There are more advantages in using 3 phase as compared to AC single phase supply system. Therefore AC 3 Phase Induction Motors running on 3 phase are used on a large scale.
Because the 3 Phase Induction Motor is more efficient, the torque and power factor at load are good, the structure is simple, simple and strong.
3 Phase Induction Motors are easy to repair, easy to start, capable of running at a constant speed, and are also inexpensive.
The 3 Phase Induction Motor contributing in this way has a huge contribution to the prosperity of the industrial sector.
In this post we will see detailed information about 3 Phase Induction Motor structure, functions, types and uses etc.
Table of Contents
Definition of 3 phase induction motor. | What is 3 Phase Induction Motor?
The device which converts AC 3 phase electrical power into mechanical power is called AC 3 Phase Induction Motor.
What is an induction motor?
A motor working on the principle of induction is called an induction motor.
On what principle does 3 phase induction motor work? | Working Principle of 3 Phase Induction Motor
When a short-circuited conductor is placed in a rotating magnetic field, it starts rotating in the direction of the rotating magnetic field. The 3 phase induction motor works on the same principle.
Imagine that a magnet is moving, if the disc shown in the figure is arranged near the magnet in such a way that it can move mechanically like a short circuited conductor,
So the magnetic lines are cut off from the disc while the magnetic field is rotating. Therefore, according to Feraday’s law of electro magnetic induction, the disk has an e.m.f. is formed, due to which eddy current flows in the disc.
Eddy currents create their own magnetic lines around the disc. The magnetic lines of the disk oppose the rotation of the main magnetic field, according to Lenz’s law, but do not destroy it completely.
Therefore the discs tend to rotate in the direction of the rotating magnetic field to capture the magnetic field.
The disk cannot reach that velocity due to the high velocity of the rotating magnetic field. This means that the velocity of the disc is always less than the velocity of the rotating magnetic field.
What is electro magnetic field?
When an electric current passes through an electric conductor, a magnetic field is generated around it. That field is called the electro magnetic field.
Structure of 3 Phase Induction Motor
The design of 3 phase induction motor consists of two parts,
What is the stator of 3 phase induction motor?
Stator means the stationary part of 3 Phase Induction Motor.
Stator It is mounted in the yoke of the motor. The stator is made of thin-layered silicon steel steppings, which laminated to each other.
Slots are made for winding in the stator. These slots are of close type, open type and semi closed type. By doing 3 phase winding in those slots, the ends of that winding are brought to the terminal box located on the yoke.
What is the rotor of 3 phase induction motor?
The rotor means the rotating part of the motor.
The rotor on the shaft is also made of thin pieces of silicon steel laminated together.
In this, close type oblique slots are made. All these slots are short-circuited by placing aluminum or copper thick conductors with aluminum rings at both ends.
The shortened ring is called the End Ring. The rotor thus shortened is called a Squirrel Cage Rotor. Squirrel cage rotor is used only in squirrel cage induction motors.
Double Squirrel Cage Rotor
This type of rotor has two types of squirrel cage winding in the slots. One inner cage winding and the other outer cage winding.
The inner cage winding at the bottom of the slot is shorted by a separate end ring with a thick aluminum or copper conductor. Hence, the resistance of this cage winding is low.
The slots are shorted through an independent end ring by placing thin conductors of brass over the outer cage winding. Because of which the resistance of this cage winding is high.
This type of rotor is used only in double squirrel cage induction motor.
What is a wound rotor?
For this type of rotor, the rotor is mounted on a hollow shaft. It has semi-close type or open type slots.
Those slots hold 3 phase windings of enamelled insulated copper. The number of pole windings in the stator winding is equal to the number of pole windings in the rotor. This rotor winding is always connected in star connection.
The shaft on which the rotor is mounted has three sliprings. The sliprings are attached to the rotor winding.
There is an arrangement to connect three externally star-connected variable resistors to the sleepings by means of brushes. The rotor winding can be shorted through a star connected resistors.
This type of rotor is used only in slippering induction motors.
Operation of three phase induction motor
When 3 phase power is supplied to the stator winding of a 3 phase induction motor, a rotating magnetic field is created around the stator winding.
Due to mutual induction, the magnetic lines of the stator are cut off from the rotor. Hence E.M.F. is created in the rotor. As a result, eddy current flows in the rotor.
Because of those eddy currents, their own magnetic field is created around the rotor.
According to Lenz’s law, the magnetic lines of the rotor oppose the rotation of the magnetic lines of the stator, and the rotor also begins to spin in an attempt to repel it with its own velocity in the direction of the stator’s magnetic field.
But the rotor speed is always less than the stator speed. Thus the rotor of the 3 Phase Induction Motor rotates and mechanical power is received on the shaft of the rotor.
Types of Three Phase Induction Motor
There are two main types of three phase induction motors.
- Squirrel Cage Induction Motor
- slipping induction motor.
There are two types of squirrel cage induction motors.
- Single Squirrel Cage Induction Motor
- Double Squirrel Cage Induction Motor..
Single Squirrel Cage Induction Motor
The stator of this motor is similar to the stator of other common induction motors, and the rotor is of a single squirrel cage type. The star or delta connection is made in the stator by 3 phase winding.
Three or six ends of these windings are brought out to the terminal box. If there are three ends, they are marked as U, V, W, and if the six ends come from the motor to the terminal box, they are
U1 U2, V1 V2, W1 W2 are marked as such.
This terminal is to be supplied by connecting three phase wires i.e.
