About AC Machines

1. EVEN SEMESTER
2017-2018
P. Maria Sheeba
ASSISTANT PROFESSOR /ECE
MOUNT ZION COLLEGE OF ENGINEERING AND
TECHNOLOGY
PUDUKKOTTAI
P. Maria Sheeba
ASSISTANT PROFESSOR /ECE
MOUNT ZION COLLEGE OF ENGINEERING AND
TECHNOLOGY
PUDUKKOTTAI 1
BE8254 - BASIC ELECTRICAL AND INSTRUMENTATION
ENGINEERING
BE8254 - BASIC ELECTRICAL AND INSTRUMENTATION
ENGINEERING

2. • Induction motors are used worldwide in many residential,
commercial, industrial, and utility applications.
• Induction Motors transform electrical energy into mechanical
energy.
2
Overview of Three-Phase Induction
Motor
Overview of Three-Phase Induction
Motor

3. 3
•
It can be part of a pump or fan.
IntroductionIntroduction

4. • A induction machine can be used as either a
induction generator or a induction motor.
• Induction motors are popularly used in the
industry
• Focus on three-phase induction motor
• Main features: cheap and low maintenance
• Main disadvantages: speed control is not easy
4
IntroductionIntroduction

5. • Torque producing mechanism
When a 3 phase stator winding is connected to a 3
phase voltage supply, 3 phase current will flow in the
windings, hence the stator is energized.
A rotating flux Φ is produced in the air gap. The flux
Φ induces a voltage Ea in the rotor winding (like a
transformer).
5
Principle of OperationPrinciple of Operation

6. The induced voltage produces rotor current, if
rotor circuit is closed.
The rotor current interacts with the flux Φ,
producing torque. The rotor rotates in the
direction of the rotating flux.
6
Principle of OperationPrinciple of Operation

7. • When a 3 phase stator winding is connected to a 3 phase voltage
supply, 3 phase current will flow in the windings, which also will
induced 3 phase flux in the stator.
• Where; p = is the number of poles, and
f = the frequency of supply
p
f
ns
120
=
7
Rotating Magnetic FieldRotating Magnetic Field

8. • These flux will rotate at a speed called a
Synchronous Speed, nSynchronous Speed, nss.
• The flux is called as Rotating magnetic Field.
• Synchronous speed: speed of rotating flux
8
Rotating Magnetic FieldRotating Magnetic Field

9. Construction
• The three basic parts of an AC motor are the rotor, stator, and
enclosure.
• The stator and the rotor are electrical circuits that perform as
electromagnets.
9
ConstructionConstruction

10. 10
Stator ConstructionStator Construction

11. 11
Squirrel RotorSquirrel Rotor

12. Construction
12
ConstructionConstruction

13. • The stator is the stationary electrical part of the motor.stationary electrical part of the motor.
• The stator core of a National Electrical Manufacturers Association
(NEMA) motor is made up of several hundred thin laminationsseveral hundred thin laminations.
13
Stator constructionStator construction

14. • Stator laminations are stacked togetherstacked together forming a hollowhollow
cylindercylinder.
• Electromagnetism is the principle behind motor operation.
• Each grouping of coilsEach grouping of coils, together with the steel core it
surrounds, form an electromagnet. The stator windings are
connected directly to the power source.
14
Stator constructionStator construction

15. • 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.
15
Rotor constructionRotor construction

16.  Squirrel cage type
 Wound rotor type
Squirrel cage typet
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
16
Induction motor typesInduction motor types

17. 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.
17
Induction motor typesInduction motor types

18. Rotor construction
18
Wound Rotor
Squirrel-Cage Rotor
/rotor winding/rotor winding
Short circuits allShort circuits all
rotor bars.rotor bars.

