Today in this topic we are going to collect some important data about 3-phase induction motor. So let’s know what are these.
Conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. In D.C. motor the electric power is conduct directly to the armature through the brushes and commutator. So, in this sense a D. C. motor is also know as conduction motor.
In A. C. motor the rotor does not receive electric power by conduction but it receive by induction in exactly the same way as the secondary of a two winding transformer receive its power from the primary. That is why such motor are know as induction motor.
An induction motor can be treat as a rotating transformer. and All the A.C. motor the polyphase induction motor is the one which is extensively use for various kinds of industrial drives.
Advantages of 3 phase induction motor
- It is very simple and rugged form ( It is almost unbreakable construction ).
- Its cost is very low and reliable.
- So, Its efficiency is sufficiently very high in normal running condition, no brushes are nee, hence frictional losses are reduce. therefore it has say reasonably good power factor.
- So, It require less maintenance.
- Induction motor can be operate easily, it can start from rest, it does not need any extra motors.
- And also it does not require to be synchronize. and It starting arrangement is almost simple.
Disadvantages of 3 phase induction motor
- Its speed cannot be varied without sacrificing its efficiency.
- It speed decrease with increase in load.
- The starting torque is somewhat inferior to that of D.C. motor.
Explain construction of three phase induction motor
An induction motor consists of two main parts. and That is (1) starter and (2) rotor.
Stator
The stator of a induction motor, it is made up of a number of stamping, which are slotted to receive the windings. Then, the stator carries 3-phase winding and it is fed by 3-phase supply.
The stator is wound for a definite number of poles, So the exact number of poles is determined by the requirement of speed. Greater the number of poles lesser is the speed and vice-versa.
When the stator winding is supply with 3-phase current then a magnetic flux is produce in it, which is constant magnitude and revolve an synchronous speed given by NS = 120ƒ / P. and This revolving magnetic flux induces an E.M.F. in the rotor by mutual induction.
Read also this what is resistance ? in electrical
Rotor
The rotor is a hollow laminated core, which is mounted on a shaft. So there are slots on its outer periphery in which the windings are placed they are called rotor winding.
There are two types of rotor that is (a) Squirrel cage rotor and (b) Slip ring rotor [Slip ring rotor is also know as phase wound or wound rotor].
Squirrel cage rotor
Squirrel cage rotor is a simple and most rugged construction and unbreakable. and The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors and it consists of heavy bars of copper, aluminum or alloys.
Then one bar is place in each slot and are inserted from the end when semi closed slots are used. So the rotor bars are bolted to two heavy short-circuiting end rings. Which gives to a picture of squirrel cage construction.
The rotor bars are permanently short-circuit on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purpose.
The rotor slots are not quite parallel to each other. but they are slight skew to reduce the magnetic hum and to reduce the locking tendency of the rotor.
Phase wound rotor
This type of rotor is provided with 3-phase double layer distributed winding consisting of coils as used in alternators. The 3 -winding terminals are fetch out and connected to 3-insulated slip rings mounted on the shaft with brushes resting on them.
These 3-brushes are externally connect to a 3-phase star connected rheostat. Their is additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor and for changing its speed torque current characteristics.
When running under normal condition the slip rings are automatically short circuited by means of a metal which is pushed along the shaft and connects all the rings together.
The brushes are automatically lift from the slip rings to reduce the frictional losses and the wear and tear. Hence under normal running condition the wound rotor are short circuit just like a squirrel cage rotor.
Read also this vector diagram of loaded alternator
Main parts of 3 phase induction motor
There are 7 main parts in 3-phase induction motor that is.
- Frame
- Stator and rotor core
- Stator and rotor winding
- Air gap
- Shaft and bearings
- Fans
- Slip ring
So now let us discuss the main parts of an induction motor one by one.
(1) Frame
It function is to support the field in a compact form and thus it prevent from mechanical injury it also prevent the leakage of magnetic flux.
(2) Stator and rotor core
These are made up of silicon with steel lamination (Silicon to prevent hysteresis loss and steel to prevent eddy current loss).
(3) Stator and rotor winding
These are make by copper, aluminum, alloy also copper bar, aluminum bar. and The winding have moisture proof insulation (mica and high quality varnish).
(4) Air gap
The stator and rotor are fitted carefully to ensure of uniformity air gap between them to allow the magnetic flux revolved around it with synchronous speed.
(5) Shaft and bearing
This are use to perform heavy duty, toruble free running for enhanced long service life. It minimize the frictional loss for wear and tear of the machine.
