Three Phase Induction motor working Principle and its Types
Electrical motor converts electrical energy into mechanical energy.
What are the types of motors?
Based on Supply Motor are basically two types: AC and DC Motor. AC motor are divided into two types :Induction motor(Speed is variable), induction motor are also known as asynchronous motor and Synchronous motor(speed is constant ).
What are the types of induction motors?
Depending upon nature of AC supply induction motor are two types (a)Single phase induction motor and (b) 3 Phase Induction motor. Here we will discuss in detail about three phase induction motor.
Click Here to Download 3 Phase induction Motor .ppt
General aspects
•The construction of induction motor and induction generator are same.
•Induction motors are popularly used in the industry.
•Main features: cheap and low maintenance.
•Main disadvantages: speed control is not easy.
Overview of Three-Phase Induction Motor
•Induction motors are used in many industrial, residential, commercial and utility applications.
•Induction Motors transform electrical energy into mechanical energy.
•It is used for pump or fan, or connected to some other form such as a conveyor, winder, or mixer.
 |
| Application of IM |
3 phase induction motors takes three phase supply from mains and used for various industrial applications because of their following advantages -
1. Three phase Induction motors are very simple and rugged construction.
2. It is are very reliable, robust and having low cost.
3. The efficiency of 3 phase IM is high and its power factor is good at full load.
4. It required minimum maintenance.
4. Three phase induction motor is self starting hence extra mechanism is not required for starting.
They also have some disadvantages-
1. The Speed of induction motor decreases with increase in load, same as a DC shunt motor.
2. If speed is to be varied, the efficiency may change.
Construction
•The three basic parts of induction motor are the rotor, stator, and enclosure.
•The stator is stationary part while rotor is rotating part of induction motor.
•The enclosure protects the stator and rotor of the motor from harmful effects of the environment in which the motor operates.
Bearings which is mounted on the shaft, support the rotor . A fan, also mounted on the shaft, which is used for cooling purpose.
 |
| Construction of Induction motor |
Construction of Stator
•The stator winding is wounded on stator core which is made up of several thin laminations.
Coils of insulated wire are inserted into stator slots of the stator core. The stator winding either connected in delta or star configuration. The stator windings of induction motor are connected to the supply mains.
Construction of Rotor
•The rotor is the rotating part of three phase induction motor.
•It can be found in two types:
–Squirrel cage
–Wound rotor
What are the types of 3-phase induction motors?
Three phase Induction motor types : Based on rotor construction three phase Induction motor are two types-
Squirrel cage induction motor:
Rotor winding is composed of rotor copper bars embedded in the rotor slots and their end terminal is shorted end rings arrangements. It is very Simple, low cost, robust and has low maintenance.
Wound rotor or slip ring induction motor:
Rotor winding is wound by wires. The rotor winding terminals can be connected to external resistance through slip rings and brushes for smooth speed control.
•However, the most common type of rotor is the “squirrel cage” rotor.
 |
| Types of Motor |
Difference between Three phase Squirrel cage induction motor and Wound Rotor induction motor:
Squirrel cage rotor
1 No Slip rings or brushes
2 External resistance can not be connected.
3 Starting torque can not be adjusted.
4 Less rotor copper loss.
5 Higher efficiency
6 Applications includes lathes , fan ,water pumps
Wound rotor or Slip ring rotor :
1 Slip rings or brushes are used
2 External resistance can be connected.
3 Starting torque can be adjusted.
4 High rotor copper loss.
5 Low efficiency.
6 Applications includes crane ,elevators ,compressors , lifts etc.
Working principle of three phase induction motor
•When a thee phase stator winding is connected to a three phase voltage supply, 3 phase current will flow in the stator winding, which will produces 3 phase flux in the stator.
The nature of this flux is rotating with constant speed and fixed amplitude. The rotating magnetic flux will rotate at a speed called as Synchronous Speed, Ns and which is given as Ns=120 f/p
Where; Ns=Synchronous speed: speed of rotating flux, p = is the number of poles, and f = the frequency of supply
How 3 phase induction motor starts :Torque producing mechanism
When a 3 phase stator winding is connected to a three phase voltage supply, 3 phase current will flow in the windings, hence the stator is energized which produces rotating magnetic flux .
