1. LECTURE 25 Controlled Rectifiers Dr. Rostamkolai
ECE 452
Power Electronics

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

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

10. 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

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

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

14. 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

15. 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

16. 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

17. 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

19. 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,

20. 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

21. 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