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Industrial ElectronicsVeriable Speed Derive

byNaveed Mazhar

(fa09-bee-143)

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What is VSD???

A way to control and adjust the workperformed by a motor to meet changing

demands and energy requirements.

It converts the electrical power supplied bythe mains into mechanical power with a

minimum loss.

Solid State Technology in electroniccontrollers is used to ensure the minimumloss.

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What is VSD???(cont.)

This technology has several differentnames:

Variable-Speed Drive (VSD)

Adjustable-Speed Drive (ASD) Variable-Frequency Drive (VFD)

Adjustable-Frequency Drive (AFD)

Frequency Converter

Inverter (a term used by manufacturers)

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What is VSD???(cont.)

Any variable speed electrical drive systemcomprises of the following components:

An electronic actuator - the controller.

A driving electrical machines - motor.A driven machine (load) - pump, fan, blower,

compressor

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DC Motor Control

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r.p.m.

RaIVKN

N

b

E

DC Motor Control Methods

The speed of a d.c. motor is given by:

where R = Ra for shunt motor

& R= Ra + Rs. for series motor

So there are three main methods of controlling the speed of a

d.c. motor:

By varying the flux per pole().(Flux Control Method)

By varying the armature resistance (R).(Armature Control Method)

By varying the applied voltage (V). (Voltage Control Method)

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I-Flux Control Method

ShI

1

1N

The shunt field rheostat reduces the shunt field current Ish and

hence the flux .

Generally, this method permits to increase the speed in the ratio 3:1.

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II-Armature Control Method

By varying the voltage available across thearmature, the back e.m.f and hence the speed ofthe motor can be changed.

This method can only provide speeds below the normalspeed. The highest speed obtainable is thatcorresponding to Rc= 0

cR

aR

aI-VN

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III-Voltage Contol Method(THYRISTOR DC DRIVES)

Voltage is best controlled by Thyristor technique.This is most expensive way to control the speed

of motor, but still used in industry due to itsefficient feed back control.

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DC Motor (Thyristor Control)

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AC Motor Control

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AC Motor

The AC motor is made of two parts: The Stator:

The stationary section that contain the windings.

The Rotor:

The rotating section that contains the conductors.

In a motor, the induction principle isutilized in reverse A live conductor is placed in a magnetic field

The conductor is influenced by a force which tries tomove it through the magnetic field

Speed = (frequency x 120) / # of poles

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Basics Construction of VFD

Inside the VFD there are 4 major sections: rectifier (AC/DC Converter),

intermediate circuit (DC Link),

inverter (DC/AC Converter) and

control/regulation (for interface)

Inverter

VFD Block Diagram

Motor

MainPower

Rectifier DC Link Motor

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Working of VFD

The operation of the drive is as follows :

Power first goes into the rectifier, where the 3-phase AC isconverted into a rippling DC voltage. The intermediatecircuits then smoothes and holds the DC Voltage at aconstant level or energy source for the inverter. The last

section, the inverter, uses the DC voltage to pulse themotor with varying levels of voltage and current dependingupon the control circuit. The control section uses thefrequency, voltage and phase angle to control the inverter.

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AC Drive

DC Link

Rectifier

Converts AC line voltageto Pulsating DC voltage

Inverter

Changes fixed DC to adjustable AC

Alters the Frequency of PWM waveform

Intermediate Circuit (DC Link)

Filters the pulsating DC to fixed DC voltage

V

T

T

VV

V

T

AC PowerSupply

Rectifier Inverter

M

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Sine Weighted PWM

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Conclusive advantages of using a VFD

Energy SavingsBetter Process Control

Reduced Maintenance

Higher System Efficiency

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