Alternating Current Machines-Synchronous Machines

1. 1

2. 2
Classroom Rules:
2.I talk, you listen
3.I don’t allow:
A.Heads on desk
B.Feet on desk
C.Going to sleep in class
D.MP3 players
4.Mobile phones are to be off, or on “silent” unless
OK’d with me, and in you bag (not on desk)
5.Should they be OK’d, and you receive a call, you
will answer it outside the classroom
6.No SMS-ing in class
7.Keith Butler’s number is: 0417 637 909

3. 3
Topics (from Learning Outcomes)
10. Three-phase synchronous machines
• operating principles
• construction feature
• application
11. Three-phase synchronous machines
• effects of load changes
• effects of excitation change
• load/current characteristics
12. Single phase synchronous machines
• alternators
• motors
• applications

4. 4
Assessment
(a)All prac’s must be completed
•A final written exam will be given on the
learning outcomes covered. Should a
student fail, they will be allowed one
further attempt during block. Should they
fail this they will be allowed one further
attempt within six weeks of completing
the block. Should they fail this they will
be in a “Show Cause” situation.

5. 5

6. 6 500 KW Genset / Generator Set, powered with a Cummins VTA28G1 Engine

7. 7

8. 8

9. 9
2 x 14MW Synchronous Motors
…apparently, they use permanent magnets!

10. 10

11. 11
3 phase Dunlite machine

12. 12

13. 13
Synchronous machines are not
just the big units, but they can
be small also.

14. 14

15. 15
V
AC Supply
R
V
I
I
Current is in phase
with voltage.
Time->

16. 16
L
AC supply
V
I V
I
Current lags the
Voltage by 90
o

17. 17
CAC Supply
But if an ammeter were placed in series it would
most definitely read a current.
Current appears to pass through the capacitor.
In reality, it is charging in one direction, and then
discharging and recharging in the other direction.

18. 18
V
I
CAC Supply
V
I
Current leads the
Voltage by 90
o

19. 19
This opposition to current flow is called:
Inductive reactance, in inductors. (X
L
)
Capacitive reactance in capacitors. (X
C
)
Both Inductors and Capacitors oppose, or “resist”
current flow when connected to AC supplies.
While it opposes current flow, it is NOT called resistance.
Current flow through resistance produces HEAT.
Current flow in inductors and capacitors doesn’t!

20. 20

21. 21
Generator / Transformer / Motor
S F
S
F
S
F
S F
S F
S F
B
A
C
AC
B

22. 22
AC
B
Motor
Why isn’t a neutral run to a balanced
three phase Star connected load?
A
B
C
N
?????

23. 23
AC
B
Motor
Because the Star point is at 0V
A
B
C
0V0VN
And the neutral is at zero volts also.
So if they were joined no current would flow.
0A
So why join it?

24. 24
AC
B
Motor
A
B
C
The neutral is not connected to a balanced
three phase star connected load.
Only connected to unbalanced loads!!!

25. 25
Transformer
S F
B
S F
C
Generator / Transformer / Motor
A
BC
F
S F
S
FS
S F
A

26. 26
Transformer
SF
B
SF
C
Generator / Transformer / Motor
A
BC
S
F S
F
SF
SF
A
Swapped

27. 27
3-phase Transformer Secondary
S F
B C
S F
A
S F
A B C
Note that all windings are connected in series, with the two
ends joined together.

28. 28
If we did that with three batteries, there would be major
problems!

29. 29
The voltmeter should read the sum of the three voltages?
Right?
V
A
V
B
V
C
A
CB
F
S F
S
FS
Transformer
V
The voltmeter reads the phasor sum of the voltages.

30. 30
The voltmeter reads, in effect, the distance between the
beginning of V
A
and the end of V
C
. ie. 0V
V
A
V
B
V
C
A
CB
F
S F
S
FS
Transformer
V
We can connect the two ends together because the
phasor sum adds up to zero!

