Fundamentals Drive

8/6/2019 Fundamentals Drive

1/22

MotorTorque Rotation

Mechanical Power

Electrical Power

DRIVE

What the Drive Does ?


2/22

Electric

SupplyStarter Motor

Belt /

Gear

Conventional Technology

Electric

SupplyMotor Machine

Power

Converter

Drive

Requirement of Modern Machines

Control over Torque & Speed of the motor ?

Control over Torque & Speed of the motor ?

NO !

YES !

Machine


3/22

Speed

Torque

T2

Speed

Torque

T1T1

Torque requirement

Speed Variation Load Variation

T2

NN1 N2


4/22

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


5/22

Speed

time

Reqd.

Speed

Controlled Acceleration & Deceleration

t1

Ramp-up

time

t2

Ramp down time


6/22

Coasting

Braking or Quick Stop

Stop alongRamp-down time

Stopping & Braking of Motors

N

time

stop

OFF2

OFF1

OFF3


7/22

Speed Control of Three Phase Induction Motor

120 f

PNs =Synchronous Speed =

Ns f

Nr < Ns , Hence % Slip =Ns

X 100

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

speed motor.

Ns-Nr


8/22

Why we require variable voltage ?

Torque is proportional to Flux and Armature

current .

T x I Vf

Thus if we reduce only frequency ,then flux will also

reduce.Therefore we need to reduce voltage proportional

to reduction in frequency.


9/22

General block diagram of Inverter .

Rectifier Inverter

+

-

3 Constant VoltageConstant Frequency

3 Variable Voltage

Variable Frequency


10/22

Rectifier Section

Uncontrolled Bridge Rectifier

R Y B

+

-

Output DC VoltageInput AC Voltage


11/22

Converting DC in to AC

S2

S3

S1

S4

+ / -

- / +

LOAD

Periodic reversal of polarity occurs by switchingS1S4 and S2S3 switches

DC Input

AC Output


12/22

Transistorized Inverter Block Diagram

Rectifier

DC Link Inverter

Supply

C

+

-

C

Motor


13/22

Pulse Width Modulated Waveform

0V

Time

Pulse Width Modulation

Voltage

Current


14/22

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


15/22

Basic Induction Motor Characteristics with V/f control

Speed

Normal Operating

Point

Torque Speed Characteristic of An Induction Motor

Slip

Pull out Torque

Torque

Variable frequency operation


16/22

Block diagram of V/f control

RFG Modulator

M

Nref

Current limiting

ControllerSlip Compensation

fref

InverterBridge

f

V

- +

V/f characteristics block

f

v


17/22

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


18/22

Vector Control

How vector control works ?

In AC Motor the stator winding current

sets the Flux and torque, therefore it is difficult

to 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 as

Vector Control.

Iload

Iflux

Itotal


19/22

Vector Control

Block diagram of Vector Control operation

RFG N I I

I

Gating

Unit Inv

Motor Model

M

w

I

RefI

I w Ref

Coordinate converter

Iw

I

Slip

Speed Controller


20/22


21/22


22/22

Documents

Fundamentals Drive