3-Phase Induction Motor &#; Definition, Working Principle, Advantages and Disadvantages

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A 3-phase induction motor is an electromechanical energy conversion device which converts 3-phase input electrical power into output mechanical power.

A 3-phase induction motor consists of a stator and a rotor. The stator carries a 3-phase stator winding while the rotor carries a short-circuited winding called rotor winding. The stator winding is supplied from a 3-phase supply. The rotor winding drives its voltage and power from the stator winding through electromagnetic induction and hence the name.

Working Principle of a 3-Phase Induction Motor

The working principle of a 3-phase induction motor can be explained by considering a portion of it as follows&#;

• When the 3-phase stator winding is fed from a balanced 3-phase supply, a rotating magnetic field (RMF) is produced in the motor. This RMF rotates around the stator at synchronous speed which is given by,

$$\mathrm{Synchronous\:Speed, \:&#;_&#; = \frac{120&#;}{&#;}}$$

• The RMF passes through the air gap and cuts the rotor conductors, which as yet are stationary. Due to the relative motion between the RMF and the stationary rotor conductors, EMFs are induced in the rotor conductors. As the rotor circuit is closed with short-circuit so currents start flowing in the rotor conductors.
• Since the current carrying rotor conductors are placed in the magnetic field produced by the stator winding. As a result, the rotor conductors experience mechanical force. The sum of the mechanical forces on all the rotor conductors produce a torque which moves the rotor in the same direction as the rotating magnetic field. Hence, in such a way the three phase input electric power is converted into output mechanical power in a 3-phase induction motor.
• Also, according to Lenz&#;s law, the rotor should move in the direction of the stator field, i.e., the direction of rotor currents would be such that they tend to oppose the cause producing them. Here, the cause producing the rotor currents is the relative speed between the RMF and the rotor conductors. Thus to reduce this relative speed, the rotor starts running in the same direction as that of the RMF.

Advantages of Three Phase Induction Motor

Following are the chief advantages of a 3-phase induction motor &#;

• It has simple and rugged construction.
• It requires less maintenance.
• It has high efficiency and good power factor.
• It is less expensive.
• It has self-starting torque.

Disadvantages of Three Phase Induction Motor

The disadvantages of a 3-phase induction motor are given as follows &#;

• The 3-phase induction motors are constant speed motors; hence their speed control is very difficult.
• 3-phase induction motors have poor starting torque and high inrush currents (about 4 to 8 times of the rated current).
• They always operate under lagging power factor and during light loads, they operate at very worst power factor (about 0.3 to 0.5 lagging).

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In this article, we are going to study the advantages and disadvantages of three phase and single phase of induction motor over synchronous motor.

Advantages and Disadvantages of Induction Motor

Let&#;s dive into the basic merits and demerits of asynchronous (i.e. induction) motors.

Advantages (Pros) of Induction Motor

1. The construction of the induction motor is relatively easy over the synchronous motor.
2. Three-phase induction motor does not require a DC excitation. Thus, it works as a single excited machine.
3. The main advantage of the induction motor (3-phase induction motor) has self-starting torque.
4. For starting induction motors, only the stator has a power supply.
5. The cost of an induction motor is less than a synchronous motor.
6. The squirrel cage induction motor has more efficiency.
7. Minimum maintenance is required for the induction motor than the synchronous motor.
8. There is no require an excitation system to start the motor.
9. The three-phase induction motor has a high overload capacity.

Disadvantages (Cons) of Induction Motor

10. The three-phase of the induction motor requires constant speed.
11. Due to the constant speed of the induction motor, speed control is very difficult to maintain.
12. In three phase induction motor, speed can not be easily controlled and changed. By using the external circuit in the rotor circuit, you can control speed (especially, in the slip ring induction motor).
13. The induction motor operates only at a lagging power factor.
14. The sensitivity of this motor torque is very high.

Further details study: Pros and Cons of Squirrel Cage Induction Motor

These are crucial advantages and disadvantages of induction motors compared to DC motors and synchronous motors.

If you have any queries or doubts, you can be given comments.

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I have completed master in Electrical Power System. I work and write technical tutorials on the PLC, MATLAB programming, and Electrical on DipsLab.com portal.

Sharing my knowledge on this blog makes me happy.  And sometimes I delve in Python programming.

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