Solid State Control of Electric Drive

Modern TrendsModern Trends
inin
Electric DrivesElectric Drives
ByBy
Mrs. Shimi S.LMrs. Shimi S.L
Assistant Professor,EEAssistant Professor,EE
NITTTR, ChandigarhNITTTR, Chandigarh

Power Electronic Systems
What is Power Electronics ?
A field of Electrical Engineering that deals with the application of
power semiconductor devices for the control and conversion of
electric power
Power Electronics
Converters
Power Electronics
Converters
LoadLoad
ControllerController
Output
- AC
- DC
Input
Source
- AC
- DC
- unregulated
Reference
sensors

Why Power Electronics ?
Power semiconductor devices Power switches
ON or OFF
+ vsw −
= 0
isw
+ vsw −
isw = 0
Ploss = vsw× isw = 0
Losses ideally ZERO !

−
Vak
+
ia
G
K
A
−
Vak
+
ia
K
A
−
Vak
+
ia
G
K
A

D
S
G
+
VDS
−
iD
G
C
E
+
VCE
−
ic

Power Electronics
Converters
Power Electronics
Converters
sensors
LoadLoad
Output
- AC
- DC
Input
Source
- AC
- DC
- unregulated
Reference
IDEALLY LOSSLESS !IDEALLY LOSSLESS !

Modern Electrical Drive Systems
• About 50% of electrical energy used for drives
• Can be either used for fixed speed or variable speed
• 75% - constant speed, 25% variable speed (expanding)
• Variable speed drives typically used PEC to supply the motors
AC motors
- IM
- PMSM
DC motors (brushed)
SRM
BLDC

Classic Electrical Drive for Variable Speed Application :
• Bulky
• Inefficient
• inflexible

Power
Electronic
Converters
Power
Electronic
Converters
LoadLoadMotorMotor
Reference
POWER IN
feedback
Typical Modern Electric Drive Systems
Power Electronic Converters
Electric Energy
- Unregulated -
Electric Energy
- Regulated -
Electric Motor
Electric
Energy
Mechanical
Energy

Example on VSD (variable speed drive) application
motor pump
valve
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Mainly in valve
Power out

Example on VSD application
Power
In
Power loss
Mainly in valve
Power out
motor pump
valve
Supply
motorPEC pump
Supply
Power
In
Power loss
Power out

Power
In
Power loss
Mainly in valve
Power out
Power
In
Power loss
Power out
motor pump
valve
Supply
motorPEC pump
Supply

Electric motor consumes more than half of electrical energy in the India
Fixed speed Variable speed
HOW ?
Improvements in energy utilization in electric motors give large
impact to the overall energy consumption
Replacing fixed speed drives with variable speed drives
Using the high efficiency motors
Improves the existing power converter–based drive systems

DC drives: Electrical drives that use DC motors as the prime mover
• Regular maintenance, heavy, expensive, speed limit
AC drives: Electrical drives that use AC motors as the prime mover
• Less maintenance, light, less expensive, high speed
Overview of AC and DC drives
• Easy control, decouple control of torque and flux
• Coupling between torque and flux – variable spatial
angle between rotor and stator flux

Before semiconductor devices were introduced (<1950)
• AC motors for fixed speed applications
• DC motors for variable speed applications
After semiconductor devices were introduced (1960s)
• Variable frequency sources available – AC motors in variable
speed applications
• Coupling between flux and torque control
• Application limited to medium performance applications –
fans, blowers, compressors – scalar control
• High performance applications dominated by DC motors –
tractions, elevators, servos, etc

After vector control drives were introduced (1980s)
• AC motors used in high performance applications – elevators,
tractions, servos
• AC motors favorable than DC motors – however control is
complex hence expensive
• Cost of microprocessor/semiconductors decreasing –predicted
30 years ago AC motors would take over DC motors

Extracted from Boldea & Nasar

Electrical DrivesElectrical Drives
An Electric Motor along with its controller
is called an Electric Drive

Electrical Drive SystemElectrical Drive System

Power Electronic Converters in ED Systems
Converters for Motor Drives
(some possible configurations)
DC Drives AC Drives
DC SourceAC Source
AC-DC-DCAC-DC-DCAC-DCAC-DC
AC Source
Const.
DC
Variable
DC
AC-DC-ACAC-DC-AC AC-ACAC-AC
DC Source
DC-ACDC-AC DC-DC-ACDC-DC-AC
DC-DCDC-DCDC-AC-DCDC-AC-DC

