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

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.

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

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

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

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?

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)

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

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

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

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

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.

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.

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.

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.

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 =

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

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