1. Induction Motor (Asynchronous Motor)
The three-phase induction motors are the most widely used electric motors in industry (small workshops to large industries)
These motors are employed in applications such as
centrifugal pumps
conveyers
compressors crushers
drilling machines etc.

2. Advantages
(i) It has simple and rugged construction.
(ii) It is relatively cheap.(Low Cost)
(iii) It requires little maintenance.
(iv) It has high efficiency
(v) Reasonably good power factor.
(vi) It has self starting torque.
(vii) absence of commutator
Disadvantages
(i) Its speed is not constant, when load is varied.
(ii) Low starting torque compared to DC shunt motor.

3. Constructional details
A 3-phase induction motor comprises two electromagnetic parts:
(i) Stationary part called the Stator
(ii) Rotating part called the Rotor
The stator and the rotor are each made up of
An electric circuit, usually made of insulated copper or aluminium winding, to carry current.
A magnetic circuit, usually made from laminated silicon steel, to carry magnetic flux

4. Stator The stator of the three phase induction motor
consists of three main parts:
1. Stator frame
2. Stator core
3. Stator winding or field winding

5. 0.4 to 0.5 mm thick

6. The stator is the outer stationary part of the motor, which consists of
• The outer cylindrical frame of the motor or yoke, which is made either of welded sheet steel, cast iron or cast aluminium alloy.
• The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the cylindrical space inside the outer frame.
The magnetic path is laminated to reduce eddy currents, reducing losses and heating.
• A set of insulated electrical windings, which are placed inside the slots of the laminated stator.
For a 3-phase motor, 3 sets of windings are required, one for each phase connected in either star or delta

7. Rotor The rotor is the rotating part of the electromagnetic circuit.
It can be found in two types:
Squirrel cage
Wound rotor
However, the most common type of rotor is the “squirrel cage” rotor.

8. Squirrel cage type: Wound rotor type:
Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings
Simple, low cost, robust, low maintenance
Wound rotor type:
Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes.
Easy to control speed, more expensive.

9. /rotor winding
Short circuits all
rotor bars.

12. Principle of operation of three phase Induction Motor

13. Rotating Magnetic Field

14. Rotating Magnetic Field

16. Slip on Induction motor

17. 2. Magnitude of Rotor induced EMF E2r = sE2
1. Rotor Frequency fr = sf
2. Magnitude of Rotor induced EMF E2r = sE2
3. Rotor Resistance and reactance X2r = sX2

18. Torque Equation T α Ф I2r Cos Ф2r
The torque produced in induction motor depends o the following factors
The part of RMF which reacts with rotor
Rotor Current in running conditions
Power factor of the rotor in running conditions
T α Ф I2r Cos Ф2r
(1)
Where,
T is the torque produced by induction motor,
Ф is flux responsible of producing induced EMF
I2r  is rotor current in running condition,
Cos Ф is the power factor  of rotor circuit in running condition
The flux Ф produced by the stator is proportional to stator E1.(Stator Voltage)
Ф ∝ E1
(2)

19. We know that transformation ratio “K” is defined as the ratio of secondary  voltage (rotor voltage) to that of primary voltage  (stator voltage)
(3)
From (2) and (3)
Ф ∝ E1
Ф ∝ E2
(4)
Thus equation (1) can be replaced by E2
T α E2 I2r Cos Ф2r
We Know
(5)
(6)

20. T α E2 I2r Cos Ф2r Substitute (4) (5) and (6) in equation (1) (7)
(7)
K constant
(8)
(9)

21. Starting Torque S=1
(10)
Rotor current I2 is defined as the ratio of rotor induced emf under running condition , sE2to total impedance, Z2 of rotor side,

22. Substitute (4) (5) and (6) in equation (1)
T α E2 I2r Cos Ф2r
T α E2 x x
(7)
K constant
(8)
(9)

23. Starting Torque S=1
(10)
Rotor current I2 is defined as the ratio of rotor induced emf under running condition , sE2to total impedance, Z2 of rotor side,

24. CONDITIONS FOR MAXIMUM TORQUE
From the torque equation, the torque depends on the SLIP at which motor is running
V T E1 E constant
SLIP is variable and controlling parameter
Maximum Torque
Differentiate T with respect to S (U/V) method

25. This is the slip at which torque is max.

26. T α s Torque SLIP Characteristics Supply voltage is constant E2
Supply voltage is constant E2
Now to Judge the nature of Torque Slip
S = 0
S = 1
Low Slip Region
R2 is constant
T α s

27. High Slip Region

28. Torque Slip Characteristics

29. From NO LOAD to full load its speed decreases SLIP Increases

30. Losses and Efficiency of Induction Motor
There are two types of losses occur in three phase induction motor. These losses are,
1. Constant or fixed losses,
2. Variable losses.
1. Iron or core losses,
2. Mechanical losses,
3. Brush friction losses.

32. Circle Diagram of an induction Motor
No Load Test or Open Circuit Test
Sl.no
NO Load Voltage
Rated Line Voltage
Vo Volts
No Load Current
Io Amps
No Load Input Power
Wo Watts

33. Short Circuit Input Power
Blocked or Locked Rotor Test or Short Circuit Test
Sl.no
Short Circuit Voltage
Vsc Volts
Short Circuit Current
Isc Amps
Short Circuit Input Power
Wsc Watts

34. From OC TEST
From SC TEST
  Wo = √3Vo Io cosΦo
  Wsc = √3Vsc Isc cosΦsc
Short Circuit Current at Normal Voltage
AG = WSN
WSN = Short Circuit Power at Normal Voltage
Torque Line

35. Y V A
Rotor
Copper loss
Output Line
ISN E
Torque Line
Stator
Copper loss
Φsc F D Φo O’ C
Fixed loss Io O X G

36. A’ Y
Full load output V A
Rotor
Copper loss
Output Line P
ISN E
Torque Line
Stator
Copper loss
Φsc Q F D R Φo O’ C
Fixed loss Io O X S G

37. A’ Y
Full load output V J A
Rotor
Copper loss
Output Line P
ISN E
Torque Line
Stator
Copper loss Q K
Φsc R F D S Φo O’ C
Fixed loss Io O X T G

38. Method to find Location E
Stator Copper loss = 3 I12 R1
Rotor Copper loss = Wsc - 3 I12 R1

39. Double Cage Induction Motor
Squirrel Cage Induction Motor has good running characteristics but starting torque is poor
Slip ring Induction Motor has good running characteristics and good starting torque but
Unsuitable for many types of application