This post covers the following topics :
What is an Induction Motor ?
Induction motor is single excited motor whose only stator contains the winding. Electrical power is only needed to be connected to the stator.
Rotor do not contains any winding. Rather; rotor is a type of solid metallic block made from silicon steel stamping.
Rotor’s stamping, Aluminum bars are melted in skewed form. While both the bar ends are shorted from both the ends.
Working Principle of Induction Motor
Induction motor works on the principle of “Faraday law of electromagnetic Induction“.
In this principle, There are 2 working phenomenon’s :
- When a changing magnetic field links with the conductor, An EMF get generated in the conductor.
- When a current carrying conductor get placed in changing magnetic field, Torque get produced around the conductor.
In a GIF below, working principle of Induction motor is been explained. A conductor is placed in between Toroidal core inductor.
When power supply is get connected across stator winding, A rotating magnetic field is get generated. Whose magnetic field further links with the conductor placed in between the stator, An EMF get generated in the conductor.
This conductor; placed in stator, is a type of closed loop conductor, Which results current get induced in it. Current carrying conductor in between rotating magnetic field, Torque get produced & rotor starts rotating.
How does an Induction Motor Work ?
As discussed, Induction motor‘s only stator contains winding. Next, Working process is discussed one by one.
- An AC power supply is applied across the stator.
- Stator’s winding produces a rotating magnetic field.
- Rotating magnetic field further links with skewed conductors of the rotor.
- Changing magnetic field linking with skewed rotor conductors, EMF get generated in rotor conductors.
- Since skewed rotor conductors are shorted through the end rings, Current get induced in the conductors due to generated emf.
- Current carrying conductors placed in between the magnetic field. Which results torque get produced, Rotor starts rotating.
Power flow diagram of Induction motor
- In an Induction motor, Electrical power work as an input.
- While rotating shaft mechanical power; is an output.
- When rated power supply is get connected across the motor, Core loss & copper loss appear across the stator’s winding.
- Power than transfers to the Rotor through the air gap.
- Similarly, Core & copper loss happen across the stator of the rotor.
- After windage & friction loss, mechanical power get available across the motor’s shaft.
Efficiency calculation of Induction motor
Next, Name plate of 22 KW Induction motor is discussed !
Now, Calculating the efficiency of the motor shown in an image above.
Efficiency = (Output/Input)*100
Efficiency of Induction motor = (Mechanical power/Electrical power)*100
The details of the motor is listed in a table below :
| Sr No. | Particulars | Value | Units |
| 1 | Rated Voltage | 400 | V |
| 2 | Rated Current | 41 | I |
| 3 | Rated power factor | 0.83 | – |
| 4 | Rated mechanical power | 22 | KW |
| 5 | Calculated Electrical power | 23.6 | KW |
| 6 | Calculated efficiency | 93 | % |
- 3 phase Electrical power to Induction motor= √3*V*I* cosΦ
- Calculated Electrical power= √3*400*41*0.83=23.6 kW
- Calculated Electrical efficiency= (Mechanical rated output power/Input Electrical power)*100
- Calculated Electrical efficiency= (22/23.6)*100=93%
Why Rotor Never Reaches Synchronous Speed?
Induction motors are also known as squirrel cage Asynchronous motor. “
Rotor contains conductor in a skewed form around the stator’s stamping. These skewed conductors resemble with a type of cage, A squirrel cage.
Rotating magnetic field generated by stator’s winding, Rotates at a particular speed around the rotor. That speed of RMF (Rotating magnetic field) is known as ‘Synchronous speed’.
Synchronous speed (Ns) is directly proportional to frequency. While, It is inversely proportional to number of poles.
Ns= (120*f)/P Where : F= Frequency / P= Poles
Impact of “frequency “& “number of poles” on synchronous speed shown in an table below :
| Number of poles (P) | Frequency (f) | Synchronous Speed (Ns) |
|---|---|---|
| 2 | 50 | 3000 |
| 4 | 50 | 1500 |
| 6 | 50 | 1000 |
| 2 | 60 | 3600 |
| 4 | 60 | 1800 |
| 6 | 60 | 1200 |
Rotor placed in between the stator winding, tries to catch the speed of RMF. But rotor do not able to attain the speed of RMF (synchronous speed).
