1
Introduction to Variable Speed Drives
Pekik Argo DahonoElectrical Energy Conversion Research Laboratory
Institute of Technology Bandung
Why Electric Drives
• Electric drives are available in any power.
• They cover a wide range of torque and speed.
• Adaptable to almost any operating conditions.
• Electric drives are operable instantaneously.
• High efficiency.
• Easily controllable.
• Can be operated in all four quadrants.
• Can be built in a variety of designs.
2Variable Speed Drives
2
Limitations of Electric Drives
• The dependence on a continuous power
supply causes problems with vehicle
propulsion.
• Due to the magnetic saturation, electric
motors are likely to have a lower power-to-
weight ratio than high pressure hydraulic
drives.
3Variable Speed Drives
Why we need variable speed drives
• Some industrial process need variable speed
drives (robots, machine tools, conveyors,
etc.)
• Some industrial machines will be more
efficient if driven by a variable speed drive
(pumps, blowers, fans, etc.)
• Vehicle propulsion system.
4Variable Speed Drives
3
Energy flow and typical losses
5Variable Speed Drives
Energy Team
6Variable Speed Drives
4
Energy Team
7Variable Speed Drives
Pumping System
• Variable –flow variable-pressure
• Constant-flow variable-pressure
• Variable-flow constant-pressure
8Variable Speed Drives
5
Pump Characteristics
( )3
221121
3
Speed Power Pump
(kWh)Energy Pump
(kW)Power Pump
)()/( (kW)Power Pump
∝
×−×=−
×××=
××=
××=
tHQHQkEE
tHQk
HQk
mheadhmFlowk
RPM2
RPM1
FLOW2
FLOW1∝
2
2
RPM2
RPM1
PRESSURE2
PRESSURE1∝
9Variable Speed Drives
Pumping System with Valve Control
PUMP
MOTOR
STARTER
MOTORIZEDVALVE
SET-POINTCONTROLLER
SENSOR
FLOW
REFERENCE
10Variable Speed Drives
6
Pumping System with VSD
PUMP
MOTORSENSOR
FLOW
REFERENCE
CONTROLLER
SET-POINTCONTROLLER
11Variable Speed Drives
Variable-flow variable-pressure
Pumping System
200 400 600 800 1000 1200 1400 1600 1800
0
0
40
80
120
160
200
240
Tota
l H
ea
d (
FT
)
Flow (GPM)
40 50 6070 75
8078
30
100% SPEED
84% Speed
operating
Point
Valve Losses
Throttled Operating
Point
Throttled
System
Curve
Unthrottled
System Curve
Pump Output =
Power
Head (Ft.) x Flow (GPM) x Specific Gravity
3960
12Variable Speed Drives
7
Energy input for variable-flow
variable-pressure pump
20
10
30
40
50
60
70
80
0
500 700 900 1100 1300 1500 1700
FLOW (GPM)
INP
UT
KW
ADJUSTABLE SPEED
OPERATION WITH
ENERGY EFFICIENT MOTOR
VALVE OPERATION
WITH STANDARD
EFFICIENTMOTOR
13Variable Speed Drives
Constant-flow variable-pressure
200 400 600 800 1000 1200 1400 1600 1800
60
0
40
80
120
160
200
240
TO
TA
L H
EA
D
(FT
.)
FLOW (GPM)
40 50 60 70 75
8078
30
20
100
140
180
220
CONSTANT FLOW
100% SPEED
PUMP POWER OUTPUT =
HEAD x FLOW x SPECIFIC GRAVITY
3960
VALVE LOSSES
REDUCED SPEED
OPERATING POINT
CONSTANT SPEED
THROTTLED OPERATING POINT
14Variable Speed Drives
8
Constant-flow variable-pressure
20
10
30
40
50
60
70
0
100 120 140 160 180 200 220 HEAD (FT.)
INP
UT
KW
ADJUSTABLE SPEED
OPERATION WITH
ENERGY EFFICIENT MOTOR
VALVE OPERATION WITH
STANDARD EFFICIENT MOTOR
15Variable Speed Drives
Variable-flow constant-pressure
PUMPMOTOR
STARTER
MOTORIZED
VALVE
SET-POINT
CONTROLLER
SENSOR
MAIN
VALVES
PRESSURE
REFERENCE
CONSTANT
16Variable Speed Drives
9
Variable-flow constant-pressure
PUMP
MOTORSENSOR
CONSTANT
PRESSURE
REFERENCE
CONTROLLER
SET-POINT
CONTROLLER
MAIN
VALVES
17Variable Speed Drives
Variable-flow constant-pressure
200 400 600 800 1000 1200 1400 1600 1800
60
0
40
80
120
160
200
240
TO
TA
L H
EA
D
(FT
.)