Converters for Motor Drives
Configurations of Power Electronic Converters depend on:
Sources available
Type of Motors
Drive Performance - applications
- Braking
- Response
- Ratings

IntelligentIntelligent
ControllersControllers
Smart Sensors –Smart Sensors –
Not Only Intelligent,Not Only Intelligent,
but Adaptablebut Adaptable
Intelligent ActuatorsIntelligent Actuators
PlantPlant

Separately Excited DC MotorSeparately Excited DC Motor

DC Motor EquationsDC Motor Equations
where J , D, and TL are the moment of inertia, damping factor
and load torque
Dynamic Steady- State

Speed control optionsSpeed control options
Va = IaRa + Eb
= IaRa + KIf ω Or
= (ω Va – IaRa)/ Kif Or
= (ω αVs – IaRa)/ Kif
So drive Speed can be controlled by:
ArmatureVoltage control
Field Flux control

For speeds less than the rated speed:
The armature current and field currents are
maintained at fixed values (hence constant torque
operation), and the armature voltage controls the
speed.
For speeds higher than the rated speed:
The armature voltage is maintained at rated value,
and the field current is varied to control the speed.
The power developed is maintained constant. This mode is
referred to as “field weakening” operation.

Speed Torque CharacteristicsSpeed Torque Characteristics

CONSTANT TORQUE LOAD
Constant torque load are those for which the output power requirement
may vary with speed of operation, but the torque does not vary.
Conveyors, rotary kilns and constant - displacement pumps are typical
examples of constant torque loads.
VARIABLE TORQUE LOAD
Variable torque loads are those for which the torque required varies with
speed of operation. Centrifugal pumps and fans are typical examples of
variable torque loads ( torque varies as the square of the speeds ).
CONSTANT POWER LOAD
Constant power loads are those for which the torque requirements are
typically changed inversely with speed. Winders, coilers are typically the
examples of constant power loads.

CharacteristicsCharacteristics

DC Series MotorDC Series Motor

Series Motor CharacteristicsSeries Motor Characteristics

DCVoltage control StrategiesDCVoltage control Strategies
Linear Regulation
Switch Mode Regulation

Linear RegulatorLinear Regulator
Transistor is operated in linear (active)
mode.
Output Voltage Vo = Vin -Vce

Equivalent CircuitEquivalent Circuit
Power loss is high at high current due to:
Po = IL
2
x RT Or
Po = Vce x IL

Life Time Motor Operating CostsLife Time Motor Operating Costs

Switching RegulatorSwitching Regulator
Transistor is operated in switched-mode:
Switch closed: Fully on (saturated)
Switch opened: Fully off (cut-off)

Equivalent CircuitEquivalent Circuit
When switch is open: no current flows in it
When switch is closed: no voltage drops across it.

Advantages of Switching RegulatorAdvantages of Switching Regulator
• Since P =VI, no losses occurs in the switch.
• Power is 100% transferred from source to
load.
• Power loss is zero (for ideal switch)
Switching regulator is the basis of all
DC - DC converters

What is a ‘Chopper’?What is a ‘Chopper’?
Chopper is an electronic switching
circuit which converts the unregulated
DC input to a controlled DC output
with a desired voltage level by switching
the supply ON and OFF.

General Block DiagramGeneral Block Diagram

Methods Of ControlMethods Of Control
The output dc voltage can be varied by
the following methods.
◦ Pulse width modulation control or
constant frequency operation.
◦ Variable frequency control.
◦ Current limit control.

