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TUTORIAL II Electric Drives 101 &
Variable-Frequency Drives
1. Basic Principles: Ned Mohan UofMN 2. Practical Aspects: Robin Priestley, Rockwell
Automation
50th MIPSYCON Minneapolis, MN
November 6, 2014 1
2
Part 1 Basic Principles – Ned Mohan Part 2 Practical Aspects - Robin Priestley
3
Part 1 Variable-Frequency Electric Drives are essential in wind-turbines and in pumps/ compressors in oil/gas industry spurred by fracking. This tutorial will present their operating principle requiring only basic EE concepts as prerequisites.
5
Efficiency Improvement by Motor Drives
Motors 51%HVAC 16%
IT
14%
Lighting 19%
Motors 51%HVAC 16%
IT
14%
Lighting 19%
Motors 51%HVAC 16%
IT
14%
Lighting 19%
MOTOR
Inlet
EssentiallyConstant Speed
Throttling Valve
Outlet
PumpConstant
frequency
AC
MOTOR
Inlet
EssentiallyConstant Speed
Throttling Valve
Outlet
PumpConstant
frequency
AC
Adjustable
Speed Drive
(ASD)Inlet
Outlet
PumpAdjustable frequency
electric converter
Constant
frequency
AC
Adjustable
speed
Adjustable
Speed Drive
(ASD)Inlet
Outlet
PumpAdjustable frequency
electric converter
Constant
frequency
AC
Adjustable
speed
Role of Electric Drives in Harnessing Wind Energy
6
7
Fig. 3-7 Wind-resource map of the United States [6].
Wind Resource in the U.S.
20% of the U.S. Power Production by 2030 can be by Wind.
8
0 69010 60
VHz
34.5kV
161kV
Power ElectronicsConverters
V
Time0
60Hz
Low-VoltageRide-Through
Generator
690V
WIND
9
GE 1.5 MW Turbine
Source:
1 / 2.25 /m s miles hr
/m s
Power from the Wind
Source: 10
31
2windP AV 31
2turbine p wind pP C P C AV
mechR
V
Wind turbine
Variable speedgenerator
VariableFrequency AC
PowerProcessing Unit
Constantfrequency AC
Utility
Variable wind
Interface for Wind Generator:
Converter
Controller
Source Load
Power ElectronicsInterface
Converter
Controller
Source Load
Power ElectronicsInterface
Wind Generator Utility Grid
© Ned Mohan, 2012 11
12
Wind Generation using an AC Generator Connected through Power Electronics
13
Wind Generation using a Doubly-Fed Induction Generator
AC
DC
DC
AC
Wound rotor
Induction Generator
Generator-side
Converter
Grid-side
Converter
Wind
Turbine
AC
DC
DC
AC
Wound rotor
Induction Generator
Generator-side
Converter
Grid-side
Converter
Wind
Turbine
14
Motor Drives
Power Processing Unit (PPU)fixed
form
measured
speed/ position
speed /
position
Motor
Electric Drive
Load
input comm and
(speed / position)
Power
Signal
adjustable
formElectric Source
(utility)
Sensors
Controller
Power Semiconductors
15
Power Electronics Basics:
16
Switch-Mode Conversion: Switching Power-Pole
as the Building Block
Figure 1-20 Switching power-pole as the building block in converters.
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
(b)
Av
0t
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
inV
+
-
(a)
+
-Av
Aq
inVAv
0
1Aq
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
(b)
Av
0t
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
inV
+
-
(a)
+
-Av
Aq
inVAv
0
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
(b)
Av
0t
(b)
Av
0t
inV
+
-
(a)
+
-Av
Aq
inV
+
-
(a)
+
-Av
Aq
inVAv
0
1Aq
17
Pulse-Width Modulation (PWM) of the Switching Power-Pole
up
A in A in
s
Tv V d V
T 0 1Ad
( / )A up sd T TFigure 1-21 PWM of the switching power-pole.