19. a Fc
-93 10 113 216
-1.5
-1
-0.5
0
0.5
1
1.5
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
Fb
Fa F
Fb
Fc
F
Fa
F
Fb
Fc Fc Fb
F
Space angle (θ) in degrees
F
Fa Fc
Fb
t = t0= t4
t = t1
t = t2 t = t3
t = t0= t4
RMF(Rotating Magnetic Field)
19

20. AC Machine Stator
20

21. a’
a’
-90 -40 10 60 110 160 210 260
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Fa
Space angle (theta) in degrees
t0
t01
t12
t2
a
MMF Due to ‘a’ phase current
21

22. 1 Cycle
Amp
timet0
t1 t2 t3 t4
t01 t12
22
Current in dc machine

23. Equivalent Circuit of the
Induction Motor
23

24. 22
2
2
jsXR
sE
I
+
=
2
2
22 RIP =
( ) 22
2
2
/ jXsR
E
I
+
=
( )
s
R
Is
s
R
RIPP ag
22
2
2
2
2
2 1 =



−+==
( )s
s
R
IPmech −= 122
2
( ) agmech PsP *1−=
( )
2
1
P
s
s
Pmech
−
= agsPRIP == 2
2
22
( )ssPPP mechag −= 1::1:: 2
24

25. Equivalent Circuit
Theveninequivalent circuit 25

26. ( )
12
1
2
1
V
XXR
X
V
m
m
th
++
=
( )2
1
2
1 mXXRIf +<<
11
1
VKV
XX
X
V th
m
m
th =
+
≈
( )
( ) thth
m
m
th jXR
XXjR
jXRjX
Z +=
++
+
=
11
11
If , then,
26

27. 27
( )2
1
2
1 mXXR +<<
1
2
1
2
1
RKR
XX
X
R th
m
m
th =







+
≅
1XXth ≈

28. Locked rotor test‑
28

29. 29
BL
BL
BL
I
P
R 2
13
=
BL
BL
fBLBL
I
V
Z
1
1
=
( )22
BLfBLBLfBLBL RZX −=
testrotorblockedatFrequency
FrequencyRated
XX fBLBLBL *=

30. 30
21 XXXBL ′+≅
NLm XXX =+1
1XXX NLm −=
Assume X1 = X2’ (at rated frequency )
Then X1 , X2 can be obtained

31. Blocked rotor equivalent circuit for improved‑
value
31

32. ( ) 22
2
2
2
2
R
XXR
X
R
m
m
′
+′+′
=
R
X
XX
R
m
m
2
2
2 




 +′
=′ 2
2
2
R
XX
X
R
m
m
′





+′
≅
1RRR BL −=
32

33. Equivalent Circuit of Induction Machines
• Conventional equivalent circuit
 Note:
● Never use three-phase equivalent circuit. Always use
per- phase equivalent circuit.
● The equivalent circuit always bases on the Yalways bases on the Y
connection regardless of the actual connection of theconnection regardless of the actual connection of the
motormotor.
● Induction machine equivalent circuit is very similar to
the single-phase equivalent circuit of transformer. It is
composed of stator circuit and rotor circuit
33

34. Equivalent Circuit of Induction Machines
• Step1 Rotor winding is open
(The rotor will not rotate)
• Note:
– the frequency of E2 is the same as that of E1 since the rotor is at
standstill. At standstill s=1.
34
f f

35. Equivalent Circuit of Induction Machines
• Step2 Rotor winding is shorted
(Under normal operating conditions, the rotor winding is shorted. The slip is s)
• Note:
– the frequency of E2 is fr=sf because rotor is rotating.
35
f fr

36. Equivalent Circuit of Induction Machines
• Step3 Eliminate f2
Keep the rotor current same:
36

37. Equivalent Circuit of Induction Machines
• Step 4 Referred to the stator sideStep 4 Referred to the stator side
37

38. • Note:
– X’2 and R’2 will be given or measured. In practice, we do
not have to calculate them from above equations.
– Always refer the rotor side parameters to stator side.
– Rc represents core loss, which is the core loss of stator
side.
38

39. Equivalent Circuit of Induction Machines
• IEEE recommended equivalent circuit
• Note:
– Rc is omitted. The core loss is lumped with the
rotational loss.
39