31. 31
Transformer
V
A
V
B
V
C
A
CB
F
S F
S
FS
The voltmeter reads, in effect, the distance between the
beginning of V
A
and the end of V
C
. ie. 0V
We can connect the two ends together because the
phasor sum adds up to zero!
No Arc!

32. 32

33. 33
Generator Load
STAR
V
L
= 3 V
PH
I
L
= I
PH
DELTA
I
L
= 3 I
PH
V
L
= V
PH
P = 3 x V
PH
x I
PH
x Cos
= 3 x V
L
/3 x I
L
x Cos
= 3/3 x V
L
x I
L
x Cos
= 3 V
L
x I
L
x Cos
P = 3 x V
PH
x I
PH
x Cos
= 3 x V
L
x I
L
/3 x Cos
= 3/3 x V
L
x I
L
x Cos
= 3 V
L
x I
L
x Cos

34. 34
P = 3 V
L
x I
L
x Cos
NOT:
P = 415 x I x pf.

35. 35
Three Single Phase Power Equations:
True Power = Watts = V x I x Cos
Apparent Power = VA = V x I
Reactive Power = VAR’s = V x I x Sin
Power Factor = Cos
where Cos = Cosine of the angle
between Voltage
and Current

36. 36
VA
Watts
Var’s

Phase angle
between current
and volts
This can be put as a triangle:
VA
2
= Watts
2
+ Var’s
2

37. 37
V=240V
Alternators, where the windings are limited by the
current through them, are rated in VA.
To rate them in watts, (ie. watts delivered to the load)
would give no idea of the current through them.
Load 3 = 14.14A at 45º
Load 1 = 10A
Load 2 = 20A at 60º
P = V x I x Cos 45º
= 240 x 14.14 x 0.707
= 2.4kW
P = V x I x Cos 0º
= 240 x 10 x 1
= 2.4kW
P = V x I x Cos 60º
= 240 x 20 x 0.5
= 2.4kW

38. 38
Q What dictates the phase angle of the current
supplied by a single alternator supplying a single load?
V=240V
Load 3 = 14.14A at 45º
Load 1 = 10A
Load 2 = 20A at 60º
P = V x I x Cos 45º
= 240 x 14.14 x 0.707
= 2.4kW
P = V x I x Cos 0º
= 240 x 10 x 1
= 2.4kW
P = V x I x Cos 60º
= 240 x 20 x 0.5
= 2.4kW
A The load

39. 39
V
Al currents here take the same power
Constant power line

40. 40
V
Al currents here take the same power
Higher power

41. 41
V
Al currents here take the same power
Constant power line

42. 42
V
Al currents here take the same power
Lower power

43. 43

44. 44

45. 45
Alternator
Mechanical Energy
Electrical Energy
L
os
se
s
Alternator
Prime
Mover
- Diesel Engine
- Steam Turbine
- Small petrol engine
Alternator:
Pout
Eff% = x 100
Pin
Alternator:
Pin = Pout + Losses

46. 46
Loss
es
Synchronous Motor
Mechanical Energy
MSB
Electrical Energy
Motor:
Pout
Eff% = x 100
Pin
Motor
Pin = Pout + Losses
Motor Load

47. 47
Synchronous Machine
Stator
- Identically wound
to an induction motor.
- Connected to supply.
Rotor
- Constant DC field
- Connected to supply
via sliprings.
Electrical
Power
DC
Supply

48. 48
•The stator produces a rotating magnetic field
exactly the same as an induction motor.
•The rotor is a magnet and locks in to the RMF
•Rotor travels at SYNCHRONOUS SPEED.
SYNCHRONOUS
MOTOR

49. 49
Synchronous Machine
If a synchronous motor is OVER driven by the load
(eg electric train going down a hill), then it will
generate power, still at synchronous speed.
If an alternator coupled to the grid is UNDER driven
by the prime mover (eg steam stops), then it will
motor, and drive the turbine at synchronous speed.
Electrical
Power
DC
Supply

50. 50
Synchronous Machine
In other words, the two machines are identical in
construction.
Electrical
Power
DC
Supply