PulseWidth ModulationPulseWidth Modulation
tON is varied keeping chopping frequency ‘f’
& chopping period ‘T’ constant.
Output voltage is varied by varying the
ON time tON

PulseWidth ModulationPulseWidth Modulation
V 0
V
V
V 0
t
t
t O N
t O N t O F F
t O F F
T

Variable Frequency ControlVariable Frequency Control
Chopping frequency ‘f’ is varied keeping
either tON or tOFF constant.
To obtain full output voltage range,
frequency has to be varied over a wide
range.
This method produces harmonics in the
output and for large tOFF load current may
become discontinuous

Variable Frequency ControlVariable Frequency Control

Current Limit ControlCurrent Limit Control

Chopper Controlled DC MotorChopper Controlled DC Motor

Quadrants of operationQuadrants of operation
Forward Breaking
Forward Motoring
Reverse Motoring
Reverse Breaking

Motoring ControlMotoring Control

Motoring ActionMotoring Action

Regenerative BreakingRegenerative Breaking
If back emf E >Va, the machine acts as a
generator
Armature current flows towards the
source
Energy stored in the machine rotor is fed
back to the source.
It causes the machine to slow down until
E =Va and then revert to motoring mode

During motoring mode, armature current
Ia = (Vt – Ea)/Ra
If Ea (= Km ωm) exceedsVt, Ia is reversed
Power is delivered to the dc bus
The motor works as a generator in the
regenerative braking mode
For loads, Such as a train going down the
hill or a descending hoist, emf Ea is more
than the source voltageVs

Two Quadrant Chopper drivesTwo Quadrant Chopper drives

T1 conducts → va = Vdc
Q1Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
−
DC DRIVES
AC-DC-DCAC-DC-DC DC-DC: Two-quadrant Converter

Q1Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
−
D2 conducts → va = 0
Va Eb
T1 conducts → va = Vdc
Quadrant 1 The average voltage is made larger than the back emf
DC DRIVES

Q1Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
−
D1 conducts → va = Vdc
DC DRIVES

Q1Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
−
T2 conducts → va = 0
Va
Eb
D1 conducts → va = Vdc
Quadrant 2 The average voltage is made smaller than the back emf, thus
forcing the current to flow in the reverse direction
DC DRIVES

Four Quadrant Chopper drivesFour Quadrant Chopper drives

leg A leg B
+ Va −
Q1
Q4
Q3
Q2
D1 D3
D2D4
+
Vdc
−
va = Vdc when Q1 and Q2 are ON
Positive current
DC DRIVES
AC-DC-DCAC-DC-DC DC-DC: Four-quadrant Converter

leg A leg B
+ Va −
Q1
Q4
Q3
Q2
D1 D3
D2D4
+
Vdc
−
va = -Vdc when D3 and D4 are ON
va = 0 when current freewheels through Q and D
Positive current
DC DRIVES

Positive current
va = Vdc when D1 and D2 are ON
Negative current
leg A leg B
+ Va −
Q1
Q4
Q3
Q2
D1 D3
D2D4
+
Vdc
−
DC DRIVES

Positive current
va = -Vdc when Q3 and Q4 are ON
va = Vdc when D1 and D2 are ON
Negative current
leg A leg B
+ Va −
Q1
Q4
Q3
Q2
D1 D3
D2D4
+
Vdc
−
DC DRIVES

DC DRIVES
Available AC source to control DC motor (brushed)
AC-DC-DCAC-DC-DCAC-DCAC-DC
Controlled Rectifier
Single-phase
Three-phase
Uncontrolled Rectifier
Single-phase
Three-phase
DC-DC Switched mode
1-quadrant, 2-quadrant
4-quadrant
Control Control

Single –Phase Half-Wave Converter Drives
Π<<+=
Π<<+=
11 0)cos1(
0)cos1(
2
αα
π
αα
π
for
V
V
for
V
V
m
f
m
o

Single –Phase Semiconverter Drives
)cos1(
)cos1(
1α
π
α
π
+=
+=
m
f
m
o
V
V
V
V

Single –Phase Full Converter Drives
+
Vo
−
α
π
α
π
α
π
cos
22
cos
2
cos
2
1
=
=
=
pf
V
V
V
V
m
f
m
o
Average voltage
over 10ms
50Hz
1-phase
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
-400
-200
0
200
400
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
0
5
10
50Hz
1-phase
Ia
Q1Q2
Q3 Q4
Vt
α<900
α >900
α=900
-Vt

0
1
11
,
2
,
1
180
0cos
2
0cos
2
=+
Π≤≤=
Π≤≤=
−
−
αα
αα
π
αα
π
for
V
V
for
V
V
mLL
o
mLL
oQ1Q2
Q3 Q4
ω
T
Forward motoring
Conv 1
Forward Reg braking
Conv 1
Reverse Reg braking
Conv 2
Reverse motoring
Conv 2
Single-Phase Dual Converter Drives