FLOW (GPM)
40 50 6070 75
8078
30
20
100
140
180
220
CONSTANT SPEEDOPERATING POINT
CONSTANT PRESSURECONTROL LINE
TOTAL FLOW
SYSTEM CURVE
BYPASS PARTIALLY
OPEN
MAIN SYSTEM CURVE #2
MAINSYSTEM CURVE #1
BYPASS CLOSEDSYSTEM
VALVES OPEN
MAINFLOWMAINFLOW
BYPASSFLOWBYPASSFLOW
CLOSING SYSTEM VALVES
OPENING BYPASS VALVE
SYSTEM CURVE #1
SYSTEM CURVE #2
18Variable Speed Drives
10
Variable-flow constant-pressure
200 400 600 800 1000 1200 1400 1600 1800
60
0
40
80
120
160
200
240
TO
TA
L H
EA
D
(FT
.)
FLOW (GPM)
REDUCED SPEED
OPERATING POINT
20
100
140
180
220 CONSTANT SPEEDOPERATING POINT
100% SPEED
PUMP OUTPUT POWER =HEAD x FLOW x SPECIFIC GRAVITY
3960
40 50 6070 75
8078
30
90% SPEED
BYPASS
FLOW
BYPASS
LOSSES
MAIN
FLOW
TOTAL
FLOW
19Variable Speed Drives
Variable-flow constant-pressure
20
10
30
40
50
60
70
0
FLOW (GPM)
INP
UT
KW
ADJUSTABLE SPEED
OPERATION WITHENERGY EFFICIENT MOTOR
BYPASS OPERATION WITHSTANDARD EFFICIENT MOTOR
500 700 900 1100 1300 1500 1700
80
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Comparison of two pumping systems
21Variable Speed Drives
Two-Stages Pumping System
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Benefits
• High availability
• Fast and precise control
• Minimized energy consumption
• Reduced emission
• Minimized actuator equipment
• Soft starting
Variable Speed Drives 23
Power Consumption
Variable Speed Drives 24
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Investment and Energy Costs
Variable Speed Drives 25Energy is calculating for 1300 kW motor and 3 years operation
Fans
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Rotary screw air compressor
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Lifts
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Lift Energy Comparison
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Mechanical Variable Speed Drives
Conveyor Belt LoadCentrifugal-Acting
In-Line Fluid CouplingFull-SpeedAC Motor
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Mechanical Variable Speed DrivesV-Belt
Hand crank opens and closes sheave tomove belt up or down.
Beltridesup or
down basedon groove
setting
Available up to 100 kW
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Mechanical Variable Speed Drives
MotorizedSpeed Changer
PIV Transmission BoxFull-Speed Input
Reduced-SpeedOutput
PrimeMover
Operator-ControllerPotentiometer forSpeed Selection
ACMotor
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Conventional Variable Speed Drives
Full-Speed AC Motor
Full-Speed Shaft
Variable-SpeedShaft
Magnetic fieldfor Flux Control
FieldControl
Speed Potentiometer
33Variable Speed Drives
Conventional Variable Speed DC Drives
source AC
φ3
Motor
G
Generator
Exciter
M
Motor
Load
34Variable Speed Drives
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Modern Variable Speed DC Drives
M
Motor
Load
source AC
ConverterQuadrant -Four
supply Field
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Conventional Variable Speed AC Drives
• Pole changing
• Stator voltage control
• Rotor resistance control
• Slip-recovery drive system
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Modern Variable Speed AC Drive System
Converter AC-ACsou
rce
A
C
φ3
supply AC
frequency-variable
voltage-Variable
Motor AC
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Advantages of Variable Speed AC Drives
• Commutatorless and brushless
• High-power and high-speed
• Simple design and robust
• High-efficiency
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Classifications of Variable Speed AC Drives
• Motor types : induction and synchronous
motors
• Power supplies : inverter, cycloconverter,
matrix converter.
• Inverter types : voltage source, current
source, square type, PWM.
39Variable Speed Drives
Variable Voltage Inverter System
φ3
Motor AC
outv
source AC
rectifier
Controlled
inductor
Smoothing
capacitor
Smoothing
inverter
wave-Square
• DC voltage is variable and controlled by the rectifier.
• The output voltage of inverter is quasi square-wave.
• The output frequency is determined by the inverter.
• The output voltage is determined by the rectifier.
• An LC filter is required in the dc link.
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PWM Inverter System
φ3
Motor AC
outv
source AC
rectifier
edUncontroll
capacitor
Smoothing
inverter PWM
• The dc voltage is constant. No controller is required by the rectifier.
