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Introduction to Electric Drives

(EEC - 604)

Unit - 3

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Chopper

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Introduction

Variable speed dc and ac drives are being increasinglyused in all industries. These drives takes power from the

dc voltage sources.

In many cases, conversion of dc source voltage to

different levels is required. For example, subway cars, trolley buses or battery

operated vehicles requires power from a fixed voltage dc

source.

However, their speed control requires conversion of fixedvoltage dc source to variable voltage dc source for the

armature of dc motor.

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Techniques to obtain variable dc voltage

Resistance Control: A variable resistance is inserted between the load and the

source.

High wastage of energy

For different values of load current, different values ofresistances will be required.

Motor Generator set:

Separate generator excitation gives a voltage which can be

varied from zero to rated value with either polarity. Set up is bulky, costly, slower in response and less efficient.

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Contd

A.C. link Chopper ( Inverter rectifier): dc is first converted to ac by an inverter (dc to ac converter).

The obtained ac is then stepped up or down by a transformerand then rectified back to dc by a rectifier.

As the conversion is in two stages, this technique is, therefore,

costly, bulky and less efficient. D. C. Chopper ( dc to dc power converters):

It is a static device (switch) used to obtain variable d.c. voltagefrom a source of constant d.c. voltage through one stageconversion.

The d.c. chopper offers greater efficiency, faster response,lower maintenance, small size, smooth control, and for manyapplications, lower cost than motor-generator sets or gastubes approaches.

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Step up/down Chopper

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Cycloconverter

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Introduction

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A cycloconverter is a type of power electronic controller inwhich an alternating voltage at one frequency is converteddirectly to an alternating voltage at other frequency withoutany intermediate stage.

Basically, cycloconverters are of two types, namely

Step down cycloconverters Step up cycloconverters

In step down cycloconverters, the output frequency is lowerthan the supply frequency and in step up cycloconverters,the output frequency is higher than the input frequency.

The majority of cycloconverters are naturally commutated.

A cycloconverter is controlled through the timings of its firingpulses, so that it produces an alternating output voltage.

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Introduction Contd

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By controlling the firing angles of the converters, it ispossible to control the frequency and amplitude of the

output voltage.

The applications of cycloconverters include the following:

Speed control of high-power ac drives Induction heating

Static VAR generation

For converting variable-speed alternator voltage to constant

frequency output voltage for use as power supply in aircraft ofshipboards.

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Single Phase to Single phase

Cycloconverter

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In this cycloconverter, the input and output both are singlephase.

The input a.c. voltage of supply frequency 50 Hz is convertedinto lower or higher frequency a.c. output.

There are mainly two configurations for this type ofcycloconverter: Centre tapped transformer configuration

Bridge configuration

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(i) Step Down cycloconverter with ResistiveLoad [fo= (1/3)*fs]

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(ii) With Inductive Load[fo= (1/4)*fs]

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(a) For Discontinuous Load Current

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(b) For Continuous Load Current

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Bridge Configuration

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Step up cycloconverter with resistive load

(fo= 6*fs)

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Three Phase Half Wave Cycloconverters

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A three phase cycloconverter works on the sameprinciples as single phase cycloconverter.

The type of three phase cycloconverter depends on the

number of pulses used.

The amount of ripple content can be reduced byincreasing the number of pulses used.

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(i) 3 phase to 1 phase Cycloconverters

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Three phase to three phase cycloconverter