∏<<+=
∏<<+=
−
−
11
,
,
0)cos1(
2
3
0)cos1(
2
3
αα
π
αα
π
for
V
V
for
V
V
mLL
f
mLL
o
α
α 1α
Three –Phase Semiconverter Drives

+
Vo
−
1
,
,
cos
3
cos
3
α
π
α
π
mLL
f
mLL
o
V
V
V
V
−
−
=
=
Average voltage
over 3.33 ms
50Hz
3-phase
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
-500
0
500
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
0
10
20
30
α α1
50Hz
3-phase
Three –Phase Full Converter Drives

DC DRIVES
+
Vo
−
+
Vo
−
α
π
= cos
V2
V m
o
90o
180o
π
mV2
π
− mV2
90o
π
− m,LLV3
π
− − m,LLV3
α
π
= −
cos
V3
V m,LL
o
Average voltage
over 10ms
Average voltage
over 3.33 ms
50Hz
1-phase
50Hz
3-phase
180o
AC-DCAC-DC

DC DRIVES
AC-DCAC-DC
Ia
Q1Q2
Q3 Q4
Vt
3-phase
supply
+
Vt
−
ia
- Operation in quadrant 1 and 4 only

DC DRIVES
AC-DCAC-DC
Q1Q2
Q3 Q4
ω
T
3-phase
supply
3-
phase
supply
+
Vt
−
0
1
11
,
,
180
0cos
3
0cos
3
=+
Π≤≤=
Π≤≤=
−
−
αα
αα
π
αα
π
for
V
V
for
V
V
mLL
f
mLL
o

DC DRIVES
AC-DCAC-DC
Q1Q2
Q3 Q4
ω
T
F1
F2
R1
R2
+ Va -
3-phase
supply

DC DRIVES
AC-DCAC-DC
Cascade control structure with armature reversal (4-quadrant):
Speed
controller
Speed
controller
Current
Controller
Current
Controller
Firing
Circuit
Firing
Circuit
Armature
reversal
Armature
reversal
iD
iD,ref
iD,ref
iD,
ω
ωref + +
_
_

DC DRIVES
AC-DC-DCAC-DC-DC
controlUncontrolled
rectifier
Switch Mode DC-DC
1-Quadrant
2-Quadrant
4-Quadrant

DC DRIVES
AC-DC-DCAC-DC-DC
control

DC DRIVES
AC-DC-DCAC-DC-DC
vAB
Vdc
-Vdc
Vdc
0
vB
vA
Vdc
0
2vtri
vc
vc
+
_
Vdc
+
vA
-
+
vB
-
Bipolar switching scheme – output
swings between VDC and -VDC

DC DRIVES
AC-DC-DCAC-DC-DC
Unipolar switching scheme – output
swings between Vdc and -Vdc
Vtri
vc
-vc
vc
+
_
Vdc
+
vA
-
+
vB
-
-vc
vA
Vdc
0
vB
Vdc
0
vAB
Vdc
0

DC DRIVES
AC-DC-DCAC-DC-DC
Bipolar switching scheme
0.04 0.0405 0.041 0.0415 0.042 0.0425 0.043 0.0435 0.044 0.0445 0.045
-200
-150
-100
-50
0
50
100
150
200
Unipolar switching scheme
0.04 0.0405 0.041 0.0415 0.042 0.0425 0.043 0.0435 0.044 0.0445 0.045
-200
-150
-100
-50
0
50
100
150
200
• Current ripple in unipolar is smaller
• Output frequency in unipolar is effectively doubled
Vdc
Vdc
Vdc
DC-DC: Four-quadrant Converter
Armature
current
Armature
current

MeritsMerits
 Versatile control characteristics.
 High starting torque.
 Control over a large speed range.
 Speed control methods are simpler and
cheaper as compared to AC machines.

De-meritsDe-merits
 Bulky as compared to AC counterpart.
 Commutator sparking – not suitable for
petrochemicals, mine and chemical
applications.
 Above 500 kW, manufacturing of machine
itself is tedious.