51. 51

52. 52
3000RPM1500RPM 1000RPM750RPM
185kw 310A, .88pf 315A, .86pf 343A, .80pf348A, .78pf
220kw 362A, .89pf 375A, .86pf 408A, .78pf412A, .78pf
150kw 242A, .90pf 265A, .87pf 279A, .80pf278A, .77pf
Lower the RPM,
• Larger value I
S
• More lagging I
S
Characteristic of WEG
®
Induction Motors.
110kw 182A, .90pf 200A, .84pf 205A, .80pf203A, .81pf
22kw 39A, .87pf 41A, .83pf 42A, .80pf 47A, .74pf
4kw 7.8A, .87pf 8.2A, .82pf 9A, .74pf 11A, .63pf
What is the
tendency as RPM
gets lower?

53. 53
So why use a Synchronous Motor?
Uses:
–Low Speed Drives. Low speed induction motors
draw very large currents at poor power factors.
This cannot be altered or corrected. In
synchronous motors, the p.f. can be altered to
cause the motor to draw minimum current. (The
alternative is to use a high speed induction
motor through a gearbox.)
–Power Factor Correction
–Constant Speed drives

54. 54

55. 55

56. 56

57. 57
Salient Pole Rotor Cylindrical Rotor
2 basic types:
Cylindrical rotor
Salient Pole
-Low speed
-Diesel Prime Mover
-Hydro systems
-High speed
-Steam Turbine

58. 58

59. 59

60. 60

61. 61

62. 62

63. 63

64. 64
www.tecowestinghouse.com
Small salient pole synchronous machine rotor

65. 65

66. 66

67. 67

68. 68

69. 69

70. 70

71. 71

72. 72

73. 73

74. 74

75. 75
Loss
es
Synchronous Motor
Mechanical Energy
MSB
Electrical Energy
Motor:
Pout
Eff% = x 100
Pin
Motor
Pin = Pout + Losses
Motor Load

76. 76
Same as an induction motor.

77. 77
A1
A2
2-Pole Machine
ie. 3000RPM

78. 78

79. 79
A1
A2
B1
B2
C1
C2
2-Pole Machine
ie. 3000RPM
In reality, the coils
span more slots in
a 2-pole motor.
N
S
Notice that for a
two pole stator we
have a 2-pole rotor

80. 80
N N
S
S
A
A
AA
B
B
B
B
C
C
C
C
4-pole machine
A four pole stator
must have a four
pole rotor

81. 81
Flux
+
-
Time->
1
Resultant flux =
1.5 x flux of one phase
N
S
N
S
S
N

82. 82
Flux
+
-
Time->
2
Resultant flux =
1.5 x flux of one phase
N
S
N
S
N
S

83. 83
Flux
+
-
Time->
3456

84. 84
Flux
+
-
Time->
345612
So the flux rotates one full rev in one cycle,
for our two pole machine.

85. 85
Flux
+
-
Time->
345612
Because the flux is a constant value, it gives:
1. Very quiet operation
2. Constant torque as the rotor rotates.

86. 86
Flux
+
-
Time->
345612
This rotating magnetic field rotates at:
3000RPM for a 2-pole motor
1500RPM for a 4-pole motor

87. 87
Flux
+
-
Time->
345612
To reverse the direction of rotation:
reverse any two phases to the motor.

88. 88
where N = RPM
f = frequency
P = Number of poles (per phase).
N = 120f/P
So the speed of the rotating magnetic field is
affected by:
Frequency, and
Number of poles.

89. 89
As the rotating magnetic field rotates, the
rotor is locked in synchronism with it and is
dragged along for the ride.

90. 90
N
S
As the rotating magnetic field rotates, the
rotor is locked in synchronism with it and is
dragged along for the ride.

91. 91
What will happen as a load is put on the shaft?
N
S

92. 92
N
S
What will happen as a load is put on the shaft?