L1, L2, L3.
Operation of Single Squirrel Cage Induction Motor
When 3 phase power is supplied to the stator winding, a rotating magnetic field is created in the stator. Mutual induction generates EMF in the rotor and causes current to flow.
As a result, the rotor starts rotating with the torque of the rotor
And the mechanical power starts getting on the rotor shaft.
Properties of Single Squirrel Cage Induction Motor
- Since the rotor resistance is low, the starting current is high. This current is about 6 to 10 times more than the total load current.
- The lower the rotor resistance, the lower the starting torque.
- The load rotates at an almost synchronous speed.
- No load torque is less because there is less slip at noload.
- At no load the power factor is low, it is usually 0.2 to 0.3 lagging.
- Both power factor and torque improve upon load. The power factor is about 0.8 to 0.85 lag at full load.
- The speed is almost constant.
Where is Single Squirrel Cage Induction Motor used?
Single Squirrel Cage Motor is used in places where high starting torque is not required such as water pump, drill machine, grinder, saw machine, flour mill, blower machine etc.
Double Squirrel Cage Induction Motor
The stator of this motor is the same as the stator of a single squirrel cage induction motor, and the rotor is of double squirrel cage type.
Working of Double Squirrel Cage Induction Motor
When the stator winding receives 3 phase power supply, the stator flux is disconnected from the upper conductor of the rotor itself, and the rotor starts rotating with torque.
Since the resistance of the upper cage winding is high, the rotor starts rotating with good starting torque.
The stator flux reaches the lower cage winding of the rotor until the motor picks up speed and is cut off by the lower cage wineing,
As the resistance of the lower cage winding decreases, the rotor current (I2) increases, so the rotor starts to spin, adding more running torque to the rotor.
Properties of Double Squirrel Cage Induction Motor
- Single squirrel cage takes less starting current than induction motor.
- Both starting and running torque are good.
- Power factor is good.
- The speed is almost constant.
Where is double squirrel cage induction motor used?
Where good starting torque is required
This motor is used in places like Crane, Punching Machine, Press Machine, Planer, Elevator, Rolling Mill etc.
How can the rotation direction of a 3 phase induction motor be changed?
If the direction of rotation of an induction motor is to be changed, the phase sequence of the 3 phase supply to the motor has to be changed.
If any 2 of the 3 phases received by the motor are interchanged, then the direction of rotation of that motor changes.
Slipping Induction Motor
The stator of this motor is similar to squirrel cage induction motor. The rotor, however, is a wound rotor rather than a squirrel cage.
There are as many pole windings in the stator as there are pole windings in the rotor. The stator winding is usually connected with the delta connection and the rotor winding is always connected with the star connection.
The rotor winding is connected to a slippering mounted on the rotor shaft. There is a provision for attaching a star connected variable revostat from outside in that slippering.
While starting this motor, the rotor circuit has to supply 3 phase supply to the stator winding by keeping the star connected variable revostat completely in the circuit.
Working of Sleeping Induction Motor
When a 3 phase power supply is supplied to the stator winding, initially the resistance of the rotor circuit is high, so the motor draws less starting current. And the motor starts the motor by getting more starting torque.
As the motor starts accelerating, the rotor resistance has to be gradually reduced. The rotor winding has to be shorted through the handle of the motor revostat.
This means that the rotor winding has a high resistance at the very beginning, as the external resistance is present only in the rotor circuit at the beginning and then again after the external resistance is removed from the circuit when the motor has attained its full speed. The rotor becomes short circuited like a squirrel cage rotor. And because of this, this motor starts working like a squirrel cage induction motor.
Properties of slipping induction motor
- Starting torque is of high quality.
- Starting current is reduced.
- Power factor is good.
- The speed decreases on load.
- The speed can be changed up to 5% by adding an external resistance to the rotor circuit.
- There is a possibility of sparks on slippering.
- Due to the increased I2R losses in the rotor, the efficiency is reduced.
- Squirrel cages are more expensive than motors.
- It is more complicated to turn on than a squirrel cage motor.
- The design is more complex than the Squirrel Cage Motor.
Where is sleeper induction motor used?
In places where high starting torque is required,
This motor is used to drive multiple shafts simultaneously in a crane, textile mill, shear machine, planer, elevator, rolling mill, punching machine etc.
Difference Between Squirrel Cage Induction Motor and Slipping Induction Motor
|Squirrel Cage Induction Motor||Slipping Induction Motor|
|1. It has squirrel cage rotor.||1. It consists of a wound rotor.|
|2. Starting current takes more. This is about 6 to 10 times more than the full load current.||2. Takes less starting current. This is almost twice the full load current.|
|3. Rotor resistance is low||3. The rotor resistance is high.|
|4. Cannot add external resistance to rotor circuit||4. External resistance can be added to the rotor circuit.|
|5. Lowering power factor at no load, improving power factor at no load.||5. Power factor is good on both the loads.|
|6. Efficiency is high.||6. Efficiency is less.|
|7. The speed is almost constant.||7. Speed decreases on load.|
|8. The speed can be changed by changing the number of poles.||8. The speed can be changed up to 5% by adding an external resistance to the rotor circuit.|
|9. The design is simple and cheap.||9. Complicated & expensive.|
|10. Easy to start.||10. Some caution is needed.|
|11. Maintenance cost is low.||11. Requires more maintenance.|
|12 is useful for continuous rotation.||12. Suitable for high starting torque.|