online current calculator. Also, I recommend installing my Free android electrical calculator, I use it all the time to calculate motor current, no matter I have power in KW or HP.

I created it my self, to help you do your calculations. Try it now.

Current formula for three phase induction motor is, I = P/(1.73*V*pf)


I : Motor current

P: Motor power in Watt

V: Motor operating voltage

pf: Motor power factor

You can find motor’s data, voltage, power and pf, on the motor nameplate. I have written a detailed article about Motor Name Plate, You Can Find It here, for more information.

If we don’t know the power factor of the motor, we assume it’s 0.85

I designed my android app to be easy and help you so, no matter what motor power you have KW or HP, the app converts it to watt internally.

What Causes An Electric Motor Start Slow?

There are different factors that affect the speed of the motor and cause it to slow down.  It is very important to know about these factors. As it will badly affect the performance of the motor. So we are discussing each factor in detail.

  • Electrical Load

Load is one of the main factors that slow down the motor. If an electrical load attached with the motor is greater than the rated load of the motor. It will slow down the motor.

Therefore, it is important to keep in mind, always attach a load to a motor that is less than the rated load of the motor.

The phenomenon behind the electrical overload is that it will require more current to flow across the windings of the motor. Which will exceed the safety limit of an electric motor.

  • Low supply voltage

A supply voltage is a source for operating an electric motor. If the supply voltage is less than the rated input voltage, it will not produce the amount of enough current to produce the required torque in the rotor.

While the motor will try to maintain its rated speed.  As a result, the motor will start slow.

  • Low resistance

Low resistance causes degradation of insulation flux. As a result,it will cause overheating and flux leakage and the motor will start slow.

It is worth mentioning to discuss overheating. As overheating is the main cause behind motor failure and slowdown. Keep in mind that with every 10oc temperature increase, the insulation life decrease by 50%.

  • Motor installation

Speed of motor also depends upon the installation of the motor. If a motor is installed on an uneven surface, it will result in vibrations.

Similarly, loose bearing or any other issue can cause vibrations. And as a result, the motor slows down.

What Happens If An Electric Motor Loses Power Under Load?

Three Phase Loss

Electric motors are designed to rotate freely with the help of bearings. This makes it safe to rotate in case of power loss (i.e 3 phase power loss).

The motor keeps rotating with the load connected. Its rotating speed decreases gradually to zero.

As power is a combination of electric current and voltage. In case, If power is cut off or an electric motor loses power under load.

It means that the amount of induced EMF or flux will reduce in the insulation winding. As a result, the motor will stop.

Single Phase Loss

In case of phase loss (i.e one phase loss) while the motor is under load, The motor will continue running under lower speed and the current in the remaining phases will increase.

If the motor has no phase loss protection, the increased current will burn the winding.

Do Electric Motors Slow Down With Age?

In most cases “No”. Because, electric motors are designed in such a manner that they don’t lose their performance with the passage of time.

The reason that can cause the motor to slow down with age is a physical or mechanical reasons, such as bad bearing, rusted shaft due to long period of stop or increase of mechanical load.

What is squirrel cage induction motor?

Squirrel cage induction motor is that AC induction motor type which function is based on electromagnetism principle. This motor can be single or three phase motor.

Why is it called squirrel cage?

This name of squirrel cage motor comes from the fact that the rotor of the motor looks like the cage of a squirrel (squirrel-cage rotor).