3 phase Induction Motor || Operating principle

Have you visited an industry? Have you think about industrial motor? Have you any idea which type of motors are used in the industry? Then, you are at right place. In the industry, mostly motors they used are three phase induction because it is cheap, easy construction, easily available. More details are following below. 3 phase induction motor is the motor which operate on three phase supply i.e. there is 3 phase connection points. These motors operates on the principle of electromagnetic induction. 3 phase induction motor is widely used in industrial area because it is cheap, efficient and reliable. Also it has high starting torque, and requires less maintenance.

Parts of 3 phase induction motor

3 phase induction motor mainly consists of two parts , one is stator and another is rotor.  The rotating part of induction machine is called rotor which carry short circuited winding and the stationary part is called stator which carry 3 phase winding.  They are separated by a small air gap, generally 0.4 to 4 mm (depends on motor size/power). 

Here, stator is made of high-grade alloy of steel which is laminated to reduce eddy current loss. The stator conductor are connected in such a way to form 3 phase winding (either star or delta) and these conductors are placed in the slot on stator. 

The rotating part, rotor, is made of thin laminated steel as stator. It shape is looks cylindrical (having two types of rotor, one is squirrel cage rotor and next one is wound rotor or slip ring rotor). It is placed at the center inside of stator, which will rotate when supply passes from stator due to electromagnetic induction.

Types of rotors :-

There are 2 type of rotors of 3 phase induction motor. They are following as given below :-

1. Squirrel cage rotor

It consists of a cylindrical core , which is laminated, with parallel slots to the shaft axis. In all slot, aluminum or copper is placed which is not insulated. On the both end of rotor, there is a end ring which is heavy and used to short circuit the rotor bar conductor. By the connection of conductor and the end ring, it form cage which is used for keeping squirrels so it is named squirrel cage rotor. It produce more uniform torque and produce less noise. Also , its locking tendency is reduced which is a advantage of this rotor. But it suffer from low starting torque because it is permanently shorted so can't add resistance to the rotor circuit.

2. Wound rotor

It consist of slotted armature in which insulated conductors are placed and connected to form three phase double layer winding. The rotor winding is uniform and star connected. The open end of star circuit is brought outside of rotor and connected to the slip ring which is insulated. These 3 slip ring are placed on the rotor shaft on which brushes are connected to the slip ring. At last, these brushes are connected to the variable resistances which is in star connection. For external resistance connection, slip ring and brushes are used.

Advantage of 3 phase induction motor :-

1. It is cheap and efficient as compared to others.

2. It has high starting torque.

3. It has good overload handling capacity.

4. It requires less maintenance and has simple construction.

Disadvantage of 3 phase induction motor :-

1. For speed change VFD, gearbox are required which is not economical.

2. It has less starting torque as compared to DC shunt motor.

On conclusion, it has more advantages as compared to disadvantage so that it is widely used for different work in any industry that helps to grow more to them.

Operating principle of 3 phase induction motor

When three phase supply is given to the stator, the rotating magnetic filed (magnetic flux) is created on stator with synchronous speed i.e. speed (Ns) = (120 * f / P ). This rotating filed will cut the rotor conductor by passing through the air gap between rotor and stator. The rotor is still in stationary condition, so emf will be induced because of relative speed between rotating magnetic flux and stationary rotor i.e Faraday's law of electromagnetic induction. As we know, the rotor circuit is short circuited with or without external resistance (i.e. wound or squirrel cage rotor), current will start to flow in the rotor conductor due to induced emf. This rotor conductor that carry current are placed in the magnetic field that is produced by the stator. As a result, the mechanical forces is developed in the rotor conductor in tangential direction. The sum of this mechanical forces on all rotor conductor produce the torque, as a result, the rotor will start to move. The direction of  produced magnetic field, mechanical forces is given by left-hand rule and the direction of current and stator magnetic filed is given by right-hand rule as shown in figure below.

                                        Figure :- Flaming's right hand rule

                                            Figure :- Flaming's left hand rule

We know, the stator rotating magnetic filed is rotate in synchronous speed but the rotor can't run at synchronous speed. Let's assume,  the rotor rotates at synchronous speed then, the stator magnetic flux won't cut the rotor conductor and no emf will induced. This results no induced torque on the rotor so rotor will not move. Hence, rotor will rotate slightly less than stator flux.

Slip speed in 3 phase induction motor

The difference between synchronous speed and actual speed is known as slip speed and given by,

                            Slip speed = N_s – N_r rpm (rotation per minutes)

Where, N_s = synchronous speed

            N_r = rotor actual speed

percentage slip =  ((N_s – N_r ) / N_s ) x 100

Here, synchronous speed and actual rotor speed can be present in r.p.s.

n_s = synchronous speed in r.p.s.

nr = actual speed in r.p.s. 

percentage slip = (( n_s - n_r ) / n_s ) x 100

In this way, we can calculate percentage slip for the motor.

Generally, slip varies about 5% for small motor at full load and about 2% for larger motor. Also slip is present in percentage and denoted by s.

Slip speed & frequency relation at different conditions

As we have already discussed about motor speed. 

speed (Ns) = (120 * f / P )

From that equation, we can find frequency of the induction motor by following formula,

f = PNs / 120

This is the supply frequency which is also equal to the stator frequency ( frequency of current and voltage in stator). But in rotor winding, frequency varies and it depends on synchronous speed and rotor speed as given by,

 fr = P(Ns - Nr) / 120

Rotor frequency depends on difference between synchronous speed and rotor speed which is equal to slip speed.

so,

fr / f = (Ns - Nr) / Ns

or, fr / f = s  [ ∵ (Ns - Nr) / Ns = s slip ]

∵ fr = sf

Hence, rotor frequency = slip x supply frequency

At standstill of rotor, fr = f

In this way, we can calculate rotor frequency for motor.