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

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

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

Topic 1: Introduction to Electric Drives

Spring 2004

ECE 8830 - Electric Drives

Page 2: 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”

Page 3: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric 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

Page 4: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 5: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Power Devices

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

Page 6: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 7: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Power Diodes

Circuit Symbol:

Current-Voltage Characteristics:

iiDD++vvDD

--

AA

KK

iiDD

vvDD

II

vvFF

vvratedrated

reversereverseblockingblocking

iiDD

vvDD

RealReal IdealIdeal

reversereverseblockingblocking

Page 8: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Diode Switching Characteristics

Reverse Forward

Forward Reverse

00

IIFF

iiDD

00

IIFF

iiDD

tt

tt

-I-IFF

ttrrrr

QQrrrr

Page 9: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 10: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 11: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 12: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 13: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 14: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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).

Page 15: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 16: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Switching Waveforms for GTOs

tt

tt

iiGG

tt

iiAA

vvSS

large in magnitude ~ 1/3 ilarge in magnitude ~ 1/3 iAA

Page 17: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 18: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 19: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 20: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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

Page 21: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Motor Drive Schematic

Ref: R. Krishnan, “Electric Drives: Modeling, Analysis and Control”

Page 22: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Subdisciplines of Electrical Engg.

Semiconductor Devices Magnetic Materials Power Electronics Control Systems Electromagnetics Sensors Analog and Digital Electronics Signal Processing

Page 23: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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”

Page 24: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric 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

Page 25: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 26: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

Selection Criteria for Electric Machines

Cost Thermal Capacity Efficiency Torque-speed profile Acceleration Power density, volume of motor Ripple, cogging torques Peak torque capability

Page 27: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 28: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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.

Page 29: Topic 1: Introduction to Electric Drives Spring 2004 ECE 8830 - Electric Drives

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