What is Three Phase Induction Motor and Working Principle

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The motor that runs at synchronous speed is called a synchronous motor, while the motor that runs at asynchronous speed is called an induction motor. It is a type of AC motor. This motor is the superstar of the world of electrical engineering, which is the beating heart of every factory, every machine, and every big industry. In this article, the working principle, types, advantages, disadvantages, specific features, and applications of the three-phase induction machine have been studied in detail.

What is Three Phase Induction Motor

A three-phase induction motor is an AC electric motor that works on a three-phase Alternating Current (AC) supply. The stator of this motor, which is the stationary part, is given an AC supply, and the rotor, which is its rotating part, is not given any supply. It rotates on the induction principle, which is why it is called an “induction motor.” An induction motor is a machine that converts electrical energy into mechanical energy. This motor is most commonly used in industries, factories, and large equipment due to its simplicity, strength, and low maintenance.

Three Phase Induction Motor Working Principle

When a three-phase supply is connected to the stator of an induction motor, a three-phase revolving magnetic field is generated in the stator winding, which runs at synchronous speed. The winding on the rotor, which is located below the stator, cuts the revolving field. As Faraday’s law of electromagnetic induction states, when a conductor cuts a magnetic field at a relativistic velocity, an electromotive force is generated across the conductor. If this conductor forms a closed circuit, current flows through it.

On this principle, electric current flows in the rotor winding of an induction motor. This rotor current interacts with the revolving magnetic field of the stator, causing the rotor to rotate in the direction of the stator’s revolving magnetic field. This interaction can be understood better from Lenz’s law. According to Lenz’s law:

The direction of the induced current produced in the rotor always opposes the cause due to which it is created.
Since the induced current in the rotor is caused by the change in magnetic flux, the current generated in the rotor always tries to oppose the rotating magnetic field of the stator.
Due to this, a torque is generated on the rotor, which tries to reduce the relative velocity between the rotor conductor and the revolving magnetic flux, so the rotor starts rotating in the direction of the flux.

Three Phase Induction Motor Contruction

A three-phase induction motor is less complex than a DC motor and a Synchronous motor; it consists of two main parts: the stator and the rotor. Let us know all its parts in detail.

Frame: The stator is the outer part of the motor, which supports the stator and stator windings and also protects the inner parts.
Stator Core: The stator construction of a three-phase induction motor is similar to that of a 3 phase synchronous motor or a three-phase alternator. Slots are made in the inner periphery of the stator core in which the stator winding is carried.
Stator Winding: In a three-phase induction motor, the three-phase winding is placed in the slots of the stator core. This winding produces a three-phase revolving magnetic field.
Rotor: The rotor is the rotating part of the induction motor. It is of two types: squirrel cage rotor and phase wound rotor. This type of rotor features a slot in its outer periphery, where the rotor conductor is placed and shorted.
Rotor Winding: There are two types of windings used in rotor slots, squirrel winding and rotating winding.
Shaft and Bearings: The rotor and rotor windings are mounted on a shaft. This shaft is supported on bearings.

Types of Three Phase Induction Motors

Among electric motors, the three-phase induction motor is the most widely popular in both industrial and domestic sectors. This motor is used to convert electrical energy into mechanical energy. It is widely used due to its robustness, simplicity, low cost, and low maintenance. These motors are mainly divided into two major types based on the structure of the rotor. Let us understand their types.

1. Squirrel Cage Induction Motor

The squirrel cage induction motor rotor resembles a cage, hence its name, and is the most common and widely used type of motor. The reason for its wide usage is its robust construction, lower maintenance costs, and a cheaper rate. It is used in pumps, fans, compressors, conveyor belts, and general industrial machinery. Even though the starting torque is low, it is sufficient for general industrial use. Has high efficiency and long life.

2. Wound Rotor Induction Motor

It is also called a slip ring induction motor because the rotor winding of this motor is connected to the external resistance through slip rings, which are fitted in the shaft. It gives high starting torque from the motor and also allows better speed control. It is widely used in cranes, lifts, compressors, pumps, mills, and rolling machines.

Application of Three Phase Induction Motors

Three-phase induction motors are used in a wide range of applications from small household appliances to large industrial plants and are therefore known as the industrial workhorse.

