1. Dr John Fletcher John.Fletcher@eee.strath.ac.uk
Thyristor Rectifiers
Dr John Fletcher

2. Single-phase Rectifiers
Most common is the bridge configuration
- uncontrolled (4 diodes)
- half-controlled (2 thyristors and 2 diodes plus flywheel)
- fully-controlled (4 thyristors optional flywheel)
Fully-controlled rectifier provides bidirectional power flow.

3. Half-controlled rectifier
Flywheel diode is typically used.
Current can be continuous or discontinuous in the load and supply.
Higher mean output voltages because of flywheel
Current usually discontinuous

4. Fully-controlled Rectifier
4 thyristors
Optional flywheel
If α<Φ then conduction is continuous.
Continuous current, no flywheel.

5. What is this circuit? Full or half-wave? Flywheel or no flywheel?
Note how load current waveform defines the supply current waveform.

6. Three-phase Half-wave and full-wave versions
Controlled and uncontrolled versions
Half-wave, controlled shown
Usually assume load L is large (that is it looks like a current source of constant current.
Freewheel diode optional.
Looking at the waveforms is there a freewheel diode?

7. Three-phase – half-wave, controlled
Consider if α=0o (triggered continuously). The thyristor connected to phase with the largest voltage will conduct.
α is measured from the crossover between A and C phases.
Each device will conduct for 120o of the cycle.
α is now delayed. Va is now triggered at (30+α), Vb at (150+α) etc.
Waveforms are as shown in the diagram. 120o periods on conduction. Phase A conducts from (30+α), Vb at (150+α)

8. Three-phase – half-wave, controlled
The phase voltages are
The average output voltage:

9. Three-phase – half-wave, controlled
Note: Source current is square.
Lots of harmonics!
And a DC component (bad for supply transformers).

10. 3-phase, fully-controlled, full-wave
Six thyristors.
Assume load is ‘highly inductive’ – constant current
Thyristor legs are triggered with a trigger angle α

11. Conduction occurs across lines that have the largest magnitude.
Each phase conducts for two periods
- a, two 120o segments
- b, two 120o segments
- c, two 120o segments
Line currents are bi-polar with no dc component but still have high harmonic content.
We can show that the average output voltage is

12. Output voltage waveform
Diagram below shows the output voltage waveform (in bold)
As α increases the area of shaded portion A gets smaller hence the average voltage decreases
If α increases beyond 90o the output voltage goes negative.

13. Bidirectional capability
This shows what happens as α is varied from 0o to 180o.
The shaded portion represents the average voltage.
This can be varied from
The fully-controlled rectifier can also act as an inverter.

14. Bidirectional capability
For example, if α=180o the dc load voltage is negative but has the same polarity as the load current.
Hence the load is providing power to the supply.
This can only happen if (a) there is no flywheel diode present and (b) the dc load has an energy source.

15. Commutation Overlap
Current conduction switched from one diode/thyristor to another in a finite time.
This is as a result of finite circuit inductance.
During phase commutation a short circuit occurs between the outgoing and incoming phases. See below.
This leads to a reduction in the average output voltage which is related to the overlap angle, γ.

16. Rectifier Quality Factors
Ripple Factor
A measure of the quality of the current or voltage waveform.
The RIPPLE FACTOR is a measure of the departure from ideal and is defined
RF = rms value of all alternating components
mean value of wave
eg for a rectifier voltage:
RF = = VAC/VDC =

17. Rectifier Quality Factors
Power Factor and Displacement Factor
In sinusoidal systems the power factor of any load is defined
In rectifier systems, currents are non-sinusoidal. Assuming voltages are sinusoidal then
- the fundamental current component delivers power
- the harmonic currents do not

18. Rectifier Quality Factors
Power Factor and Displacement Factor
Hence
where Φ1=angle between the fundamental current i1 and the supply voltage.
The power factor is now
Distortion Factor
Displacement Factor