Diode Thyristor
Power Devices
Uncontrolled On-controlled On-Off Controlled
GTO
Transistor
BJT
MOS & FETS
IGBT

Operating RangeOperating Range

Electric Car Speed ControlElectric Car Speed Control

Electric Trains and TramsElectric Trains and Trams

Steel Rolling MillsSteel Rolling Mills

Modeling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
INDUCTION MOTOR DRIVES
Scalar ControlScalar Control Vector ControlVector Control
Const. V/HzConst. V/Hz is=f(ωr)is=f(ωr) FOCFOC DTCDTC
Rotor FluxRotor Flux Stator FluxStator Flux Circular
Flux
Circular
Flux
Hexagon
Flux
Hexagon
Flux
DTC
SVM
DTC
SVM

AC DRIVESAC DRIVES
The AC motor have a number of advantages :
• Lightweight (20% to 40% lighter than equivalent DC motor)
• Inexpensive
• Low maintenance
The Disadvantages AC motor :
* The power control relatively complex
There are two type of AC motor Drives :
1. Induction Motor Drives
2. Synchronous Motor Drives
AC motor Drives are used in many industrial and
domestic application, such as in conveyer, lift, mixer,
escalator etc.

INDUCTION MOTOR DRIVES
Three-phase induction motor are commonly used in adjustable-speed
drives (ASD).
Basic part of three-phase induction motor :
• Stator
• Rotor
• Air gap
Stator
Rotor

Three Phase rotating magnetic fieldThree Phase rotating magnetic field

The stator winding are supplied with balanced three-phase AC voltage,
which produce induced voltage in the rotor windings. It is possible to
arrange the distribution of stator winding so that there is an effect of
multiple poles, producing several cycle of magnetomotive force (mmf) or
field around the air gap.
The speed of rotation of field is called the synchronous speed ωs , which
is defined by :
p
s
ω
ω
2
=
ωs is syncronous speed [rad/sec]
Ns is syncronous speed [rpm]
p is numbers of poles
ω is the supply frequency [rad/sec]
f is the supply frequency [Hz]
Nm is motor speed
p
f
Ns
120
=
or

The rotor speed or motor speed is : )1( Ssm −=ωω
Where S is slip, as defined as :
S
mS
S
ω
ωω −
= Or S
mS
N
NN
S
−
=
The motor speed

Equivalent Circuit Of Induction MotorEquivalent Circuit Of Induction Motor
Where :
Rs is resistance per-phase of stator winding
Rr is resistance per-phase of rotor winding
Xs is leakage reactance per-phase of the
winding stator
Xs is leakage reactance per-phase of the
winding rotor
Xm is magnetizing reactance
Rm is Core losses as a reactance

Performance Characteristic of InductionPerformance Characteristic of Induction
MotorMotor
Stator copper loss : sscus RIP
2
3=
'2'
)(3 rrcur RIP =Rotor copper loss :
m
s
m
m
c
R
V
R
V
P
22
33 ≈=Core losses :

S
R
IP r
rg
'
2'
)(3=
)1()(3
'
2'
S
S
R
IPPP r
rcurgd −=−=
)1( SPP gd −=
- Power developed on air gap (Power fropm stator to
rotor through air gap) :
Performance Characteristic of
Induction Motor
- Power developed by motor :
or
- Torque of motor :
m
d
d
P
T
ω
=
s
g
S
g P
S
SP
ωω
=
−
−
=
)1(
)1(
or
m
d
d
N
P
T
π2
60
=or

mssi IVP φcos3=
gcusc PPP ++=
Input power of motor :
Induction Motor
loadnodo PPP −=
gcusc
loadnod
i
o
PPP
PP
P
P
++
−
==η
Output power of motor :
Efficiency :

)( cuscg PPP +>>
loadnod PP >>
S
P
SP
P
P
g
g
g
d
−=
−
=≈ 1
)1(
η
If
and
so, the efficiency can calculated as :
Induction Motor

)(
222
ssm XRX +>>
Generally, value of reactance magnetization Xm
>> value Rm (core
losses) and also
So, the magnetizing voltage same with the input voltage : sm VV ≈
Therefore, the equivalent circuit is ;
Xm
Induction Motor