(a) (b)
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
(a) (b)
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Ad
(a) (b)
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
(a) (b)
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Av
inV+
-
+
-
Ai
1or 0Aq
A sd T
dAi
upT
sT
Aq
Av
0
0t
t
1
inV
Av
Ad
18
Switching Power-Pole in a Buck DC-DC Converter:
An Example
o A A inV v d V 0 o inV V Figure 1-22 Switching power-pole in a Buck converter.
inV
Aq
Av
oV
ini
Li
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
(a)
(b)
inV
Aq
Av
oV
ini
Li
inV
Aq
Av
oV
ini
Li
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
(a)
(b)
AvinV
Aq
Av
oV
ini
Li
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
(a)
(b)
inV
Aq
Av
oV
ini
Li
inV
Aq
Av
oV
ini
Li
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
A sd T
sT
Aq
Av
Li
ini
0
0
0
0t
inV
t
t
t
1
(a)
(b)
Av
Synthesis of Low-Frequency AC:
aNv
0 t
dV
aNv
aNv
aNv
0
0sT
aNv
0 t
dV
aNv
aNv
aNv
0
0sT
20
Inverter Inverter
N
dcV bc
dcV
0.5 dcV
0
a
aNv bNv cNv
21
Interface for Wind Generator Inverter
N
dcV bc
a
Gen
Electric Machines Basics:
0 69010 60
VHz
34.5kV
161kV
Power ElectronicsConverters
V
Time0
60Hz
Low-VoltageRide-Through
Generator
690V
22
AC Machines
Synchronous Machines
Induction Machines
axisa
axisb
axisc
2 / 3
2 / 3
2 / 3
bi
ai
ci
axisa
axisb
axisc
2 / 3
2 / 3
2 / 3
bi
ai
ci
23
24
Basic Principles
Electromagnetic Force:
[ ] [ ][ ][ ]
emT mANm
f B i
(a) (b)add
subtract
external fieldB
emf
resultant
emf
(c)
i
emf
B
(a) (b)add
subtract
external fieldB
emf
add
subtract
external fieldB
emf
resultant
emf
resultant
emf
(c)
i
emf
B
i
emf
B
25
Induced Voltage:
[ ][ ] [ ][ / ]TV m m s
e B l u
qf
u
(into paper)B
qf
u
(into paper)B
(b)(a)
qf
qf
u
(into paper)B
qf
qf
u
(into paper)B
26
Three-Phase Stator Windings:
27
Synchronous Generator:
http://www.ece.umn.edu/users/riaz/animations/alternator.html
28
( )2 2
syn pf
© Copyright N. Mohan 2010 29
af syne
em aT I
At the Condition for Max Torque/Amp:
Production of Magnetic Field
http://www.ece.umn.edu/users/riaz/animations/spacevectors.html
(a)
(b)(c)
(d)(e)
(f)(g)
to360o180o120o60 o240 o300
a b c a
ot 0
a
a
c b
cb
ot 60
o60
a
a
c b
cb
ot 120
o120
a
a
c b
cb
ot 120
o120
a
a
c b
cb
ot 180
o180
a
a
c b
cb
ot 180
o180
a
a
c b
cb
ot 240
o240
a
a
c b
cb
ot 240
o240
a
a
c b
cb
ot 300
o300
a
a
c b
cb
ot 300
o300
a
a
c b
cb
o0
mai mbi mci
30
31
af me
,a af a ARe e e
Equivalent Circuit
,a AR ae I
Current Control
Figure 12-1 Voltage-link system.
