Fundamentals AC Drive-V2

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A&D2/ATU AC-DRV-BASIC

Motor Torque Rotation

Mechanical Power

Electrical Power

DRIVE

What the Drive Does ?

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A&D2/ATU DRV-BASIC

Electric Supply Starter Motor Belt /

Gear

Conventional Technology

Electric Supply Motor MachinePower

ConverterDrive

Requirement of Modern Machines

Control over Torque & Speed of the motor ?

Control over Torque & Speed of the motor ?

NO !

YES !

Machine

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A&D2/ATU AC-DRV-BASIC

Speed

Torque

T2

Speed

Torque

T1 T1

Torque requirement

Speed Variation Load Variation

T2

N N1 N2

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A&D2/ATU AC-DRV-BASIC

Few expectations from the Drive • Variable speed at the motor shaft maintaining torque producing capability of the motor• Speed regulation

• Controlled acceleration & deceleration• Quick & safe reversal and braking• Current limiting• Protection against abnormal conditions

• Auto-optimization

• Easy fault diagnostics

• Communication capable • Energy Saving

Speed

time

Reqd.Speed

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Controlled Acceleration & Deceleration

t1Ramp-uptime

t2Ramp down time

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Coasting

Braking or Quick Stop

Stop alongRamp-down time

Stopping & Braking of Motors

N

time

stopOFF2

OFF1

OFF3

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A&D/ATU DRV-BASIC

Speed Control of Three Phase Induction Motor

120 f

PNs =Synchronous Speed =

Ns α f

Nr < Ns , Hence % Slip = Ns

X 100

Normally 4 to 6 % Slip is common for all practical applications. And we say that induction motor is constant speed motor.

Ns-Nr

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A&D/ATU DRV-BASIC

Why we require variable voltage ?

Torque is proportional to Flux and Armature current .

T α Φ x I Φ α V f

Thus if we reduce only frequency ,then flux will also reduce.Therefore we need to reduce voltage proportional to reduction in frequency.

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General block diagram of Inverter .

Rectifier Inverter

+

-

3 Φ Constant Voltage Constant Frequency

3 Φ Variable Voltage Variable Frequency

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Rectifier Section

Uncontrolled Bridge Rectifier

▲▲

▲▲

▲▲

▬▬

▬ ▬▬ ▬

R Y B

+

-Output DC VoltageInput AC Voltage

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A&D/ATU DRV-BASIC

Converting DC in to AC

S2

S3

S1

S4

+ / -

- / +

LOAD

Periodic reversal of polarity occurs by switching S1S4 and S2S3 switches

DC Input

AC Output

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A&D/ATU DRV-BASIC

Transistorized Inverter Block Diagram

Rectifier

DC Link Inverter

Supply

C

+

-

C

Motor

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A&D/ATU DRV-BASIC

Pulse Width Modulated Waveform

0V

Time

Pulse Width Modulation

Voltage

Current

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Pulse Width Modulated Waveform

• Voltage going to the inverter is chopped rectangular shape

• Pulse Width is varying .

• PWM voltage is of high frequency chopped pulses.

• Due to motor inductance the current flowing through winding will be of sinusoidal nature

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A&D/ATU DRV-BASIC

Basic Induction Motor Characteristics with V/f control

Speed

Normal OperatingPoint

Torque Speed Characteristic of An Induction Motor

Slip

Pull out TorqueTorque

Variable frequency operation

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A&D/ATU DRV-BASIC

Block diagram of V/f control

RFG Modulator

M

Nref

Current limitingController

Slip Compensation

fref

Inverter Bridge

f

V

- +

V/f characteristics block

f

v

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A&D/ATU DRV-BASIC

• V/f Control is called as open loop control.• Vector control operation is performed because it has following advantages. • Speed Accuracy • Torque Accuracy• Speed range• Dynamic Performance• Performance at low speed• Shock load operation

Vector Control

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Vector Control

How vector control works ? • In AC Motor the stator winding current sets the Flux and torque, therefore it is difficultto control Flux and Torque separately.In drive above independent control of Flux and Torque producing components is achieved with the help of high speed microprocessor and the same is called asVector Control.

I load

Iflux

Itotal

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Vector Control

Block diagram of Vector Control operation

RFG N I I

I

Gating

Unit Inv

Motor Model

M

w

μμI

μRefI

I w Ref

Coordinate converter

Iw

IμSlip

Speed Controller

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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A&D/ATU DRV-BASIC

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