)(
)()(
'
'
'
'
rsm
r
s
r
smrsm
i
XXXj
S
R
R
S
R
RjXXXX
Z
++++
+++−
=
Total Impedance of this circuit is :
Induction Motor
Xm
The rotor current is :
( )
2
1
2'
2'
'








++





+
=
rs
r
s
s
r
XX
S
R
R
V
I

( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω
Torque – speed Characteristic

Three region operation :
1. Motoring :
2. Regenerating :
3. Plugging :
10 ≤≤ S
0<S
21 ≤≤ S

Starting speed of motor is ωm = 0 or S = 1,
Induction Motor
Starting torque of motor is :
( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
st
XX
S
R
R
VR
T
ω
Slip for the maximum torque Smax can be found by setting : 0=
dS
Td d
So, the slip on maximum torque is :
( ) ( )[ ]2
1
2'2
'
max
rss
r
XXR
R
S
++
±=

( ) 


 +++
=
2'2
2
max
2
3
rssss
s
XXRR
V
T
ω
Induction Motor
Torque maximum is :
And the maximum regenerative torque can be found as :
( ) 


 +++−
=
2'2
2
max
2
3
rssss
s
XXRR
V
T
ω
Where the slip of motor s = - Sm

( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω
Speed-Torque Characteristic :
( )
2'
2'






+>>+
S
R
RXX r
srs
( )2'
2'
3
rss
sr
d
XXS
VR
T
+
=
ω
( )2'
2'
3
rss
sr
st
XX
VR
T
+
=
ω
For the high Slip S. (starting)
So, the torque of motor is :
And starting torque (slip S=1) is :

( ) s
r
rs R
S
R
XX >><<+
'2'
rs
s
d
R
SV
T
'
3 2
ω
=
For low slip S region, the motor speed near unity or synchronous
speed, in this region the impedance motor is :
So, the motor torque is :
( ) ( )[ ]2
1
2'2
'
max
rss
r
XXR
R
S
++
±=And the slip at maximum torque is :
The maximum motor torque is :
( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω

Stator Voltage Control
Controlling Induction Motor Speed by
Adjusting The Stator Voltage
( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω

Frequency Voltage Control
Controlling Induction Motor Speed by
Adjusting The Frequency Stator Voltage
( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω

VSI
Rectifier
3-phase
supply IM
Pulse
Width
Modulatorωs* +
Ramp
f
C
V
Modeling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

CONTROLLING INDUCTION MOTOR SPEED USING
ROTOR RESISTANCE
(Rotor Voltage Control)

Wound rotor induction motor applications
cranes

CONTROLLING INDUCTION MOTOR SPEED USING
ROTOR RESISTANCE
(Rotor Voltage Control)
Equation of Speed-Torque :
( )








++





+
=
2'
2'
2'
3
rs
r
ss
sr
d
XX
S
R
RS
VR
T
ω
rs
s
d
R
SV
T
'
3 2
ω
=In a wound rotor induction motor, an external
three-phase resistor may be connected to its
slip rings,

These resistors Rx are used to control motor starting and stopping
anywhere from reduced voltage motors of low horsepower up to
large motor applications such as materials handling, mine hoists,
cranes etc.
The most common applications are:
AC Wound Rotor Induction Motors – where the resistor is wired into the
motor secondary slip rings and provides a soft start as resistance is
removed in steps.
AC Squirrel Cage Motors – where the resistor is used as a ballast for soft
starting also known as reduced voltage starting.
DC Series Wound Motors – where the current limiting resistor is wired to
the field to control motor current, since torque is directly proportional to
current, for starting and stopping.

The developed torque may be varying the resistance Rx
The torque-speed characteristic for variations in rotor resistance
This method increase the starting torque while limiting the starting current.
The wound rotor induction motor are widely used in applications requiring
frequent starting and braking with large motor torque (crane, hoists, etc)

The three-phase resistor may be replaced by a three-phase diode rectifier and
a DC chopper. The inductor Ld acts as a current source Id and the DC
chopper varies the effective resistance:
)1( kRRe −=
Where k is duty cycle of DC chopper
The speed can controlled by varying the duty cycle k, (slip power)

The slip power in the rotor circuit may be returned to the supply by replacing
the DC converter and resistance R with a three-phase full converter
(inverter)

Solid State Control of Electric Drive

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