LECTURE 25 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics

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

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

The phase-control converters are classified as: 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

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

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

Semiconverter Operation Consider the following circuit, where α is the delay or firing (ignition) angle

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

The average output voltage will be maximum when α is 0 The rms output voltage is:

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

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

The average output voltage will be maximum when α is 0 The rms output voltage is:

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

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

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

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

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,

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

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