Whenever we think about the motor, An image of an iron block get created inside our mind in which a metallic shaft freely moves inside that iron block. Electric motors are available in different types such as DC motors, Synchronous motors & Induction motors.
In this post, we will talk about 3 phase induction motors, Working of induction motor, Name plate of induction motor & Parts of induction motor.
INDUCTION MOTOR
Induction motor is single excited motor where only stator contains the winding while rotor don't have any type of winding. Stator contains slots which is made up of silicon steel stamping in which winding is been placed.
Rotor freely suspend inside the stator with the help of bearings or bushes through the end covers. Rotor contains a solid block at a center which is type of silicon steel stamping in which aluminum conductor bars are melted in skewed form. Power to the rotor get delivered through induction.
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| 0.18 KW 3Phase Induction Motor By ABB |
WORKING PRINCIPLE OF INDUCTION MOTOR
When a rotating/changing magnetic field links with the conductor, An Emf get generated in the conductor. If that conductor is closed loop circuit than a current flows through the conductor due to the generated emf. This is the phenomenon is known as faraday law of Electromagnetic induction.
A closed loop current carrying conductor in between the magnetic field; A torque get produced on the conductor which results Conductor starts rotating. This is the phenomenon behind the working of Induction motor.
Stator's winding of the motor produces the rotating magnetic field while rotor behave as closed loop conductor.
PARTS OF INDUCTION MOTOR
Before going through the parts of induction motor we should read the name plate of induction motor first Which is been described next.
Name Plate
Whatever the type of work we are doing on induction motor whether it is motor connection OR motor testing, Name plate play the most important part. It tells about the working capacity, power supply ranges like voltage, frequency & power factor In short we can say name plate provides the safe limits of the induction motor.
The name plate of the Induction motor includes :
- Manufacturer name & trade mark
- Frame number
- KW rating
- RPM
- Efficiency %
- Rated Voltage
- Rated Current
- Power factor rating
- Ambient Temperature
- Bearing Details
- Duty type
- Insulation class
- IP rating
- Weight
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| 0.18 KW 3Phase Induction Motor By ABB |
Stator
Stator is the stationary part which contains the winding which produces the rotating magnetic field inside the motor. Stator's frame is generally made with cast iron or aluminum. In small induction motor, The outer periphery of the stator is designed in a special manner with outer fins for proper air cooling by increasing the effective surface area.
Winding is a set of coils made with enameled insulated wires, the wire is either made from copper OR aluminum. A stamping made with silicon steel carrying number of slots is being fitted inside the stator. Inside the slots of stamping, winding wires been placed. Inside the slot; behind the coil; insulation paper is being laid. On compete winding, For better insulation & proper binding of coils, insulated varnish is coated on entire winding.
The type of winding & which gauge wire is going to install, all depend upon the winding design of that particular power rating motor. Since system is 3 phase, 3 winding sets are created finally in every 3phase induction motor. Each winding set contains 2 open ends that means in total, we have 6 open winding ends. By connecting these 6 winding ends in particular manner, we can create two winding connections; one is STAR & another one is DELTA.
STAR CONNECTIONS
In star connections, one end of each winding set is connected as one common point & Rest three ends becomes terminals for power supply. That common point of the winding becomes the neutral which means each winding set get a power supply of single phase(phase-neutral)
DELTA CONNECTIONS
In delta connections, total 6 windings are connected in series manner so that 2 open ends each from different sets are connected to one common point which results finally 3 terminals get obtained. In delta connections, each winding ser get double phase power supply.
Rotor
Generally, Rotor of induction motor contains parallel number of conductors molted inside the slots of silicon steel stamping with common end rings. If we only consider these parallel conductors & neglecting the complete rotor's stamping than it appears like a cage, A Squirrel cage ! That's why Induction motors are also known as squirrel cage induction motor.
Both the end sides of the rotor are kept held with the end covers through bearings from both the sides, So that Rotor rotates freely inside the stator. There is very minimal gap in between the rotor stamping & stator stamping.
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(Top most Image) Rotor holding through End covers with bearings (Middle Image) Rotor inside the Stator (Bottom Image) Rotor, Stator & End Covers |
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| Rotor of 3.7KW- 3phase Induction motor by SIEMENS |
Connection Box
Connection box contains terminal plate where all the motor terminals been connected. Power supply to the motor is being connected out there.
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| Connection plate of 3phase Induction motor |
Cooling System
Small rating induction motor contains integrated air cooling system with a fan attached on the rotor's shaft at the back side of induction motor inside the back cover. Outer periphery fins all around the stator increases the effected surface area for better cooling.
In large induction motors for better cooling; cooling pump & cooling pipes are fitted across the motor.
IMPORTANT TERMS OF INDUCTION MOTOR
SYNCHRONOUS SPEED (Ns)
The magnetic field of the stator winding rotates at a speed known as synchronous speed(Ns).
Ns=(120*f)/P
If f is 50Hz & Pole is 4 than synchronous speed becomes, Ns=(120*50)/4 ; Ns=1500rpm
Where f is frequency & P is Pole of the winding
If f is 50Hz & Pole is 4 than synchronous speed becomes, Ns=(120*50)/4 ; Ns=1500rpm
Where f is frequency & P is Pole of the winding
ROTOR SPEED (Nr)
Rotor of the motor actually rotates at a speed lower than the synchronous speed & the difference between the synchronous speed & rotor speed known as slip.
As per shown in an image below, Motor speed; which is the rotor speed; is 2900 rpm
Motor's rated frequency is 50Hz.
Motor's winding pole is 2
Than synchronous speed becomes 3000 rpm
Slip which is equal to [(Ns-Nr)/Ns]*100
Where Ns is synchronous speed
Nr is rotor speed
s=3.3%
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| Name plate of 5.5 KW, 2900rpm Induction Motor |
In induction motor, rotor speed is not equal to the synchronous speed that's why it is known as Asynchronous motor.
Hope, You like the post. If you have any doubt, suggestion or query please do comments !