14453188123456789101121314151617181920.ppt

Engineering

1. Induction MotorInduction Motor
(Asynchronous 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 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. StatorStator
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. RotorRotor
•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:Squirrel cage 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: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/rotor winding
Short circuits allShort circuits all
rotor bars.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. 1. Rotor Frequency f
r
= sf
2. Magnitude of Rotor induced EMF E
2r
= sE
2
3. Rotor Resistance and reactance X
2r = sX
2

18. Torque Equation
The torque produced in induction motor depends o the following factors
1.The part of RMF which reacts with rotor
2.Rotor Current in running conditions
3.Power factor of the rotor in running conditions
(1)
Where,
T is the torque produced by induction motor,
Ф is flux responsible of producing induced EMF
I
2r

 
is rotor current in running condition,
Cos Ф
2
is the
 power factor  of rotor circuit in running condition
The flux Ф produced by the stator is proportional to stator E
1
.(Stator Voltage)
Ф E
∝
1
(2)
T α Ф I
2r Cos Ф
2r

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)
(4)
Thus equation (1) can be replaced by E
2

Ф E
∝
1
Ф E
∝
2
T α E
2
I
2r
Cos Ф
2r

We Know
(5)
(6)

20. Substitute (4) (5) and (6) in equation (1)
T α E
2 I
2r Cos Ф
2r

K constant
(7)
(8)
(9)

21. Starting Torque S=1
(10)
Rotor
 current I
2
 
is defined as the ratio of rotor induced emf under running
condition , sE
2
to total impedance, Z
2
 
of rotor side,

22. Substitute (4) (5) and (6) in equation (1)
T α E
2 I
2r Cos Ф
2r

(7)
K constant
(8)
(9)
T α E
2 x x

23. Rotor
 current I
2
 
is defined as the ratio of rotor induced emf under running
condition , sE
2
to total impedance, Z
2
 
of rotor side,
(10)
Starting Torque S=1

24. CONDITIONS FOR MAXIMUM TORQUE
From the torque equation, the torque depends on the SLIP at which motor is running
V T E1 E2 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. Supply voltage is constant E
2
Now to Judge the nature of Torque Slip
S = 0
S = 1
Low Slip Region
T α s
R
2
is constant

Torque SLIP Characteristics

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

33. Blocked or Locked Rotor Test or Short Circuit Test
Sl.noShort Circuit Voltage
Vsc Volts
Short Circuit Current
Isc Amps
Short Circuit Input Power
Wsc Watts

34.  

W
o
 
= √3V
o
 
I
o
 
cosΦ
o
 

W
sc
 
= √3V
sc
 
I
sc
 
cosΦ
sc
From OC TEST From SC TEST
Short Circuit Current at Normal Voltage
AG = W
SN
W
SN
= Short Circuit Power at Normal Voltage
Torque Line

35. Φ
o
X O
V
Y
I
o
O’
D
Φ
sc
I
SN
A
O
utput L
ine
C
F
G
Rotor
Copper loss
Stator
Copper loss
Fixed loss
E
Torque Line

36. Φ
o
X O
V
Y
I
o
O’
D
Φ
sc
I
SN
A
O
utput L
ine
C
F
G
Rotor
Copper loss
Stator
Copper loss
Fixed loss
E
Torque Line
A’
P
Q
R
S
Full load output

37. Φ
o
X O
V
Y
I
o
O’
D
Φ
sc
I
SN
A
O
utput L
ine
C
F
G
Rotor
Copper loss
Stator
Copper loss
Fixed loss
E
Torque Line
A’
P
Q
R
S
Full load output
T
J
K

38. Method to find Location E
Stator Copper loss = 3 I
1
2
R
1
Rotor Copper loss = Wsc - 3 I
1
2
R
1

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