Both single-phase and three-phase motors are used in water pumps, irrigation pumps, and industrial pumps. Its high efficiency and continuous operation capability make it unique, as it can work for a long time without stopping.
They are used as fans and blowers in large industrial fans, exhaust fans, and ventilation systems. The stable speed and reliability of these motors help in maintaining air flow.
Three-phase induction motors are used in large and small air compressors and refrigeration systems. This motor gives good performance even with heavy loads and maintains constant pressure.
These motors are used to run conveyor belts to carry goods in factories and mines. Their simplicity and robustness make them useful even under challenging conditions.
Slip ring induction motors are used in heavy lifting machines like cranes, hoists, and elevators because of the high starting torque they provide.
Squirrel cage induction motors are used in drilling machines, lathe machines, milling machines, and other production equipment because these machines require constant speed.
Paper mills, textile mills, spinning and weaving machines run continuously for a long time, so this motor is suitable here.
In mining and metal industry where pollution is very high, such motors work very efficiently.
These motors are used in appliances like domestic washing machines, water pumps, and large air conditioners.
Three-phase induction motors are used as generators in wind power plants.

Advantages of Three Phase Induction Motors

Three-phase induction motors are the backbone of modern industries. Whether it’s a pump, compressor, or a big crane, this motor is unmatched, and it shows its power everywhere.

Due to its simple and rugged construction, this motor can work in harsh conditions and for a long time without getting damaged.
Compared to other motors, they are cheaper, and their maintenance cost is also very low because they have fewer moving parts.
Three-phase induction motors are self-starting and do not require any additional starting devices.
Its speed remains almost constant, and there is minimal effect on an increase or decrease in load.
It does not have parts like a commutator, like a DC motor, hence the chances of failure are very low.
Its efficiency is higher at higher power ratings.
It does not require frequent repairs or maintenance.
It has a wide range of application areas.

Disadvantages of Three Phase Induction Motors

Three-phase induction motors also have some limitations and drawbacks, which prevent their use in all situations.

Controlling the speed of an induction motor is difficult and expensive compared to a DC motor. Additional equipment has to be used to control the speed accurately.
Due to this motor, a low power factor may be faced, and additional equipment (Capacitor Bank) has to be installed for power factor correction, which results in additional expense.
A three-phase squirrel induction motor has low starting torque, which may cause problems in driving heavily loaded machines directly.
This motor draws 5 to 7 times more current than its full load current during starting, which increases the load on the electricity bill.
This motor becomes overheated when run continuously for a long time; therefore, it requires proper cooling and ventilation to maintain its efficiency over time.

Conclusion

The three-phase induction motor is the backbone of modern industry and technology. It has certain limitations, such as difficulty in speed control and a low power factor, but despite this, it is helpful in many applications. A squirrel cage induction motor is better suited for long continuous runs, whereas a slip ring induction motor is more suitable for heavy machinery. The efficiency of a medium-sized induction motor is about 90 percent, and its power factor is 0.89. Speed control of an induction motor is more costly and complex than that of a DC motor.

FAQ:

1. Why Three Phase Induction Motor is Self Starting

A three-phase induction motor is self-starting because when the stator windings are connected to a power supply, a rotating magnetic field is generated in the stator. This rotating magnetic field induces an electric current in the rotor due to electromagnetic induction, and this current generates its own magnetic field. Now, by Lenz’s law, the interaction between the rotating stator field and the rotor magnetic field produces torque, causing the rotor to rotate without any external force. That is why this motor is self-starting.

2. Are Three Phase Induction Motors called Alternators

No, a three-phase induction motor cannot be called an alternator because a three-phase induction motor is a motor that converts electrical energy into mechanical energy, while an alternator converts mechanical energy into electrical energy. Both working principles are different, although both use a rotating magnetic field.

3. Are Three Phase Induction Motors called a Transformer?

No, a three-phase induction motor cannot be called a transformer. Although both transform electrical energy, and working principles are different. A transformer converts electrical energy from one voltage level to another, while a three-phase induction motor converts electrical energy into mechanical energy. While both use electromagnetic principles, their purpose and working methods are different.

4. Can Three Phase Induction Motor Run at Synchronous Speed

No, a three-phase induction motor can never run at constant synchronous speed (the same speed). Its rotor speed is slightly lower than synchronous speed because at synchronous speed, the rotor cannot generate any torque. It is called a slip, which provides the torque necessary for the motor to operate.

5. Can we run a Three-Phase Induction Motor on Single-Phase?

A three-phase induction motor cannot be run directly on single-phase power, as single-phase power does not produce the stable rotating magnetic field required to rotate the rotor. However, by using some measures such as condensers or special circuits, it can be run on single-phase power, but its performance and efficiency are reduced.

AnandKumar

I am an electrical engineer and also a blogger. I write informative blog posts on topics related to electrical and electronics engineering. If you are interested in these topics, you are welcome to my site to read these articles.

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