Control of Drives

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CONTROL OF ELECTRICAL DRIVES

CONTROL OF ELECTRICAL DRIVES

Controlling of electrical drives is a vast area. Now a days electrical drives are mostlyused in the industries.

Normally we are going to electrical drives is to vary the speed. Electrical drives are mostefficient to vary the speed when comparing to the other drives.

Consumption of Electricity in INDIA in 2006

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CLASSIFICATION OF ELECTRICAL DRIVES

Drives are normally classified in to two types namely

AC Drives

DC DrivesNow a day we are mostly using the AC drives.

Varying the speed is very easy in DC MOTORS. But we have only AC source. If we want

to use DC motors we must place a rectifier circuits. It will become uncomfortable.

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Electrical Drives

Drives are systems employed formotion control

Require prime movers

Drives that employ electric motors as

Prime movers are known as Electrical Drives

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Advantages Inter-disciplinary

Several research area

Expanding

Small

Efficient

Flexible

A modern variable-speed drive has four components:

Electric machines - ac or dc

Power converter - rectifiers, choppers,

inverters and cycloconverters

Controllers -matching the motor and

power converter to meet the load

requirements

Load

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Modern electric drives

Non-linear controlReal-time controlDSP applicationPFCSpeed sensorless

Power electronicconverters

Utility interfaceRenewableenergy

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Components in electric drives

Motors

DC motors - permanent magnet wound field

AC motors induction, synchronous (IPMSM, SMPSM), brushless DC

Applications, cost, environment

Power sources

DC batteries, fuel cell, photovoltaic - unregulated

AC Single- three- phase utility, wind generator unregulated

Power processor

To provide a regulated power supply

Combination of power electronic converters

More efficient

Flexible

Compact

AC-DC DC-DC DC-AC AC-AC

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Before semiconductor devices were introduced (

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Electrical motors for control Electrical motors for control- velocity

drives (1990)

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Electrical motors for control Electrical motors for control- velocity

drives (2001)

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Flow chart of reluctance motor control system

Advantages of Switched Reluctance Motor

Simplicity and reducibility of construction

Low manufacturing and operating costs

Fault tolerance

High torque/inertia ratio and efficiency

Disadvantages of Switched Reluctance Motor Reluctance Motor

High level of torque ripple

High level of acoustical noise

Normally we are using mechanically operated switches to turn on or to off the motors. Nowa day we are using electronic devices to switch off or to on.

In this paper we are going to see about switching control of electrical machines.

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

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Circuit Symbol:

Current-Voltage Characteristics:

iiDD

++vv

DD

--

AA

KK

iiDD

vvDD

II

vvFF

vvratedrated

ReverseReverse

BlockingBlocking

iiDD

vvDD

RealReal IdealIdeal

ReverseReverse

BlockingBlocking

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II00

++--vvdd

iiTT

++

vvTT

--

ControlControl

SwitchSwitch

The current sourceThe current source

Approximates theApproximates the

Current that wouldCurrent that would

Actually flow due toActually flow due to

Inductive current storage.Inductive current storage.

ControllableControllable

switchswitch

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Gate Turn-Off Thyristors (GTOs)

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Switching Waveformsoff on offoff on off tt

SwitchSwitch

controlcontrol

signalsignal

vvTT,, ii

TT

tt

IISS

ttDD(on)(on) tt

DD(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

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Switching Waveforms for GTOs

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 10 kHz.

An example for controlling the electrical drives

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Consider an AC motor. Figure shows the flow chart.

EMI filter

L1, L2, C1, C2, C3 and C4 constitute a LP filter

Keeps asymmetrical conduction interferences within the converter

Uncontrolled rectifier

Uncontrolled rectifier with a C filter

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DC link

EMI

filterInv

erter

L 2

L 1

C 1

I N 2

C 2

C 3

E A R T H G N D

I N 1

C 4

E

d

c

D 2

Idc

+ C d c

D 3D 1

D 4

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EDC is a DC voltage with a ripple. CDC is chosen to minimize the ripple (under1% EDC)

DC link

Minimizes the ripple of the DC voltage

Contains a shunt resistor RS used to gain

information on DC link current (this informationis used to compute rotor speed)

Inverter

Six IGBT transistors with anti-parallel diodes

Microprocessor controls the driver circuit, which turns transistors no or off

These are the types of switching control of electrical drives.

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C

2

+

C

1

R S

Q 6

Q 1 Q 5

Q 2 Q 4

Q 3

W

Q 6

Q 1 Q 5

V

Q 2 Q 4

U

Q 3