Difference between Induction motor and Synchronous motor

What is a Motor?

A motor is a device that converts Electrical energy into Mechanical energy. The motor will operate in DC and AC power supply.

What are the types of Motors?

There are various types of motors available as classified below

Motor types

In AC supply, single-phase and three-phase motors are available.

In DC supply, DC motors are available.

What is Induction Motor (or) Asynchronous motor?

A Three-phase induction motor is a single excited AC machine. Only stator is given 3 phase AC supply. The rotor is not given any supply.

Why Induction motors are called Asynchronous motors?

Because their rotors never run with the synchronous speed

Working principle of induction motor

Electromagnetic induction

An induction motor works in the principle of electromagnetic induction, EMF (Electromotive force) is a phenomenon that is induced across the electrical conductor when it is placed in a rotating magnetic field.

Rotating magnetic field

In 3 phase Induction, a motor Rotating Magnetic Field (RMF) is produced by supplying currents to a set of stationery winding (or) stator with the help of a 3 phase power supply. The current-carrying windings produce the magnetic field or flux, three sets of flux produced due to power supply, resultant flux has a constant magnitude and its axis rotating in space without physically rotating the windings

Field or flux having constant amplitude but whose axis continuously rotating in a plane with a certain speed, so if the arrangement is made to rotate a permanent magnet, then the resulting field is a rotating magnetic field.

Construction of induction motor

A cross-section of the Induction motor is shown below,

induction motor

An Induction Motor basically consists of two parts

1. Stator

2. Rotor


 It is the stationary part of the Induction Motor which consists of a series of wire windings, The stator is made up of a steel frame which supports a hollow cylindrical core, The core consists of stacked laminations that have evenly spaced slots where the stator winding is placed.

induction motor stator


It is a rotating part of the Induction Motor which is placed inside the stator, an air gap between the stator and rotor separates them from each other allowing the rotor to rotate freely.

There are two types of Rotor:

Wound RotorSquirrel cage rotor  
 It consists of Three-phase winding in the rotor, Rotor windings are uniformly distributed and are connected in star or Y shape. The ends of windings are brought out at both ends and connected to the slip rings, The rheostats are connected between the slip rings and the winding ends.It consists of copper or aluminum bars embedded in the semi-closed slots resembling a squirrel cage. The rotor slots in this type of rotor are not parallel to the shaft instead they are skewed for obtaining a quick and smooth operation The rotor bar is connected at both the ends and short-circuited by solid rings made of copper, brass, or aluminum. Mostly used in Single phase Induction motor 
Difference between wound rotor and squirrel cage rotor
Wound rotor and squirrel cage rotor

What are the types of Induction motors?

types of induction motor

Advantages of Induction motors

  1. Very simple and almost unbreakable construction in squirrel cage type
  2. It requires a minimum of maintenance.
  3. Its cost is low and it is very reliable.
  4. It starts up from rest and needs no extra starting motor and has not to be synchronized, its starting arrangement is simple in squirrel cage type motor.
  5. It is a highly efficient motor; The efficiency of the Induction Motor is from 85 to 90%.
  6. Only an AC source is required to operate, it doesn’t require DC excitation.

Disadvantages of Induction motors

  1. Its speed cannot be varied without sacrificing some of its efficiency
  2. Its speed decreases with the increase in load
  3. The power factor is very low during light load conditions.
  4. A Single-phase induction motor is not self-starting, It requires some support for starting.
  5. The motor can’t use in such applications where high starting torque is necessary.

What is the application of 3-Phase Induction Motors?

The induction motor is mostly used in industrial applications. The squirrel cage induction motors are used in residential as well as industrial applications especially where the speed control of motors is not needed such as:

  1. Pumps and submersible
  2. Pressing machine
  3. Lathe machine
  4. Grinding machine
  5. Conveyor
  6. Flour mills
  7. Compressor
  8. And other low mechanical power applications

The slip ring motors are used in heavy load applications where the high initial torque is needed such as:

  1. Steel mills
  1. Lift
  2. Crane Machine
  3. Hoist
  4. Line shafts
  5. Other heavy mechanical workshops etc.

What is a Synchronous motor?