40. Equivalent Circuit of Induction Machines
• IEEE recommended equivalent circuit
Note: can be separated into 2 PARTS
• Purpose :
– to obtain the developed mechanical
40
I1 1R1X
mX
'
2X '
2R
s
s
R
−1'
21V
s
R2
s
sR
R
s
R )1(2
2
2 −
+=

41. Efficiency
WattxWhpxP
IVP
otherwise
PPP
PPP
givenarePif
P
P
out
ssin
mo
lossesino
losses
in
out
746746
cos3
,
,
%100
=×=
=
−=
−=
×=
θ
η
µ
41

42. Torque-Equation
• Torque, can be derived from power equation in term of
mechanical power or electrical power.
n
P
THence
srad
n
whereTPPower
π
π
ωω
2
60
,
)/(
60
2
,,
=
==
r
o
o
r
m
m
n
P
TTorqueOutput
n
P
TTorqueMechanical
Thus
π
π
2
60
,
2
60
,
,
=
=
42

43. Single phase Induction Motors
43

44. Introduction
• Three-phase induction motors are the most common
and frequently encountered machines in industry
– simple design, rugged, low-price, easy
maintenance
– wide range of power ratings: fractional
horsepower to 10 MW
44

45. – run essentially as constant speed from no-load to
full load
– Its speed depends on the frequency of the power
source
• not easy to have variable speed control
• requires a variable-frequency power-electronic
drive for optimal speed control
45

46. Construction
• An induction motor has two main parts
– a stationary stator
• consisting of a steel frame that supports a hollow,
cylindrical core.
• core, constructed from stacked laminations , having a
number of evenly spaced slots, providing the space for
the stator winding.
46

47. Construction
– A revolving rotor
• Composed of punched laminations, stacked to create
a series of rotor slots, providing space for the rotor
winding.
• One of two types of rotor windings
• .
47

48. • Conventional 3-phase windings made of insulated wire
(wound-rotor) » similar to the winding on the stator.
• Aluminum bus bars shorted together at the ends by
two aluminum rings, forming a squirrel-cage shaped
circuit (squirrel-cage)
48

49. Types of single phase motor
Two basic design types depending on the rotor design
•squirrel-cage
•wound-rotor
– squirrel-cage: conducting bars laid into slots and
shorted at both ends by shorting rings.
49

50. wound-rotor:
•Complete set of three-phase windings exactly as the
stator.
• Usually Y-connected, the ends of the three rotor wires
are connected to 3 slip rings on the rotor shaft.
50

51. Construction
Squirrel cage
rotor
Wound rotor
Notice the slip
rings
51

52. Cutaway in a
typical wound-
rotor IM. Notice
the brushes and
the slip rings
Brush
es
Slip
rings
52

53. Double filed revolving theory
• Balanced three phase windings, i.e.
mechanically displaced 120 degrees form
each other, fed by balanced three phase
source
120 e
sync
f
n rpm
P
=
53

54. • A rotating magnetic field with constant magnitude is
produced, rotating with a speed N.
.Where fe is the supply frequency and P is the no. of
poles and nsync is called the synchronous speed in rpm
(revolutions per minute)
54

55. Rotating Magnetic Field
55

56. Rotating Magnetic Field
56

57. Rotating Magnetic Field
( ) ( ) ( ) ( )net a b cB t B t B t B t= + +
sin( ) 0 sin( 120 ) 120 sin( 240) 240M M MB t B t B tω ω ω= ∠ °+ − ° ∠ °+ − ∠ °
ˆsin( )
3
ˆ ˆ[0.5 sin( 120 )] [ sin( 120 )]
2
3
ˆ ˆ[0.5 sin( 240 )] [ sin( 240 )]
2
M
M M
M M
B t
B t B t
B t B t
ω
ω ω
ω ω
=
− − ° − − °
− − ° + − °
x
x y
x y
57

