AC Motors are mainly of two types, one is synchronous motor, and the other is an asynchronous motor. Asynchronous motor is also known as an induction motor. Both Motors are widely used electric motors. Here we will explain the difference between induction motor and synchronous motor with the help of certain major factors such as the type of excitation used for their operation, the speed of the motor, starting, and operation, the overall efficiency of both the motors, its cost of operation, usage, applications, and their operating frequency.
Difference between Induction Motor and Synchronous Motor in tabular form
The comparison between three-phase induction motor and three-phase synchronous motor is summarised in the following table.
| Induction Motor | Synchronous Motor |
| The induction motor is self-starting. Because it has high starting torque. | It is not self-starting. It has to be run up to synchronous speed by any means for its operation. |
| The induction motor always runs below the synchronous speed (Ns) i.e. $N_s=\frac{120f}{P}$ | The synchronous motor always runs at synchronous speed (Ns) |
| If the load increased then the speed of the induction motor decreases. | The speed is independent of the load. |
| The induction motor is a single excited machine. | The synchronous motor is a doubly excited machine. |
| The induction motor operates only at a lagging power factor. | The synchronous motor can be operated with lagging and leading power factors. |
| Its stator winding is energized from an ac source. | Its field winding is energized from a dc source and its armature winding is energized from an ac source. |
| Its efficiency is lesser than that of the synchronous motor | It is more efficient than an induction motor |
| An induction motor is cheaper than a synchronous motor. | A synchronous motor is costlier than an induction motor. |
| The induction motor is the most widely used motor for all domestic and commercial purposes. | The synchronous motor can be used for power factor correction and also used for supplying torque to drive mechanical loads. |
What is an Induction motor?
An induction motor is a singly-excited ac machine. Its stator winding is directly connected to the ac source, whereas its rotor winding receives its energy from the stator by means of induction. It is also known as an asynchronous motor because it never runs at synchronous speed (Ns) i.e.
$N_s=\frac{120f}{P}$
Types of induction motor rotors
1. Squirrel–cage rotor
- Consist of a cylindrical laminated core with slots nearly parallel to the shaft axis or skewed.
- Each slot contains an un-insulated bar conductor of aluminum or copper.
- At each end of the rotor, rotor bar conductors are short-circuited by heavy end rings of the same material.
2. Wound Rotor or Slip-ring rotor
- Consists of the slotted armature.
- Insulated conductors are put in the slots and connected to a form of a 3 phase double-layer distributed winding similar to the stator winding.
- The open ends of the star circuit are bought outside the rotor and connected to 3 insulated slip rings.
What is a Synchronous motor?
A Synchronous motor is a double exciting ac machine because its field winding is energized from a dc source and its armature winding connected to an ac source. It rotates with the speed of the revolving field i.e., Synchronous speed (Ns).
In synchronous machine windings are always connected in star and armature winding is placed in stator and field winding is in the rotor.
Key differences between 3-phase Induction Motor and Synchronous Motor
Three-phase Synchronous Motor
- Synchronous motor run at a constant speed i.e. at synchronous speed
- Synchronous motor required DC excitation to its rotor.
- It is not self-starting.
- Power factor can be changed from lagging to leading and vice versa.
- The speed of a synchronous motor is constant at all loads.
- Synchronous motor rotate because of magnetic locking between rotor poles and stator poles.
- Synchronous motor cost is high and required regular maintenance.
Three-phase Induction Motor
- The induction motor always runs less than synchronous speed.
- The induction motor does not require any extra DC excitation source.
- It is self-starting.
- The induction motor always operates at a lagging power factor.
- When the load on the induction motor increases its speed decreases.
- Three-phase induction motor operates on induction principle.
- It is less costly and requires less maintenance.
Author
Susmita Ghosh
