58
Electric Motors and Drives A T1 A
T3
T1
T2
T2 B
Vdc
Vdc T3
T4
T4 B
(a)
Figure 2.7
(b)
Single-phase 2-pulse (full-wave) fully-controlled rectiWer
We are naturally interested to Wnd what the output voltage waveform on the d.c. side will look like, and in particular to discover how the mean d.c. level can be controlled by varying the Wring delay angle (a). This is not as simple as we might have expected, because it turns out that the mean voltage for a given a depends on the nature of the load. We will therefore look Wrst at the case where the load is resistive, and explore the basic mechanism of phase control. Later, we will see how the converter behaves with a typical motor load.
Resistive load Thyristors T1 and T4 are Wred together when terminal A of the supply is positive, while on the other half-cycle, when B is positive, thyristors T2 and T3 are Wred simultaneously. The output voltage waveform is shown by the solid line in Figure 2.8. There are two pulses per mains cycle, hence the description ‘two-pulse’ or full-wave. At every instant the load is either connected to the mains by the pair of switches T1 and T4, or it is connected the other way up by the pair of switches T2 and T3, or it is disconnected. The load voltage therefore consists of rectiWed
V dc V dc 45"
135"
Figure 2.8 Output voltage waveforms of single-phase fully-controlled rectiWer with resistive load, for Wring angle delays of 458 and 1358