58. Rotating Magnetic Field
1 3 1 3
ˆ( ) [ sin( ) sin( ) cos( ) sin( ) cos( )]
4 4 4 4
3 3 3 3
ˆ[ sin( ) cos( ) sin( ) cos( )]
4 4 4 4
net M M M M M
M M M M
B t B t B t B t B t B t
B t B t B t B t
ω ω ω ω ω
ω ω ω ω
= + + + −
+ − − + −
x
y
ˆ ˆ[1.5 sin( )] [1.5 cos( )]M MB t B tω ω= −x y
58

59. Rotating Magnetic Field
59

60. Principle of operation
• This rotating magnetic field cuts the rotor windings and
produces an induced voltage in the rotor windings
Where τind is the induced torque and BR and BS are the magnetic
flux densities of the rotor and the stator respectively
ind R skB Bτ = ×
60

61. • Due to the fact that the rotor windings are short
circuited, for both squirrel cage and wound-rotor,
and induced current flows in the rotor windings
• The rotor current produces another magnetic field
• A torque is produced as a result of the interaction of
those two magnetic fields
61

62. Induction motor speed
• At what speed will the IM run
– Can the IM run at the synchronous speed.
– If rotor runs at the synchronous speed, which is
the same speed of the rotating magnetic field.
– When the speed falls, the rotating magnetic field
will cut the rotor windings and a torque is
produced
62

63. Induction motor speed
• So, the IM will always run at a speed lower
than the synchronous speed.
• The difference between the motor speed and
the synchronous speed is called the Slip.
63

64. 64
slip sync mn n n= −
Where nslip= slip speed
nsync= speed of the magnetic field
nm = mechanical shaft speed of the motor

65. The Slip
sync m
sync
n n
s
n
−
=
Where s is the slip
Notice that : if the rotor runs at synchronous speed
65

66. s = 0
if the rotor is stationary
s = 1
Slip may be expressed as a percentage by multiplying
the above eq. by 100, notice that the slip is a ratio
and doesn’t have units
66

67. Torque
• While the input to the induction motor is
electrical power.
• Its output is mechanical power and for that we
should know some terms and quantities
related to mechanical power.
67

68. • Any mechanical load applied to the motor
shaft will introduce a Torque on the motor
shaft.
• This torque is related to the motor output
power and the rotor speed.
68
.out
load
m
P
N mτ
ω
= 2
/
60
m
m
n
rad s
π
ω =

69. Horse power
• Another unit used to measure mechanical
power is the horse power .
• It is used to refer to the mechanical output
power of the motor.
69

70. • Since we, as an electrical engineers, deal with
watts as a unit to measure electrical power,
there is a relation between horse power and
watts.
70
746hp watts=

71. STARTING METHODS OF 1PHASE IM
• Two separate motor windings
• Good running efficiency
• Medium amount of starting torque
• Speed typically ranges from 1800 – 3600 rpm
71

72. • Motor speed is determined by the number of
poles
• Slip is the difference between the calculated
and actual motor speeds
72

73. START WINDING
Small Wire
Large Number of Turns
High Resistance
RUN WINDING
Larger Wire
Small Number of Turns
Low Resistance
L1
L2
73

74. 120 Volts
Rotor
RUN
RUN
START
START
74

75. THE CENTRIFUGAL SWITCH
• Commonly used on open motors to de-energize
the start winding
• Opens its contacts when the motor reaches
about 75% of its rated speed
• When the contacts open and close, a spark is
created (arcing)
75
MZCET/ECE/III SEM/EE
6352-UNIT 3

76. START WINDING
Small Wire
Large Number of Turns
High Resistance
RUN WINDING
Larger Wire
Small Number of Turns
Low Resistance
L1
L2
THE CENTRIFUGAL SWITCH
76

77. CAPACITOR-START MOTOR
• Split phase motor with start and run windings.
• Start capacitor assists the motor starting by increasing
the starting torque.
• Start capacitor is wired in series with the motor’s start
winding
77

78. • Start capacitor is removed from the circuit
when the start winding is removed.
• Start capacitor increases the phase angle.
78