• The inverter is controlled by pulse width modulation (PWM) technique.
• The output current is almost sinusoidal.
• The output voltage and frequency are controlled by the inverter.
• Only a dc capacitor is used as the filter in the dc link.
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Current Source Inverter System
φ3
Motor AC
outi
source AC
rectifier
Controlled
inductor
Smoothing
inverter
wave-Square
• The dc link current is controlled by the rectifier.
• The inverter output current is quasi square-wave.
• The output current frequency is controlled by the inverter.
• The magnitude of the output current is controlled by the rectifier.
• Only an inductor is used as the filter in the dc link.
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Six-Pulse CSI System
Variable Speed Drives 43
Twelve-Pulse CSI System
Variable Speed Drives 44
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PWM CSI System
Variable Speed Drives 45
Cycloconverter System
source AC
Motor AC
• No dc link.
• The output voltage magnitude and frequency are controlled
by the cycloconverter.
• The output frequency is less than the ac supply frequency.
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Load Commutated Inverter System
φ3
Motor
sSynchronou AC
outi
source AC
rectifier
Controlled
inductor
Smoothing
inverter
commutated Load
• The dc link current is controlled by the inverter.
• The output current waveform is quasi square-wave.
• The thyristors of inverter are commutated by the help of load voltage.
• The inverter frequency is determined by the motor speed.
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Large LCI System
Variable Speed Drives 48
25
Slip Recovery Induction Drivemotorinduction
rotor Wound
RectifierInverter
sou
rce
A
C
• Converter rating is smaller than motor rating
• Only subsynchronous operation is possible
• Wound rotor motor is required
49Variable Speed Drives
VSD with Unity Power Factor Rectifier
dC
boostL
inCinL
IM
• Input current waveform is almost sinusoidal and
unity power factor
• Insensitive to input voltage variation
• Applicable up to 100 kW
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VSD with PWM Rectifier
dC
source AC
IM
rectifier PWM inverter PWM
• Power flow is bidirectional
• Input current waveform is almost sinusoidal
• Input power factor is adjustable
• Insensitive to input voltage variations
51Variable Speed Drives
Matrix Converter
IM
⇒
sou
rce
A
C
• No dc link
• Output frequency is less than the input frequency
• Input current is almost sinusoidal
• Input power factor is adjustable
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Additional Advantages of using VSD
• Soft starting (reducing system kVA requirement and lengthening rotating equipment life).
• Increased motor life
• Shortened compressor startup time
• Process ride through
• Increased speed range
• Extended operating range before surge line
• Suitable for automation
• Can be used to overpowering the equipment without endangering the motor
53Variable Speed Drives
Medium Voltage ABB Drives
Variable Speed Drives 54
28
Toshiba System
Variable Speed Drives 55
VSD Price
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Technical Barriers
• VSD may reduce the motor efficiency
• VSD draws nonsinusoidal input current
• VSD may induce overvoltage to motor winding when the cable is longer than 50 meters
• VSD may induce bearing currents
• VSD may overloading shunt capacitor in the supply system
• Special technicians, engineers, and operators may required
57Variable Speed Drives
Motor efficiencies
00
80
85
90
95
25 50 75 100
% E
FF
ICIE
NC
Y
% FULL-LOAD TORQUE
100 HPENERGY EFFICIENT
MOTOR
100 HPSTANDARD EFFICIENT
MOTOR
60 HZ
60 HZ
30 HZ
60 HZ
60 HZ
30 HZ
58Variable Speed Drives
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Chances to save energy
• Are there motors that drive fans, pumps, or
compressors which are modulated by dampers or
valve?
• Are there motor using speed control devices?
• Is there machinery that can be operated at other
than its current speed?
• For the motors identified by the above three
questions, what is the duty cycle and load profile?
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Power Line Considerations
Variable Speed Drives 60
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Power Line Considerations
Variable Speed Drives 61
Power Line Considerations
Variable Speed Drives 62
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Power Line Considerations
Variable Speed Drives 63
Implementation cost factors
• How will the VSD improve quality?
• What costs associated with existing motor drive inefficiencies?
• What are the costs of maintaining existing equipment? Are they obsolete and in need of replacement?
• Do other problems of equipment reliability cause production delays and higher production costs? How can they be eliminated by VSDs with self-diagnostic features?
• Is there opportunity to create additional space by removing large mechanical equipment with the installation of VSD?
• Can plant noise be reduced by using VSD?
• What shutdown arrangements are required to provide time to install a VSD?
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Factors that influence first cost
• Rectifier and inverter type
• Harmonic effects
• Isolation requirements
• Control specifications
• Special requirements
• Training requirements
65Variable Speed Drives
Thank You