93. 93
The load tries to slow it down.
But it must do synchronous speed!
So it stretches the lines of flux.
N
S

94. 94
N
S
C/L of RMF
C/L of Rotor Field
Torque Angle 

95. 95
If the lines stretch to breaking point (ie too
much load), then the rotor stalls
This is referred to as “Pull Out Torque”.
N
S

96. 96
What would the Torque Curve look like?
RPM
Ns0
Torque Curve for an
induction motor
Torque

97. 97
What would the Torque Curve look like?
Torque
RPM
Ns0
Torque “Curve” for a
Synchronous Motor
Pull out
Torque
Zero Torque below
synchronous speed

98. 98
1. Amortisseur winding
Starting a Synchronous Motor?

99. 99

100. 100
Rotor Construction
Squirrel Cage

101. 101

102. 102

103. 103
Starting a Synchronous Motor?
1.Amortisseur winding
This gets the motor up to speed as an
induction motor. When it is close to
synchronous speed it will lock in.
2.Shorting the rotor DC winding and
starting it as a wound rotor motor.
When it is close to synchronous speed,
the short is removed and DC is applied
to the rotor. It will (hopefully) lock
in.

104. 104
3.Using a pony motor to get the
synchronous motor up to speed, then
applying AC to the stator and DC to
the rotor.
(Not applicable if there is a high
starting torque load connected)
Note that these starting methods will only
work if the load on the motor at start can be
reduced or eliminated.
Starting a Synchronous Motor?

105. 105
• Amortisseur windings also reduce hunting.
• Hunting is rhythmic fluctuations of the
RPM around an average value.
• If not subdued, hunting can cause
the rotor to swing out of synchronism.

106. 106
Revs
Time

107. 107
N
S
N
S
N
S
N
S
N
S
And all this while it is whizzing
around at synchronous speed!

108. 108
And all this while it is whizzing
around at synchronous speed!
N
S

109. 109

110. 110
V
supply
V
induced
Induced in the
stator from
the rotor
Phasor Diagram of Synchronous Motor

111. 111
V
supply
V
induced
Torque angle

I
supply
V
R
Phasor Diagram of Synchronous Motor

112. 112
Phasor diagram for increased load:
(Excitation current held constant)
V
supply
V
induced

I
supply
V
R
Increased load = Increased Torque Angle
V
induced
Increasing the load increases the power taken from supply

113. 113
V
supply
V
induced
V
induced
Phasor diagram for increased excitation:
(Constant Load)

Constant load = Constant Power line

114. 114
Phasor diagram for increased excitation:
(Constant Load)
V
supply
V
induced

Constant load = Constant Power line

115. 115
V
supply
Phasor diagram for increased excitation:
(Constant Load)
V
induced V
R
I
supply
So to force the supply current leading,
we INCREASE excitation

Constant load = Constant Power line

116. 116
V
supply
V
induced
I
supply
V
R
Phasor diagram for decreased excitation:
(Constant Load)

Constant load = Constant Power line

117. 117
V
supply

I
supply
V
R
Phasor diagram for decreased excitation:
(Constant Load)
V
induced
So to force the supply current lagging,
we DECREASE excitation
Constant load = Constant Power line

118. 118
V
supply
V
induced
I
supply
V
R

V
supply

I
supply
V
R
V
induced
V
induced
I
supply
V
R

Constant load = Constant Power line
V
supply

119. 119
V
supply
With a constant load, changing excitation changes
the phase angle and value of supply current.
I
supply
By increasing the DC excitation current to the rotor,
the synchronous motor can act as a capacitor
It can be used for power factor correction.
Constant load = Constant Power line

120. 120
Excitation Current
Sta
tor
Curre
nt
5
0% loa
d
Unity pf
Lag Lead

121. 121
Deductions From Vee Curves
•At any particular load there is a certain value of rotor
current which gives a minimum value of stator current and
unity pf.
•If the rotor current is altered either way, the stator current
will increase, and pf will decrease away from 1.
•For any given load there is a certain value of rotor current
below which the rotor will fall out of synchronism.
•For any given load there are two values of rotor current that
will give identical values of stator current. The lower value
gives a lagging pf, and the higher value gives a leading pf.