conv1 conv2
controller
utility Load
conv1 conv2
controller
utility Load
32
33
Squirrel-Cage Induction Machine
axisa
axisb
axisc
/2 3
/2 3
/2 3
bi
ai
ci
axisa
axisb
axisc
/2 3
/2 3
/2 3
bi
ai
ci
Wind turbine
Variable speedgenerator
VariableFrequency AC
PowerProcessing Unit
Constantfrequency AC
Utility
Variable wind
Short-circuited Rotor
Transformer Analogy
m 2i i
m
1N 2N1v
2i
Load
m 2i i
m
1N 2N1v
2i
Load
34
Electrically Open-circuited Rotor
aibi
ci
cv
bv
av
bv
cvn
av
aibi
ci
cv
bv
av
bv
cvn
av mcImbI
maI
cV
bV
aV
mcImbI
maI
cV
bV
aV
35
36
Production of Magnetic Field
http://www.ece.umn.edu/users/riaz/animations/spacevectors.html
(a)
(b)(c)
(d)(e)
(f)(g)
to360o180o120o60 o240 o300
a b c a
ot 0
a
a
c b
cb
ot 60
o60
a
a
c b
cb
ot 120
o120
a
a
c b
cb
ot 120
o120
a
a
c b
cb
ot 180
o180
a
a
c b
cb
ot 180
o180
a
a
c b
cb
ot 240
o240
a
a
c b
cb
ot 240
o240
a
a
c b
cb
ot 300
o300
a
a
c b
cb
ot 300
o300
a
a
c b
cb
o0
mai mbi mci
a av e2
2 ( )syn fp
Induced Voltages in Rotor Bars: Motoring Mode
m
msB
syn
syn
axisa
at t 0
sv
m
msB
syn
syn
axisa
at t 0
sv
37
m syn
( )slip syn m
bar slipe
Induced Currents in Rotor Bars: Motoring Mode
m
msB
syn
syn
axisa
at t 0
sv
( )bari
barR
back end-ring front
end-ring
bare
m
msB
syn
syn
axisa
at t 0
sv
m
msB
syn
syn
axisa
at t 0
sv
( )bari
barR
back end-ring front
end-ring
bare
( )bari
barR
back end-ring front
end-ring
bare
38
Max Torque/Amp
m syn
bar bar slipi e
Torque – Speed Characteristics
,syn rated
m
,m rated
,em ratedT emT0
,slip rated
,syn rated m
,m rated
,em ratedT
0
,slip rated
,syn rated
m
,m rated
,em ratedT emT0
,slip rated,syn rated
m
,m rated
,em ratedT emT0
,slip rated
,syn rated m
,m rated
,em ratedT
0
,slip rated,syn rated m
,m rated
,em ratedT
0
,slip rated
http://www.ece.umn.edu/users/riaz/animations/sqmoviemotgen.html
39
ConstantV
f
em slipT
Slip frequency (fslip) in the rotor circuit
- slip syn m
slipf s f
40
slip
syn
s
How does an induction motor work?
• Load Torque goes up
• Speed slows down
• Slip speed goes up
• Rotor-bar induced voltages go up
• Rotor-bar currents go up
• Electromagnetic torque goes up
41
42
Generator Mode: Induced Voltages in Rotor Bars
m
msB
syn
syn
axisa
at t 0
sv
m
msB
syn
syn
axisa
at t 0
sv
m syn
Induced Currents in Rotor Bars
43
m syn
Torque – Speed Characteristics
,syn rated
m
,m rated
,em ratedT emT0
,slip rated
,syn rated m
,m rated
,em ratedT
0
,slip rated
,syn rated
m
,m rated
,em ratedT emT0
,slip rated,syn rated
m
,m rated
,em ratedT emT0
,slip rated
,syn rated m
,m rated
,em ratedT
0
,slip rated,syn rated m
,m rated
,em ratedT
0
,slip rated
http://www.ece.umn.edu/users/riaz/animations/sqmoviemotgen.html
44
MMF Due to Rotor Bar Currents
m
msB
syn
syn
axisa
at t 0
sv
m
msB
syn
syn
axisa
at t 0
sv
45
m syn
46
Controlling Torque
0 m
emT
47
Doubly-Fed Induction Generator
AC
DC
DC
AC
Wound rotor
Induction Generator
Generator-side
Converter
Grid-side
Converter
Wind
Turbine
AC
DC
DC
AC
Wound rotor
Induction Generator
Generator-side
Converter
Grid-side
Converter
Wind
Turbine
Doubly-Fed Induction Generator
48
49
r slip rR j L
rv
ri
re
RotorPEConverter
,s sP Q
emP
,r rP Q
slip , s
(Speed)
sP
(mode)
rP
rQ '
rQ
+
(sub-syn) sP
(generating)
+ + -
+
(sub-syn) sP
(generating)
+ - +
-
(super-syn) sP
(generating)
- - -
-
(super-syn) sP
(generating)
- + +
50
Effect of Reactive Power on Bus Voltage
bus Th ThV V jX I
ThV
ThjX
busV
I
(a)
busV
I
ThjX IThV
busV
I
ThjX I
ThV
(b)
ThjX I
Summary of Generator Types: • Synchronous Generators (torque is controlled by
stator current) – Permanent Magnet
– DC Excitation
• Induction Generators – Squirrel-cage (torque is controlled by stator
frequency)
– Wound Rotor DFIG (torque is controlled by injected rotor currents)
51
