LECTURE 25 Controlled Rectifiers

Dr. Rostamkolai

ECE 452Power Electronics

1

Introduction

In Chapter 3, we have seen that diode rectifiers provide a fixed output voltage

To obtain controlled output voltages, phase- control thyristors are used instead of diodes

A phase-control thyristor is turned on by applying a short pulse to its gate, and is turned off due to natural or line commutation

2

In case of a highly inductive load, it is turned off by firing another thyristor of the rectifier

The phase-control rectifiers are cheap, and their efficiency is above 95%

Since these rectifiers convert form ac to dc, they are called ac to dc converters

3

They are used extensively in industrial applications, especially in variable speed drives

The phase-control converters are classified as: Single-phase converters Three-phase converters

Each type is divided into: Semiconverter Full converter Dual converter

4

A semiconverter is a one-quadrant converter and it has one polarity of output voltage and current

A full converter is a two-quadrant converter and the polarity of its output voltage can be either positive or negative, but one direction of current

A dual converter can operate in four quadrants, and both the output voltage and the output current can be either positive or negative

5

In some applications, converters are connected in series to operate at higher voltages, and to improve the input power factor

The method of Fourier series can be applied to analyze the performance of the phase controlled converters

6

Semiconverter Operation Consider the following circuit, where

α is the delay or firing (ignition) angle

7

If the frequency of the supply is fs, the lowest frequency of the output ripple voltage will be fs

Now:

The dc voltage can be varied from Vm/π to 0 by varying from 0 to π

)cos1(2

)(sin2

1

m

mdc

VtdtVV

8

The average output voltage will be maximum when α is 0

The rms output voltage is:

2/1

2/122

)]2

2sin(1[

2

)](sin2

1[

mrms

mrms

VV

tdtVV

9

Single-Phase Full Converters

The circuit for a single-phase full converter is shown below

The load is assumed to be highly inductive, so the load current is continuous and ripple free

The converter operates in the rectification and inversion modes

10

11

The average output voltage is:

The dc voltage can be varied from 2Vm/π to -2Vm/π by varying α from 0 to π

cos

2)(sin

2

2 mmdc

VtdtVV

12

The average output voltage will be maximum when α is 0

The rms output voltage is:

sm

rms

mrms

VV

V

tdtVV

2

)](sin2

2[ 2/122

13

During the period from α to π, the input voltage and current are positive Power flows from the supply to load Converter operates in rectification

mode

During the period from π to α + π, the input voltage is negative, and input current is still positive Power flows from the load to supply Converter operates in inversion mode 14

The operation of the converter can be divided into two identical modes

Mode 1: T1 and T2 conduct Mode 2: T3 and T4 conduct

This converter is used in industrial applications up to 15 kW

15

Single-Phase Dual Converter

In previous section, we have seen that the single-phase full converters with inductive loads allow only two-quadrant operation

If two of these converters are connected back to back, both the output voltage and the output current can be reversed

This system will provide four-quadrant operation and it is called a dual converter

16

Dual converters are used in high-power variable-speed drives

If α1 and α2 are the delay angles of converters 1 and 2, the corresponding average output voltages will be Vdc1 and Vdc2

17

18

The delay angles are controlled such that one converter operates as a rectifier and the other converter operates as an inverter

However, both converters produce the same average output voltage

Therefore,

22

11

cos2

cos2

m

dc

mdc

VV

VV

19

One converter is rectifying and the other one is inverting, therefore:

Since the instantaneous output voltages of the two converters are out of phase, there will be an instantaneous voltage difference between the two converters

12

112

21

)cos(coscos

dcdc VV

20

This will result in a circulating current between the two converters

The dual converters can be operated with or without a circulating current

In case of operation without the circulating current, only one converter operates at a time and carries the load current

The other converter is completely blocked by inhibiting gate pulses 21