79. CAPACITOR-START MOTOR
START
CAPACITOR
L2
L1
START
RUN
79

80. Alternators
•The stator is similar in construction that of a induction
motor
•The rotor can be Salient or Non-Salient (cylindrical
rotor)
80

81. 81
•Field excitation is provided on the rotor by either
permanent or electromagnets with number of poles
equal to the poles of the RMF caused by stator
•Non-excited rotors are also possible as in case of
reluctance motors

82. Synchronous Machine Construction
82

83. Synchronous Motor-Principle
The rotor acting as a bar magnet will turn to line up with the
rotating magnet field. The rotor gets locked to the RMF and rotates
unlike induction motor at synchronous speed under all load condition
83

84. An increase in the load will cause the rotor to lag the stator field but still maintain
synchronous speed. Increase in load has increased the torque component, but the
field strength has decreased due to the increase in length of the air gap between
the rotor and the stator.
Lightly
loaded motor Heavily loaded
motor
Changing The Load
84

85. Types of synchronous motor
 Salient-pole synchronous machine
 Cylindrical or round-rotor synchronous machine
85

86. 1. Most hydraulic turbines have to turn at low speeds
(between 50 and 300 r/min)
2. A large number of poles are required on the rotor
Hydrogenerator
Turbine
Hydro (water)
D » 10 m
Non-uniform
air-gap
N
S S
N
d-axis
q-axis
Salient-Pole
86

87. L » 10 m
D » 1 mTurbine
Steam
Stato
r
Uniform air-gap
Stator winding
Rotor
Rotor winding
N
S
 High speed
 3600 r/minÞ2-pole
 1800 r/minÞ4-pole
î Direct-conductor cooling (using hydrogen
or water as coolant)
î Rating up to 2000 MVA
Turbogenerator
d-axis
q-axis
Cylindrical-Rotor
87

88. Cylindrical-Rotor
Stator
Cylindrical rotor 88

89. Salient-Pole
Stator
89

90. Salient-Pole Rotor
Excitation
Windings
90

91. Salient-Pole Rotor with brushless excitation
91

92. EMF Equation
Consider following
•Φ= flux per pole in wb
•P = Number of poles
•Ns = Synchronous speed in rpm
•f = frequency of induced emf in Hz
•Z = total number of stator conductors
92

93. • Zph = conductors per phase connected in series.
• Tph = Number of turns per phase.
Assuming concentrated winding, considering one conductor
placed in a slot.
According to Faraday's Law electromagnetic induction,
93

94. • The average value of emf induced per conductor in one
revolution
• eavg = dΦ /dt
• eavg = Change of Flux in one revolution/ Time taken for one
revolution
Change of Flux in one revolution =pΦ
Time taken for one revolution
= 60/Ns seconds.
94

95. • Hence eavg = (p x Φ ) / ( 60/Ns)
= p x Φ x Ns / 60
We know f = PNs /120
hence PNs /60 = 2f
Hence eavg = 2 Φ f volts
Hence average emf per turn = 2 x 2 Φ f
= 4Φf volts
95

96. • If there are Tph, number of turns per phase connected in
series, then average emf induced in Tph turns is
• Eph,avg = Tph x eavg = 4 f Φ Tph v
• Hence RMS value of emf induced
E = 1.11 x Eph, avg
= 1.11 x 4 Φ f Tph volts
= 4.44 f Φ Tph volts
Eph,avg= 4.44 f Φ Tph volts
96

97. Synchronous Motor Starting
• Get motor to maximum
speed (usually with no
load)
• Energize the rotor with a
DC voltage
97

98. no-load and loaded conditions
Angle δ is the power angle, load angle, or torque angle
98

99. Rotating Field Flux and Counter-emf
• Rotating field flux Φf due to magnetic field in the rotor. A
“speed” voltage, “counter-emf”, or “excitation” voltage Ef is
generated and acts in opposition to the applied voltage.
99

100. • Gross torque =9.55 Pm/Ns
• Pm=Gross motor output in watts
• Ns = Synchronous speed in R.P.M
100
Torque equationTorque equation

101. V curves
101