ECE 8830 - Electric Drives

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ECE 8830 - Electric Drives
Topic 1 Introduction to Electric Drives
Spring 2004
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Introduction

• Nearly 65 of the total electric energy
  produced in the USA is consumed by electric
  motors.
• - 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 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.

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Power Devices

• Power Diode
• Power BJT
• SCR/Thyristor
• Gate Turn-Off Thyristor (GTO)
• Power MOSFET
• Insulated Gate Bipolar Transistor (IGBT)
• MOS Controlled Thyristor (MCT)

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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.

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Power Diodes
A

• Circuit Symbol
• Current-Voltage Characteristics

iD
vD
-
K
iD
iD
I
vrated
vD
vD
vF
reverse blocking
reverse blocking
Ideal
Real
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Diode Switching Characteristics

• Reverse Forward
• Forward Reverse

iD
IF
0
t
iD
IF
trr
0
t
Qrr
? -IF
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Thyristors
A
iA

• Circuit Symbol
• Current-Voltage Characteristics

vAK
G
-
K
iA
iA
ON
reverse breakdown voltage
OFF ON if gate voltage applied
ON-state
OFF ON if gate voltage applied
OFF
vAK
vAK
reverse blocking
forward blocking
reverse blocking
forward breakdown voltage
Ideal
Real
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Thyristor Switching Characteristics
vs
R
iA
t


vAK
vs
-
-
iG
iG
t
trr reverse recovery time tq
circuit-commutated recovery time (the time
that the thyristor must have reverse voltage
applied before entering the forward blocking
state) Note trr ? tq
iA
trr

t
fires
vAK
tq
t
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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

The current source approximates the current
that would actually flow due to inductive
current storage.
I0

vd
-
vT -
iT
control switch
Controllable switch
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Controllable Switches (contd)

• Switching Waveforms

Switch control signal
off on
off t
vT, iT
VS
VS
IS
tD(on)
tD (off)
Von
t
tri
tfv
trv
tfi
tC(on)
tC(off)
tc cross over ON and OFF times
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Power Device Losses

• Conduction energy loss, EscISVONtontD(off)-tC(o
  n)-tD(on)
• Sum of turn-on and turn-off energy loss, Est
  ?0.5VSIStc(on)tc(off)
• Total power loss,
• where fs is switching frequency

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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).

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Gate Turn-Off Thyristors (GTOs)

• GTOs can be turned off
• by applying a negative
• gate current.

Circuit Symbol
A
Current-Voltage Characteristics
iA

vAK
G
-
K
ON OFF if negative gate voltage applied
iA
iA
ON
reverse breakdown voltage
ON-state
OFF ON if positive gate voltage applied
OFF-state
OFF
vAK
vAK
reverse blocking
forward blocking
reverse blocking
forward breakdown voltage
Ideal
Real
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Switching Waveforms for GTOs
vS
t
iG
t
large in magnitude 1/3 iA
iA
t
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GTOs (contd)

• GTOs are sensitive to dv/dt. Therefore,
  snubber circuits are used to minimize dv/dt and
  di/dt.
• GTOs are available to handle 1000s of 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, 1?sec.
  switching time.

iD
D

vDS
G
-

S
vGS
-
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MOS Controlled Thyristors (MCTs)
P-MCT

• 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

N-MCT
A
A
G
G
K
K
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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

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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 must have intimate knowledge about machine
  performance.
• - Bimal K. Bose, Modern Power Electronics and
  AC Drives

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Electric Machines (contd)

• 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

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Electric Machines (contd)

• 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.

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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 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.

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Controllers

• Controllers embody the control laws governing
  the load and motor characteristics and their
  interaction.
• Controller

Torque/speed/ position commands
Vc, fc, start, shut-out, signals, etc.
Torque/speed/ position feedback
Thermal and other feedback
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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
• x1 for frictional systems (e.g. feed drives)
• x2 for fans and pumps