Intro Electrical Drive

7/30/2019 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:


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.


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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Intro Electrical Drive