Rotor get remains behind the synchronous speed of RMF. Since rotor is not achieving the synchronous speed in Induction motor that’s why Asynchronous.
In total, Induction motor is known as Squirrel cage asynchronous motor.
The reason why rotor stays behind the synchronous speed is discussed below
- A current-carrying conductor placed in a changing magnetic field experiences a torque.
- In a motor, torque get produced due to changing magnetic field. Changing magnetic field is only achieved if rotor lags behind the speed of RMF.
- If both the speed, Rotor speed & RMF speed get equal, Than there is no net changing magnetic field.
- Changing magnetic field is only possible due to the relative speed between rotors & RMF of stator winding.
- The relative speed in between rotor & RMF is known as SLIP.
- Slip is the most basic need for the working of Induction motor.
Slip of Induction motor
” Slip is a relative speed in between rotor & RMF. Slip is the ratio of relative speed & synchronous speed. “
Slip= [(Ns-Nr)/Ns]*100
Where, Ns= Synchronous speed, Nr= Rotor speed.
- The value of slip lies in between 0 & 1.
- At starting slip is equal to 1.
- While at maximum speed rotor tries to achieve the synchronous speed which results the slip fall near around to 0.
- At full load slip falls from 0.02 to 0.05.
- At no load slip falls between 0.001 to 0.003
Applications of Induction Motor
- Pumps
- Fans
- Compressors
- Industrial machines
- Chillers
Advantages of Induction Motor
- Simple construction
- Low maintenance
- Rugged
- Cost-effective
Working Principle of Induction Motor – FAQs
Q.1 What is the working principle of an Induction motor?
An Induction motor works on the principle of electromagnetic induction. When a 3-phase AC supply is given to the stator, it produces a rotating magnetic field that induces current in the rotor conductors. Due to the interaction between the magnetic field and the induced current, torque is produced and the rotor starts rotating.
Q.2 Which law is used in the working of an induction motor?
An Induction motor mainly works on Faraday’s Law of Electromagnetic Induction, discovered by Michael Faraday. It also follows Lenz’s Law, proposed by Heinrich Lenz, which determines the direction of induced current.
Q.3 Why is it called an induction motor?
It is called an Induction motor because the rotor current is induced by the stator’s magnetic field. There is no direct electrical connection between the stator and rotor.
Q.4 What is slip in an induction motor?
Slip is the difference between synchronous speed and rotor speed.
Slip = (Ns − Nr) / Ns
It indicates how much slower the rotor runs compared to the rotating magnetic field.
Q.5 Why does the rotor never reach synchronous speed?
If the rotor reaches synchronous speed, there will be no relative motion between the rotating magnetic field and rotor conductors. As a result, no EMF will be induced, no current will flow, and no torque will be produced. Therefore, the rotor always runs slightly below synchronous speed.
Q.6 What happens when slip becomes zero?
When slip becomes zero:
- No EMF is induced in the rotor
- No rotor current flows
- No torque is produced
Hence, the motor cannot operate at zero slip.
Q.7 What is a rotating magnetic field (RMF)?
A rotating magnetic field is a magnetic field that rotates in space when a 3-phase AC supply is applied to the stator’s winding. This rotating field cuts the rotor conductors and induces current in them.
Q.8 What are the main parts involved in the working of an induction motor?
The main parts are:
- Stator (produces rotating magnetic field)
- Rotor (where current is induced and torque is produced)
Q.9 Why is an induction motor also called an asynchronous motor?
It is called an asynchronous motor because the rotor speed is not equal to the synchronous speed of the rotating magnetic field.
Q.10 What are the applications of an Induction motor?
Induction motors are widely used in:
- Pumps
- Fans
- Compressors
- Conveyors
- Industrial machinery
Conclusion
- An Induction motor is an electrical machine that operates using a rotating magnetic field generated by alternating current in its stator.
- The rotor, which is a solid metallic block without winding, contains aluminum bars that induce a current when exposed to the changing magnetic field.
- This design leads to torque production and rotor rotation, while the rotor never reaches synchronous speed due to the slip, which is essential for operation.
- Induction motors are known for their simple construction, low maintenance, and various applications, including pumps, fans, and industrial machinery.
- They are also categorized as asynchronous motors since the rotor speed lags behind the synchronous speed.