Synchronous motor rotor generally rotates at a constant speed as the revolving field in the machine.

Working principle of Synchronous motor:

The synchronous motor working principle is magnetic locking, when two, unlike poles, are brought near each other, if the magnets are strong, there exists a tremendous force of attraction between those two poles, in that case, the two magnets are said to be magnetically locked.

When Three-phase voltages are applied to electric conductors which are placed in a certain geometrical position which are specially arranged (At certain angles from one another), a rotating magnetic field produces. This RMF rotates with synchronous speed. The synchronous speed of a stator rotating magnetic field depends on the supply frequency and the number of poles for which stator winding is wound. If the frequency of the A.C. supply is Hz and the stator is wound for the P number of poles, then the speed of the rotating magnetic field is synchronous.

Construction of Synchronous motor

Synchronous motor

Consider a two-pole simple rotor (shown in the above figure). A Synchronous motor is a double exciting machine i.e., two electrical inputs are provided to it. Its stator winding which consists of a 3-phase winding is provided with a 3 phase supply and the rotor is provided with a DC supply.

When DC supply is given to the rotor it acts as an electromagnet. As we know rotating magnetic field rotates with synchronous speed when two opposite poles of the rotor and RMF locked rotor also rotates with the same speed of RMF in the direction of the rotating magnetic field. Hence synchronous motor rotates at constant speed i.e., synchronous speed. But this all depends on the existence of magnetic locking between stator and rotor poles. Practically it is not possible for stator poles to pull the rotor poles from their stationary position into magnetic locking conditions.

Methods of starting synchronous motor

  1. By using the small pony motors like a small induction motor, we can start the synchronous motor.
  2. Using Small D.C. Machine
  3. Using Damper Winding as a Slip Ring Induction Motor (Synchronous InductionMotor)

The synchronous motor is made self-starting by providing a special winding on the rotor poles, known as damper winding or squirrel-cage winding.

What are the types of Synchronous motors?

Synchronous motors can be classified into two types based on how the rotor is magnetized.

  1. Non-Excited Synchronous Motors
  2. Direct Current (DC) Excited Synchronous Motors.

Non-Excited Synchronous Motors are available in three designs:

  1. Hysteresis Synchronous Motors
  2. Reluctance Synchronous Motors
  3. Permanent Magnet Synchronous Motors
types of synchronous motor

What is the application of Synchronous Motors?

  1. Power factor correction
  2. Constant speed, Constant load drives
  3. Voltage regulation.
  4. Synchronous motor is preferred for driving the load requiring high power at low speed

Example: – Crushers, Rolling Mills, Steel Mills, Metal rolling mills, Pulps, Grinders.

Used in powerhouse and substation in parallel to the bus bars to improve power factor, for this purpose, it is run without mechanical load on it and overexcited.

In factories having a large number of induction motors or transformers operating at lagging power factor, it is used for improving power factor.

It is used in motor-generator set requiring constant speed

Advantages of Synchronous motors

1. It can be operated under a wide range of power factors both lagging and leading.

2. It can be constructed with wider air gaps than induction motors which makes it better mechanically.

3. Higher Efficiency especially in low-speed unity power factor.

Disadvantages of Synchronous motors

1. Requires DC excitation supplied from external sources. (It is not self-starting)

2. It can’t be used for variable speed as there is no possibility of speed adjustment.

3. It can’t be started under loaded conditions; its starting torque is zero.

4. Collector rings and brushes are required.

5. Additional damper winding is necessary.

What are the differences between the Induction motor and the Synchronous motor?