(6) Fans
Fans are use for circulation of cooling air in to the shaft there are especially make by cast iron and light aluminum.
(7) Slip ring
These parts are make by high quality phosphorus bronze and it function is to hold the rotor winding.
Principle of operation of induction motor
When the 3-phase stator winding are fed by a 3-phase supply then a magnetic flux of constant magnitude but rotating at synchronous speed is set up.
So, the flux passes through the air gap sweeps the rotor conductor and cuts it, which is in stationary position. Due to the relative speed between the rotating flux and the stationary conductors an E.M.F. is induce according to faradays law of electro magnetic induction.
The frequency of the induced E.M.F. is same as the supply frequency. So, its magnitude is proportional to the relative velocity between the flux and the conductor. Then its direction is given by Fleming right hand rule.
Since the rotor bars or conductors form a close circuit, then the rotor current is produce whose direction is given by Lenz’s law is such as to oppose the very cause producing it.
So, in this case the cause which produce the rotor current is the relative velocity between the rotating flux of the stator and the stationary rotor conductor.
Hence, to reduce the relative speed the rotor start running in the same direction as that of the flux and tries to catch up with the rotating flux.
Slip formula in induction motor
Definition of slip :- The difference between the synchronous speed ( NS ) and the actual speed( N ) of the rotor is know as slip.
So Slip is express as a percentage (%) and % is express as a ( s )
- Sometimes, (NS − N) is know as a Slip speed.
- Rotor speed or motor speed is N = NS ( 1 − s ).
S = NS − N / NS
S × NS = NS − N
N = NS − S × NS
So the formula is N = NS (1 − S )
The revolving flux is rotating synchronously and it is relative to the stator is synchronous and slip speed is relative to the rotor. The rotor never succeeds in catching up with the stator field.
If it really did so then there would be no relative speed between the rotor and stator field . Hence no rotor E.M.F. no rotor current and no rotor torque to maintain rotation.
That is why the rotor runs at a speed which is always less than the speed of the stator field. that is synchronous speed. and The difference of speed depends upon the load.
Rotor current frequency
Rotor frequency it is express as a ‘ƒ ‘.
NS − N = 120 f ‘ / P ———- ( 1 )
NS = 120 f / P ———— ( 2 )
f ‘ / f = NS − N / NS = S
So the Rotor frequency is f ‘ = s f
Some numerical and solution from 3-phase induction motor
Question-1 : A slip ring induction motor runs at 290 r.p.m. at full load when connect to 50 Hz supply. Then determine the number of poles and slip.
Solution :
Given data –
Speed (N) = 290 r.p.m
Frequency ( f ) = 50 Hz
Here, we assumed NS = 300 r.p.m
( i ) We know the formula NS = 120f / P
300 = 120 × 50 / P
P = 120 × 50 / 300
P = 20
( ii )
S = (300 − 290) / 300 × 100
S = 3.33 %
NOTE – Sometimes NS is near to the N and in this question the value of NS is not given so we assumed N = 300. So N is called rotor speed and NS is called synchronous speed.
Question-2 : The stator of a 3-phase induction motor has 3 slots per pole per phase if supply frequency is 50 Hz then calculate
- Number of stator poles produce and total number of slots on the stator
- Speed of the rotating stator flux.
Solution :
(i) Stator pole = 2n ( Where, n = is the number of phase )
= 2 × 6
= 6 poles
Total number of slots on the stator = 3 slots ×number of poles × 3 phase
= 3×6×3
= 54
(ii) N = 120f / P
= 120×50 / 6
= 1000 r.p.m.
Question-3 : A 4 pole 3-phase induction motor operates from a supply whose frequency is 50 Hz calculate
- The speed at which the magnetic field of the stator is rotating.
- The speed of the rotor when the slip is 0.04.
- The frequency of the rotor currents when the slip is 0.03.
- The frequency of the rotor currents at standstill.
Solution :
(i) Stator field revolves at synchronous speed ( Ns ) = 120 f / P
= 120×50 / 4
= 1500 r.p.m.
(ii) Rotor or motor speed , N = Ns( 1 − s )
= 1500 (1 − 0.04 )
= 1440 r.p.m.
(iii) Frequency of rotor current, f ‘ = sf
= 0.03 × 50
= 1.5 Hz
(iv) s = 1
So we know that f ‘ = sf
f ‘ = 1×50
f ‘ = 50 Hz
So here, in this above article we study about 3-phase induction motor. In industrial applications 3-phase induction motors are most commonly use. One more thing this is very interesting and important chapter of energy conversion. So, I hope you will get to learn a lot in this article. Thank you