Transcript Microelectromechanical Devices

ECE 8830 - Electric Drives
Topic 1: Introduction to Electric Drives
Spring 2004
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
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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
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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
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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:
iD
A
+
vD
K
Current-Voltage Characteristics:
iD
vrated
iD
I
reverse
blocking
Real
vF
vD
reverse
blocking
Ideal
vD
Diode Switching Characteristics
Reverse
Forward
iD
IF
0
t
Forward
Reverse
iD
IF
trr
0
 -IF
t
Qrr
Thyristors
iA
Circuit Symbol:
G
A
+
vAK
K
Current-Voltage Characteristics:
reverse
breakdown
voltage
iA
ON
OFF ON if gate
voltage applied
iA
ON-state
OFF ON if gate
voltage applied
OFF
reverse
blocking
Real
vAK
forward
breakdown
voltage
reverse forward
blocking blocking
Ideal
vAK
Thyristor Switching
Characteristics
vs
R
+
-
vs
iG
iA
+
vAK
-
t
iG
trr = reverse recovery time
tq = circuit-commutated
iA
recovery time
(the time that the thyristor must
have reverse voltage applied
before entering the forward
“fires”
vAK
blocking state)
Note: trr  tq
t
trr
t
tq
t
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
The current source
approximates the
current that would
actually flow due to
inductive current storage.
+
-
iT
+
vT
-
control
switch
Controllable
switch
Controllable Switches (cont’d)
Switching Waveforms
Switch
control
signal
off
vT, iT
on
VS
tri
VS
IS
tD(on)
Von
t
off
tD (off)
tfv
trv tfi
tC(on)
tC(off)
tc = cross over ON and OFF times
t
Power Device Losses
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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,
Est  Esc
Psw 
 f s ( Est  Esc )
ton  toff
where fs is switching frequency
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.
Circuit Symbol:
iA
G
Current-Voltage Characteristics:
iA
reverse
ON
breakdown
OFF ON if positive
voltage
gate voltage applied
reverse
blocking
Real
ON OFF if negative
gate voltage applied
iA
ON-state
OFF-state
OFF
vAK
forward
breakdown
voltage
A
+
vAK
K
reverse forward
blocking blocking
Ideal
vAK
Switching Waveforms for GTOs
vS
t
iG
t
large in magnitude ~ 1/3 iA
iA
t
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:
G
Characteristics:
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+
i
D
D
+
vDS
S
vGS
-
High impedance gate (similar to MOSFETs)
Von ~ 2V in a 1000V device !
Voltage ratings up to 2 kV, 100’s of A, ~
1sec. switching time.
MOS Controlled Thyristors (MCTs)
Circuit Symbols:
P-MCT
N-MCT
A
A
G
Characteristics:
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K
G
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
K
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.
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Semiconductor Devices
Magnetic Materials
Power Electronics
Control Systems
Electromagnetics
Sensors
Analog and Digital Electronics
Signal Processing
Electric Machines
“An engineer designing a highperformance drive system must
have intimate knowledge about
machine performance.”
- Bimal K. Bose, “Modern Power
Electronics and AC Drives”
Electric Machines (cont’d)
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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
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Cost
Thermal Capacity
Efficiency
Torque-speed profile
Acceleration
Power density, volume of motor
Ripple, cogging torques
Peak torque capability
Power Converters
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Controlled Rectifiers; fed from singlephase or three-phase ac mains supply
and provide dc output for motor drive.
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Inverters; convert dc output of battery
or rectified ac source to provide variable
ac voltages and currents at desired
frequency and phase.
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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.
Torque/speed/
position commands
Torque/speed/
position feedback
Thermal and
other feedback
Controller
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