122. 122
Excitation Current
Sta
tor
Curre
nt
5
0% loa
d
Lag
Lead
75
% loa
dStability
limit
pf=1

123. 123
0.8 pf lagUnity0.8 pf leadPer unit
Power output

124. 124

125. 125

126. 126
Points:
•At a set load there is a value of excitation that
will give minimum line current.
•Reducing OR increasing excitation from this value
will only increase line current.
•At any other value of line current, there are two
values of excitation current that can produce this.
•If a synchronous motor is heavily loaded, supply
current may not be able to be driven highly
leading.
•If a synchronous motor is lightly loaded, supply
current can be driven highly leading.

127. 127
Single Phase Synchronous Motors
•Used when constant speed is critical, with low
torque requirements. They have low efficiency,
hence made in small sizes.
•Application:
•clocks
•record players
•timers
•recorders
•communications
•servo installations
•Two main types:
•Reluctance motor
•Hysteresis motor

128. 128
Reluctance Motor
Stator
•Stator same as a single phase, split
phase motor.
•Centrifugal Switch operates at
75% synchronous speed to open
circuit the start winding.
Rotor
•Assembled from laminated sheets
with defined teeth cut away. This
forms salient poles.
•Windings are of the squirrel-cage
type.
•Number of rotor poles equals the
number of stator poles.
Two pole, 3000 RPM
rotor

129. 129
Reluctance Motor
•Operation
–Starts as an induction motor, with slip.
–A single phase stator has a “Start” and “Run” winding. At
75% centrifugal the centrifugal switch operates.
–As the load is light there is small slip
–The salient poles become permanently magnetised by the
stator field
–The salient poles will then lock to the stator field.
–Once locked into synchronism the motor will continue to
operate at synchronous speed.
–Not as much power output as a similar physical size 1-phase
motor.

130. 130
Hysteresis Motor
Rotor
•Constructed from hardened steel rings, instead of thin,
magnetically soft, silicon steel laminations.
•“Hysteresis” opposes any change once the flux is created, so
the rotor will lock into the RMF like a permanent magnet.
Stator
•Often a shaded pole stator principle is used.
•If the shaded pole principle is used then the motor is self
starting.
•Magnetic poles are established in the rotor.
•These poles lock to the stator poles.
•The rotor runs at synchronous speed determined by the
poles and frequency.

131. 131

132. 132
Why generate AC?… Why not DC?
DC cant be “transformed” through a transformer.
AC can go through a transformer.
Large brushless DC generators are not possible
Large brushless AC alternators are!
Why do we want to transform it?
It is easier to transmit to distant places
at higher voltages as the current
will be lower. (P=V x I)
Induction motors are simpler and cheaper than
DC motors

133. 133

134. 134
N
S
V
Generating an AC Voltage

135. 135
N S
Generating an AC Voltage
Volts
+
-
Time->

136. 136
N
S
Generating an AC Voltage
Volts
+
-
Time->

137. 137
NS
Generating an AC Voltage
Volts
+
-
Time->

138. 138
N
S
Generating an AC Voltage
Volts
+
-
Time->

139. 139
N S
Generating an AC Voltage
Volts
+
-
Time->

140. 140
N
S
Generating an AC Voltage
Volts
+
-
Time->

141. 141
N
S
V
Generating a AC Voltage
3-Phase

142. 142
N S
Volts
+
-
Time->
Generating a AC Voltage
3-Phase
Require:
Three sets of coils
physically displaced
from each other by
120º electrical.

143. 143
V
A
V
C
V
B
Generating a AC Voltage
3-Phase
N S

144. 144
Generating a AC Voltage
3-Phase
A1
A2
N S

145. 145
Generating a AC Voltage
3-Phase
N S
A1
A2
B1
B2
C1
C2

146. 146
N N
S
S
A
A
AA
B
B
B
B
C
C
C
C
4-pole machine
A four pole stator
must have a four
pole rotor
Generating a AC Voltage
3-Phase

147. 147
Alternator
•Reasons for having the three phase winding on the
stator rather than the rotor:
–More space on the stator for the three phase
winding.
–Only one, low voltage winding on the rotor.
•Easier to insulate.
•Less problems with centrifugal force.
–Only two sliprings required rather than four
(3-ph + N)

