intro electrical drive
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Topic 1: Introduction to Electric Drives
ECE 8830 - Electric Dr
ives
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Introduction
Nearly 65% of the total electric
energy produced in the USA isconsumed by electric motors.
65
"
- R. Krishnan, Electric Motor Drives
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Some Applications of Electric Drives
Electric Propulsion
Pumps, fans, compressors
Plant automation
Flexible manufacturing systems Spindles and servos
Appliances and power tools
Cement kilns
Paper and pulp mills; textile mills
Automotive applications
Conveyors, elevators, escalators, lifts
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Energy/Cost Savings /
System efficiency can be increased from 15% to 27% byintroducing variable-speed drive operation in place of
constant-speed operation. 1527.
US energy bill would be reduced by an estimated $90billion! 90!
For a large pump variable-speed drive, payback period ~ 3-5years whereas operating life is ~ 20 years
3-5~~20
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Power Devices
Power Diode
Power BJT BJT
SCR/Thyristor SCR /
Gate Turn-Off Thyristor (GTO) GTO)
Power MOSFET MOSFET
Insulated Gate Bipolar Transistor (IGBT)
IGBT)
MOS Controlled Thyristor (MCT)
MOS MCT)
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Categories of Switches
There are three categories of switches:
Diodes (rectifiers) - on/off determinedby the power circuit.
Thyristors(SCRs, Triacs) - latched onby a control signal but turned off bythe power circuit.
Controllable Switches (BJTs, MOSFETs,GTOs, IGBTs, MCTs) - turned on andoff by control signals.
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Power Diodes
Circuit Symbol:
Current-Voltage Characteristics:
iD+vD
-
A
K
iD
vD
I
vF
vrated
reverse
blocking
iD
vD
Real Ideal
reverse
blocking
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Diode Switching Characteristics
Reverse Forward
Forward Reverse
0
IF
iD
0
IFiD
t
t
-IF
trr
Qrr
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Thyristors
Circuit Symbol:
Current-Voltage Characteristics:
iA +vAK
-
A
K
iA
vAK
reverse
breakdown
voltage
reverse
blocking
iA
vAK
Real Ideal
reverse
blocking
G
ON
forward
breakdown
voltage
OFF ON if gate
voltage applied
OFF
forward
blocking
ON-state
OFF ON if gate
voltage applied
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Thyristor Switching
Characteristics
iA
+
-
vAK
R
+
-vs
vs
t
t
iG
t
trr
fires
iA
iG
vAK
t
tq
trr= reverse recovery time
tq = circuit-commutated
recovery time
(the time that the thyristor musthave reverse voltage applied
before enteringthe forward
blockingstate)
Note: trrt
q
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Controllable Switches
These devices do not depend on power
reversal to go off - they may be triggered off.
In many applications, the switch current
flows through a series inductance.
Idealized Circuit
I0
+-vd
iT+
vT-
control
switch
The current source
approximates the
current that wouldactually flow due to
inductive current storage.
Controllable
switch
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Controllable Switches (contd
Switching Waveforms
off on off t
Switch
control
signal
vT, iT
t
IS
tD(on) tD (off)
VS VS
Von
tri tfv
tC(on)
trv tfi
tC(off)
tc= cross over ON and OFF times
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Power Device Losses
Conduction energy loss,Esc=ISVON[ton+tD(off)-tC(on)-tD(on)]
Sum of turn-on and turn-off energyloss, Est0.5VSIS[tc(on)+tc(off)]
Total power loss,
where fs is switching frequency
)( scstsoffon
scstsw EEf
tt
EEP
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Transistor Switches
BJTs, Monolithic Darlingtons (MDs)and MOSFETs
MOSFETs are easier to parallel thanBJTs because of their positivetemperature coefficient of on-state
resistance (although parallelingMOSFETs is an art more than ascience).
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Gate Turn-Off Thyristors (GTOs)
GTOs can be turned off
by applying a negative
gate current.
Current-Voltage Characteristics:
iA +
vAK
-
K
iA
vAK
reverse
breakdownvoltage
reverse
blocking
iA
vAK
Real Ideal
reverse
blocking
G
ON
forward
breakdown
voltage
OFF ON if positive
gate voltage applied OFF
forward
blocking
ON-state
Circuit Symbol:
A
ON OFF if negative
gate voltage applied
OFF-state
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Switching Waveforms for GTOs
t
t
iG
t
iA
vS
large in magnitude ~ 1/3 iA
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GTOs (contd
GTOs are sensitive to dv/dt.Therefore, snubber circuits are usedto minimize dv/dt and di/dt.
GTOs are available to handle 1000sof V,A up to 10kHz.
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Insulated Gate Bipolar
Transistors (IGBTs)
Circuit Symbol:
Characteristics:
High impedance gate (similar to MOSFETs)
Von ~ 2V in a 1000V device !
Voltage ratings up to 2 kV, 100s of A, ~1sec. switching time.
D
SG +
vGS-
+
-
vDS
i
D
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MOS Controlled Thyristors (MCTs)
Circuit Symbols:
Characteristics: Current-voltage characteristics similar to GTOs
Two main advantages over GTOs:
1) Smaller turn-off current2) Faster switching speeds (~ sec)
Voltage ratings up to 1500V;
current ratings ~ few hundred Amps
A A
G
GK K
P-MCT N-MCT
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Motor Drive Components
A modern variable-speed drive hasfour components:
(i) Electric machines - ac or dc
(ii) Power converter - rectifiers,choppers,
inverters, and cycloconverters
(iii) Controllers -matching the motor and
power converter to meet the loadrequirements
(iv) Load
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Motor Drive Schematic
Ref: R. Krishnan, Electric Drives: Modeling, Analysis and Control
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Subdisciplines of Electrical Engg.
Semiconductor Devices
Magnetic Materials
Power Electronics Control Systems
Electromagnetics
Sensors Analog and Digital Electronics
Signal Processing
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Electric Machines
An engineer designing a high-performance drive system musthave intimate knowledge aboutmachine performance.
- Bimal K. Bose, Modern PowerElectronics and AC Drives
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Electric Machines (contd
DC Machines - shunt, series,compound, separately excited dcmotors and switched reluctance
machines AC Machines - Induction, wound
rotor synchronous, permanentmagnet synchronous, synchronousreluctance, and switched reluctancemachines.
Special Machines - switched
reluctance machines
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Electric Machines (contd
All of the above machines arecommercially available in fractional
kW to MW ranges exceptpermanent-magnet, synchronous,synchronous reluctance, andswitched reluctance which areavailable up to 150 kW level.
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Selection Criteria for Electric Machines
Cost
Thermal Capacity
Efficiency Torque-speed profile
Acceleration
Power density, volume of motor Ripple, cogging torques
Peak torque capability
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Power Converters
Controlled Rectifiers; fed from single-phase or three-phase ac mains supplyand provide dc output for motor drive.
Inverters; convert dc output of batteryor rectified ac source to provide variableac voltages and currents at desiredfrequency and phase.
Cycloconverters; Directly convert fixedfrequency ac voltage/current to variablevoltage/current of variable frequency for
driving ac machines.
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Controllers
Controllers embody the control lawsgoverning the load and motor
characteristics and their interaction.
Controller
Torque/speed/position commands
Torque/speed/
position feedback
Thermal andother feedback
Vc, fc, start,shut-out,
signals, etc.
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Load
The motor drives a load that has acharacteristic torque vs. speedrequirement.
In general, load torque is a function ofspeed and can be written as:
Tlmx
x=1 for frictional systems (e.g. feed drives)
x=2 for fans and pumps
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