As the name suggests, an induction motor is a combination of induction and motor, induction is influencing or altering in any way an article or object by another article or object without physical contact. The best example being, a magnet that can either attract or repulse without physical contact. The motor is known to be associated with motion. Therefore when we say induction motor it is a rotating device based on the principle of induction. This article discusses- What is Three Phase Induction Motor, Working Principle, Construction, Advantages, and Disadvantages.
Working Principle of Three Phase Induction Motor
The flow of current(Amp) always produces a magnetic field, and if, this magnetic field is rotating in nature it is being produced by a three-phase supply of current. Motors based on this type of supply are called three-phase induction motors.
To further understand the working of a three-phase induction motor we must first understand what a rotating magnetic field is
The above figure shows how three phases of current (1,2,3) are shifted by 120 degrees in real-time. If it is plotted on a circle with each phase shifted by 120 degrees (3*120=360) we will find the peak rotating in a circle, and thus producing a rotating magnetic field. This magnetic field produced due to stator current induces a current in another set of windings (Rotor) in such a way that it opposes the cause of its induction(same as two magnets opposing each other) which is depends on Lanz’s law. This set of windings i.e, Rotor is free to rotate as it is on bearings and thus starts rotating continuously till Stator is supplied with three-phase current.
3 Phase Induction Motor Construction
It can be better understood if we see the construction of 3 phase induction motor which has two major parts namely
Both these parts consist of windings. The three-phase induction motor connection diagram is shown below.
Since this motor has very wide applications and speed control of 3 phase induction motor can give an added advantage.
Now, let us see how we can achieve speed variation in this motor. The speed of an induction motor is a function of speed or no rotations of the field, in other words, the frequency and the number of magnetic poles created on the stator.
N= 120F/P where ‘N’ is the rpm of motor, ‘P’ is the number of poles created on the stator and ‘F’ is the frequency of supply fed to the stator. Thus by altering the number of poles or frequency, we can control the speed of the induction motor.
Stator windings can be connected in different ways two basic terminal connections are either star or delta.
In the delta, full phase to phase voltage is applied across the winding while instar, it is only root three times the line voltage or phase to phase voltage and gets connected across two windings. Thus affecting the torque supplied by the motor.
The nameplate of a three-phase induction motor shows two types of connections ie delta and star.
Let us now see the applications of three-phase induction motors.
3 phase induction motors have the widest applications due to their almost constant speed, torque, and low maintenance.
3 Phase Induction Motor Applications
Some of the applications are under large industrial motors.
- Mills, conveyors, elevators, separators, etc.
- Machine tools
The advantages and disadvantages of these motors are as under
The Advantages of Induction Motor include the following.
- The cost of a three-phase induction motor is low as compared to DC motor.
- The running cost or the cost of maintenance in low.
- This motor has almost constant speed and torque.
- Due to its compact size, it finds many applications
- This motor can easily speed controlled by using variable speed control drives. Also known as vvvf drives.
The Disadvantages of Induction Motor include the following.
- The starting current of these motors is high in order to generate sufficient starting torque.
- These motors are sensitive to voltage fluctuations which again directly affects the toque.
- These motors, if run on partial load reduces the power factor drastically.
Please refer to this link to know more about Three Phase Induction Motor MCQs.
1). How does a 3 phase induction motor run?
When a three-phase supply is applied to the stator of this motor, due to phase shift between the phases of 120 degrees a circulating field is created. When this circulating field interacts with rotor winding due to induction a rotating torque is created which rotates the motor.
2). How can the speed of a 3 phase induction motor be changed?
The speed of a three-phase induction can be changed either by changing the number of poles on the stator or by changing the frequency. Pole changing motors are available and generally run on duel speed. Frequency can be changed using vvvf drives.
3). Why the body of a three-phase induction motor has fins?
The stator body of this motor has fins to dissipate excessive heat generated in the motor due to the conversion of electrical energy to mechanical energy.
4). What is meant by a single phasing of a three-phase induction motor?
When one of the three phases is missing and if the motor is run in this condition, it is said to running in single This is a very undesirable condition as most of the motor burnout cases happen due to this fault.
5). What will happen if a three-phase induction motor is overloaded?
If this motor is overloaded, its slip will increase and will ultimately stop taking excessive current which will lead to motor burn out.
6). How can one protect the motor from burn out?
One can protect this induction motor by using a suitable overload relay and a single phasing preventer. However, for large motors, additional protection is provided.
7). What is the effect of voltage variation on a three-phase induction motor?
Voltage variation in this motor is not desirable as torque is directly proportional to the square of the voltage and a variation in voltage will result in very high torque variations.
Thus, this is all about an overview of a three-phase induction motor along with construction, speed control, advantages, disadvantages, and applications. Here is a question for you, what is the function of a 3 phase induction motor?