148. 148
Alternator
Stator
- Connected to load.
Rotor
- Constant DC field
- Connected to its own DC
supply via sliprings.
Electrical
Power
Mechanical
Power
M
a
gn
e
ti
c Fi
e
ld

149. 149
Alternator
Q: What keeps an alternator producing 50Hz under all
load conditions?
A: The governor on the prime mover. It detects any
drop in speed, and tries to speed the unit up.
Alternator
Petrol
Engine

150. 150

151. 151
I
FIELD
V
OUT
Alternator Excitation Curve
(No Load)

152. 152
Alt
Load
R
X
L
Internal
Impedance
Alternator

153. 153
V
OUTI
LOAD
V
R
V
Z
V
L
V
GEN
V
Z
= Internal Impedance of the alternator
V
R
= Internal Resistance of the alternator
V
L
= Internal Reactance of the alternator
Resistive Load

154. 154
Resistive Load
V
OUTI
LOAD
V
R
V
Z
V
L
V
GEN
 Notice that terminal volts DROP as load increases
 Load current and p.f. are dictated by the LOAD!

155. 155
Inductive Load
V
OUT
I
LOAD
V
R
V
Z
V
L
V
GEN
Parallel

156. 156
Inductive Load
V
OUT
I
LOAD
V
R
V
Z
V
L
V
GEN
 Now there is a greater voltage drop under load

157. 157
Capacitive Load
V
OUT
I
LOAD
V
R
V
Z
V
L
V
GEN
 Now there is a voltage RISE under load
Parallel
 Because of the voltage rise under load, it is
not desirable to run alternators at a
leading power factor.

158. 158
Leading pf
Unity pf
Lagging pf
Load Current
Output
Voltage
Effect of Power Factor on
Output Voltage

159. 159
Voltage Regulation
(V
NL
– V
FL
)
%Voltage Regulation = x 100
V
FL
eg An alternator output falls from 240V to 200V
with constant excitation.
Calculate the % voltage regulation.
(Ans: 20%)

160. 160
Summary:
When an alternator is standing by itself with a single load:
Output voltage is affected by excitation current
Output frequency is affected by input power to the
alternator.
Alternators - stand alone

161. 161
When an alternator is tied to the grid, you cannot change:
Grid voltage
Grid frequency
So the output voltage of the alternator will not change, and
the output frequency of the alternator will not change.
Notice that, for a stand alone alternator with stand alone
load, these are the two things that changed when:
(a) the excitation was altered, and
(b) the power input to the alternator was increased
(ie. Put the foot down on the prime mover)
Alternators tied to the Grid

162. 162
Alternators tied to the Grid
V
OUT
V
R
V
ZV
L
V
GEN
If excitation is increased, and V
OUT
cannot alter, V
GEN
will increase and push the triangle over.
I
LOAD
1. Altering Excitation.

163. 163
V
OUT
I
LOAD
V
R
V
Z
V
L
V
GEN
If excitation is increased, and V
OUT
cannot alter, V
GEN
will increase and push the triangle over.
Alternators tied to the Grid
1. Altering Excitation.
Note that input power to the alternator is not changing,
so output power does not change either.
Constant Power Line
(Output power of the
alternator has not
Changed)

164. 164
V
OUT
I
LOAD
V
R
V
Z
V
L
If excitation is reduced, and V
OUT
cannot alter, V
GEN
will reduce and pull the triangle back.
Alternators tied to the Grid
1. Altering Excitation.
V
GEN
This drives the load current lagging

165. 165
V
OUT
I
LOAD
V
R
V
Z
V
L
V
GEN
If excitation is reduced, and V
OUT
cannot alter, V
GEN
will reduce and pull the triangle back.
Alternators tied to the Grid
1. Altering Excitation.
This will drive the load current leading

166. 166
If input power is reduced, and frequency and V
OUT

cannot alter, output power will reduce.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD

