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Topic 1: Introduction to Electric Drives
Spring 2004
ECE 8830 - Electric Drives
Introduction
“Nearly 65% of the total electric energy produced in the USA is consumed by electric motors.”
- R. Krishnan, “Electric Motor Drives”
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
Energy/Cost Savings
System efficiency can be increased from 15% to 27% by introducing variable-speed drive operation in place of constant-speed operation.
US energy bill would be reduced by an estimated $90 billion!
For a large pump variable-speed drive, payback period ~ 3-5 years whereas operating life is ~ 20 years.
Power Devices
Power Diode Power BJT SCR/Thyristor Gate Turn-Off Thyristor (GTO) Power MOSFET Insulated Gate Bipolar Transistor (IGBT) MOS Controlled Thyristor (MCT)
Categories of Switches
There are three categories of switches: Diodes (rectifiers) - on/off determined
by the power circuit. Thyristors (SCRs, Triacs) - latched on by
a control signal but turned off by the power circuit.
Controllable Switches (BJTs, MOSFETs, GTOs, IGBTs, MCTs) - turned on and off by control signals.
Power Diodes
Circuit Symbol:
Current-Voltage Characteristics:
iiDD++vvDD
--
AA
KK
iiDD
vvDD
II
vvFF
vvratedrated
reversereverseblockingblocking
iiDD
vvDD
RealReal IdealIdeal
reversereverseblockingblocking
Diode Switching Characteristics
Reverse Forward
Forward Reverse
00
IIFF
iiDD
00
IIFF
iiDD
tt
tt
-I-IFF
ttrrrr
QQrrrr
Thyristors
Circuit Symbol:
Current-Voltage Characteristics:
iiAA ++vvAKAK
--
AA
KK
iiAA
vvAKAK
reversereversebreakdown breakdown voltagevoltage
reversereverseblockingblocking
iiAA
vvAKAK
RealReal IdealIdeal
reversereverseblockingblocking
GG
ONON
forwardforwardbreakdownbreakdownvoltagevoltage
OFF ON if gateOFF ON if gatevoltage appliedvoltage applied
OFFOFF
forwardforwardblockingblocking
ON-stateON-stateOFF ON if gateOFF ON if gatevoltage appliedvoltage applied
Thyristor Switching Characteristics
iiAA++
--vvAKAK
RR
++--
vvss
vvss
tt
tt
iiGG
ttttrrrr
““fires”fires”
iiAA
iiGG
vvAKAK
ttttqq
ttrrrr = reverse recovery time = reverse recovery time
ttqq = circuit-commutated = circuit-commutated
recovery timerecovery time(the time that the thyristor must(the time that the thyristor musthave have reverse voltagereverse voltage applied appliedbefore enteringbefore entering the the forwardforwardblockingblocking state) state)
NoteNote: t: trrrr t tqq
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
II00
++ --vvdd
iiTT++vvTT
--
controlcontrolswitchswitch
The current source The current source approximates the approximates the current that would current that would actually flow due to actually flow due to inductive current storage.inductive current storage.
ControllableControllable switchswitch
Controllable Switches (cont’d)
Switching Waveforms
off on off off on off tt
SwitchSwitchcontrolcontrolsignalsignal
vvTT, , iiTT
tt
IISS
ttDD(on)(on) ttDD (off) (off)
VVSS VVSS
VVonon
ttriri ttfvfv
ttCC(on)(on)
ttrvrv ttfifi
ttCC(off)(off)
ttcc = cross over ON and OFF times = cross over ON and OFF times
Power Device Losses
Conduction energy loss, Esc=ISVON[ton+tD(off)-tC(on)-tD(on)]
Sum of turn-on and turn-off energy loss, Est 0.5VSIS[tc(on)+tc(off)]
Total power loss,
where fs is switching frequency
)( scstsoffon
scstsw EEf
tt
EEP
Transistor Switches
BJTs, Monolithic Darlingtons (MDs) and MOSFETs
MOSFETs are easier to parallel than BJTs because of their positive temperature coefficient of on-state resistance (although paralleling MOSFETs is an art more than a science).
Gate Turn-Off Thyristors (GTOs)
GTOs can be turned off
by applying a negative gate current.
Current-Voltage Characteristics:
iiAA ++vvAKAK
--KK
iiAA
vvAKAK
reversereversebreakdown breakdown voltagevoltage
reversereverseblockingblocking
iiAA
vvAKAK
RealReal IdealIdeal
reversereverseblockingblocking
GG
ONON
forwardforwardbreakdownbreakdownvoltagevoltage
OFF ON if positive OFF ON if positive gate voltage appliedgate voltage applied OFFOFF
forwardforwardblockingblocking
ON-stateON-state
Circuit SymbolCircuit Symbol::AA
ON OFF if negative ON OFF if negative gate voltage appliedgate voltage applied
OFF-stateOFF-state
Switching Waveforms for GTOs
tt
tt
iiGG
tt
iiAA
vvSS
large in magnitude ~ 1/3 ilarge in magnitude ~ 1/3 iAA
GTOs (cont’d)
GTOs are sensitive to dv/dt. Therefore, snubber circuits are used to minimize dv/dt and di/dt.
GTOs are available to handle 1000’s of V,A up to 10kHz.
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, 100’s of A, ~
1sec. switching time.
DD
SSGG
++vvGSGS --
++
--vvDSDS
iiDD
MOS Controlled Thyristors (MCTs)
Circuit Symbols:
Characteristics: Current-voltage characteristics similar to GTOs Two main advantages over GTOs: 1) Smaller turn-off current 2) Faster switching speeds (~ sec) Voltage ratings up to 1500V; current ratings ~ few hundred Amps
AA AA
GG
GGKK KK
P-MCTP-MCT N-MCTN-MCT
Motor Drive Components
A modern variable-speed drive has four 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 load requirements
(iv) Load
Motor Drive Schematic
Ref: R. Krishnan, “Electric Drives: Modeling, Analysis and Control”
Subdisciplines of Electrical Engg.
Semiconductor Devices Magnetic Materials Power Electronics Control Systems Electromagnetics Sensors Analog and Digital Electronics Signal Processing
Electric Machines
“An engineer designing a high-performance drive system must have intimate knowledge about machine performance.”
- Bimal K. Bose, “Modern Power Electronics and AC Drives”
Electric Machines (cont’d)
DC Machines - shunt, series, compound, separately excited dc motors and switched reluctance machines
AC Machines - Induction, wound rotor synchronous, permanent magnet synchronous, synchronous reluctance, and switched reluctance machines.
Special Machines - switched reluctance machines
Electric Machines (cont’d)
All of the above machines are commercially available in fractional kW to MW ranges except permanent-magnet, synchronous, synchronous reluctance, and switched reluctance which are available up to 150 kW level.
Selection Criteria for Electric Machines
Cost Thermal Capacity Efficiency Torque-speed profile Acceleration Power density, volume of motor Ripple, cogging torques Peak torque capability
Power Converters
Controlled Rectifiers; fed from single-phase or three-phase ac mains supply and provide dc output for motor drive.
Inverters; convert dc output of battery or rectified ac source to provide variable ac voltages and currents at desired frequency and phase.
Cycloconverters; Directly convert fixed frequency ac voltage/current to variable voltage/current of variable frequency for driving ac machines.
Controllers
Controllers embody the control laws governing 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.
Load
The motor drives a load that has a characteristic torque vs. speed requirement.
In general, load torque is a function of speed and can be written as:
Tl mx
x=1 for frictional systems (e.g. feed drives)
x=2 for fans and pumps