A rotating flux Φ is produced which links with rotor and according to faradays law of electromagnetic induction principle a voltage is induces in the rotor winding (like a transformer).
The induced voltage produces rotor current. The rotor current interacts with the flux and as we know that current carrying conductor placed in magnetic field it experiences torque.
This current will flow in such a direction that the rotor will experience a force that accelerates it in the same direction as that of RMF.
Slip and Rotor Speed
What is slip in induction motor?
Slip (s): The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The percentage difference between synchronous speed and rotor speed is called as slip.
s=(Ns-Nr)/Ns or Nr= Ns(1-s)
Where; ns = synchronous speed (rpm), Nr = mechanical speed of rotor (rpm)
Under normal operating conditions, slip lies between 1% -5%, which is very small.
Rotor Speed : When the rotor rotates with a speed of Nr (rps), the stator flux will circulate the rotor conductor at a speed of (Ns-Nr) rps. Hence, the frequency of the rotor can be derived as:
Where; s = slip, f = supply frequency
Torque Equation-The torque produced by IM depends on :
1.The rotor Power factor under running condition= CosØ 2
2.The rotor current under running condition= I2r
3.The Part of rotating magnetic field which induces e.m.f in rotor winding= Ø
Therefore, T α Ø Ir CosØ 2 ----------------(i)
But Ø α E1 Where E1 = Stator voltage But E1/E2 =K= Transformation ratio
Therefore, E2 α E1 Hence, E2 α Ø and E2 = Er
 |
| Equivalent circuit |
•Here, k=3/2Ï€Ns Ns = Synchronous speed in RPS= Ns/60
This expression indicates that:
1.For a slip ring IM we can connect external resistance by modifying R2 in the rotor circuit.
2.Hence we can adjust Starting torque by varying rotor resistance R2.
3.Modification of R2 in Cage type IM is not possible.
Relation between Air gap power(Pg),Rotor copper loss (Pcu) and Mechanical Power output(Pm) :
 |
| Power flow diagram |
a).Relation between Pg and Pcu:
Pg= Air gap Power
Pcu= Rotor copper loss
Pm= Mechanical Power
Let, Ns= synchronous speed and Nr= Rotor Speed
As we know, P= ω T (Angular Freq x Torque)
Pg = ω Tg
Pg = 2Ï€NsTg -------------------(1)
Similarly, Pm= 2Ï€NrTg ------(2)
Pcu = Pg-Pm
Pcu = 2Ï€NsTg - 2Ï€NrTg
Pcu = 2Ï€Tg (Ns – Nr) -------------(3)
Dividing equation 1 and 3
Therefore Pcu = sPg --------------(4)
b.Relation between Pm and Pcu:
Pg= Pm + Pcu
Pcu(1/s) = Pm + Pcu
Pcu = sPm + sPcu
Pcu – sPcu = sPm
Pcu(1-s) = sPm
Therefore Pcu/Pm = (s/(1-s) ---------(5)
Comparing Pcu = sPg -----------------(4) and (5)
Pg : Pcu : Pm = 1 : S : (1-S)
Need of Starter for three phase Induction motor: 3 Phase IM is self starting, because there is a starting torque.
•Let V1 be the stator voltage /phase
•Ze1 be the equivalent impedance/ phase ref to stator.
Ist = V1/ Ze1
•At the time of starting Ze1 is very very small, hence short circuit current Isc is very high. Generally it is 5 times greater that full load current.
•This higher current in stator winding may damage the winding due to overheating. Hence at the time of starting starters are required in three phase induction motor.
Types of Starters
Starters for Squirrel cage rotor induction motors
i) Direct on line (D.O.L.) starter
ii) Stator – resistance starter
iii) Auto transformer starter
iv) Star delta starter
Starters for Slip Ring Induction Motor
i) Rotor resistance starter