Induction MotorSynchronous Motor
It works on the principle of electromagnetic induction.It works on the principle of magnetic locking.
Construction of Induction Motor is relatively easy.Construction of Synchronous Motor is Difficult.
The induction motor does not require an excitation system to start motor.The Synchronous Motor requires the DC excitation system (or prime mover) to start the motor (i.e. for the rotor rotating).
The motor is cheap, especially cage rotors and maintenance-freeThe motor is costly and requires frequent maintenance.
It can’t be used as a synchronous condenserIt can be used as a synchronous condenser for power factor improvement.
Self-startingNot self-starting
The speed is always less than synchronous but never synchronous.The speed is always less than synchronous irrespective of the load.
As load increases- speed decreasesAs load increases, load angle increases keeping speed constant at synchronous
The induction motor is a single exciting machineSynchronous motor is Double exciting machine
Speed control is possible through difficultSpeed control is not possible
Own self-starting TorqueNo self-starting torque, so some other auxiliary means have to be provided for starting the synchronous machine.
This is comparatively less efficientThis is comparatively more efficient than an induction motor
Its stator winding is energized from an AC SourceIts field winding is energized from a DC source and Armature winding is energized by AC Source
It has only a lagging power factor.It has unity or lagging or leading power factor.  
Mostly it works on the unity power factor.
Relative motion is required between stator and rotorRelative motion is not required between stator and rotor
Different slip occurs in Induction Motor
Zero slip occurs in Synchronous Motor
The induction motor always runs below the synchronous speed Ns=120f/pThe synchronous motor always runs at Synchronous speed Ns.
Application: Used in pumps, compressors, paper mills, textile mills, lathes, Drilling machines industrial drives.Application: 1. Used in substations and industries to improve the power factor 2. Control voltage at the end of the transmission line by varying the excitations.
Difference between induction motor and synchronous motor

Some useful Questions and Answers:

  1. Who Invented Motor?

     Michael Faraday.

2. Which motor is used in the grinder?

 The synchronous electric motor which works on AC current is used in domestic grinder, the motor is winding is done with copper wire to produce a magnetic effect to the rotor, the rotor, in turn, revolves with a high speed as per the capacity and power utilized by them.

3. Which motor is used in the washing machine?

     Universal motor

4. Why Induction motors are called Asynchronous motors?

Because their rotors never run with the synchronous speed.

5. Can a 3phase motor be run on a single-phase line?

    Yes, it can run, but a phase-splitter will be essential in this case.

6. What is meant by a phase-splitter?

       It is a device consisting of a number of capacitors connected in the motor circuit, it produces three output waves from a single input wave which differ in phase from each other.

7. What are the Indications of winding faults in an Induction Motor?

  1. Excessive and unbalanced starting currents
  2. Peculiar noises
  3. overheating

8. What are factors need to be considered while choosing a motor?

1) Site and operating conditions.

2) Reference to this standard, that is, ‘Indian standard/European standard

3) The degree of protection provided by enclosure; namely IP 54, IP 23.

4) Type of duty.

5) Method of cooling.

6) Type of construction and mountings.

7) Frequency in Hz.

8) Number of phases.

9) Mechanical output in kW.

10) Rated voltage and permitted variation.

11) Class of insulation.

12) Speed, in RPM (revolutions per minute).

13) Direction of rotation.

14) Unidirectional or bidirectional rotation requirements.

 15) The maximum temperature of the cooling air and water in the place in which the motor is intended to work.

16) The altitude of the place in which the motor is intended to work if it exceeds 1000m

17) System of earthing.

18) Rotor, whether squirrel cage or slip-ring;

19) Details of shaft extension required;

20) Method of starting to be employed:

21) Method of drive and Type of coupling

22) Any specific requirement.

9. How to make the single-phase Induction motor self-starting?

To make motor self-starting, during starting period the motor is temporarily converted into a two-phase motor, For this purpose stator of a single-phase motor is provided as extra winding, known as starting (or) auxiliary winding addition to the main or running winding. These two windings are spaced 90˚ electrically apart and are connected in parallel across the single phase.

Single phase induction motor


post-graduate in Electronics & communication.

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