167. 167
If input power is reduced, and frequency and V
OUT

cannot alter, output power will reduce.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD

168. 168
If input power is reduced, and frequency and V
OUT

cannot alter, output power will reduce.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD
Size of triangle
reduces

169. 169
If input power is reduced, and frequency and V
OUT

cannot alter, output power will reduce.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
Z
V
L
V
GEN
I
LOAD
Size of triangle
reduces

170. 170
If input power is increased, and frequency and V
OUT

cannot alter, output power will increase.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD

171. 171
If input power is increased, and frequency and V
OUT

cannot alter, output power will increase.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD

172. 172
If input power is increased, and frequency and V
OUT

cannot alter, output power will increase.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
V
R
V
ZV
L
V
GEN
I
LOAD

173. 173
If input power is increased, and frequency and V
OUT

cannot alter, output power will increase.
Alternators tied to the Grid
2. Altering input power to the alternator.
V
OUT
VR
VZ
VL
V
GEN
I
LOAD

174. 174
Alternators - tied to the Grid
Summary:
Changing excitation changes the pf of output current.
Changing input power changes output power
• Increasing excitation drives load current lagging
• Reducing excitation drives load current leading
• Increasing input power increases output power
• Reducing input power reduces output power
•Output frequency and voltage do not change.

175. 175
Alternators – stand alone
Summary:
•Changing excitation changes output voltage.
•Changing input power changes RPM, which changes
output frequency.
• Here, output frequency and voltage do change

176. 176
Paralleling Alternators
To parallel alternators (or parallel one onto the
grid), the following criteria must be met:
• Output voltage must be the same
• Output frequency must be the same
• Phase rotation must be the same
• Supply voltage must be in phase
It is understood that they must both produce the
same waveform – a sine wave!

177. 177
Alternator Rating
Alternators are rated according to:
Frequency
Voltage
Current
kVA
The frequency dictates the RPM (3000, 1500, etc).
Voltage and Current give the kVA rating.

178. 178
Efficiency
Losses:
•By far the main loss in an alternator is HEAT loss.
•If an alternator can be kept cool, more power can
be obtained from it. ie. Instead of a 300MW
machine, it will become a 500MW machine.
•More power must be put into it to get this
increased output power.
•Cooling large alternators is a big deal! They are
often cooled using hydrogen.

179. 179
Efficiency
Losses:
• Copper Losses:
I
2
R losses in the stator winding
I
2
R losses in the rotor winding
• Iron Losses:
Hysteresis loss in stator
Eddy current Loss in stator
• Friction and windage

180. 180
Single Phase Alternators
Electrical
Power
Mechanical
Power
M
a
gn
e
ti
c Fi
e
ld
Regulator
S
ta
tor

181. 181

182. 182
Single Phase Alternators
•These are usually low rated units for portable use.
•Prime mover is usually a small petrol or diesel engine.
•The engine speed is kept constant by a governor.
•This speed will usually be either 3000RPM or 1500RPM
•Output voltage is kept constant using an automatic voltage
regulator. This senses the output voltage and adjusts the
rotor excitation current automatically.
•They are usually self exciting, so if the load is left on at
start, they may not build up output voltage.
•Many small alternators are brushless.
•Usually, neither side is earthed. This is called a FLOATING
system.

183. 183

184. 184
Rotor
Brushless Alternators
AC is
sampled
Regulator DC
Field P.S.
Note: Self Excited
3-phase
out

185. 185
Brushless Alternators
Rotor
Regulator
3-phase
out
Prime
Mover
3-phase
out

186. 186
Small Alternators
-Factors when choosing:
•Voltage: 240V / 415V (1-phase or 3-phase)
•kVA rating
•RPM (3000RPM or 1500RPM)
•Petrol or Diesel
•Brushless or brushes
•Ability to start loads such as motors
•Extras: Soundproofing, starting, power outlets,
mounting holes, 12VDC / welding output

187. 187

188. 188

189. 189

190. 190

191. 191

192. 192

193. 193

194. 194

195. 195

196. 196

197. 197

198. 198

199. 199