Page 421 - Power Electronics Handbook

410 Power semiconductor circuit applications

Three-phase resistive loads can be controlled by using three-phase
bridge circuits, fully controlled and half-controlled systems being shown in
Figure 8.6. The power into the load will vary with the firing angle delay,
and as this delay changes the number of phases to which power is
simultaneously supplied also varies between two and three. For a
star-connected load and a fully controlled bridge, the power is given by
equations (14.14) to (14.16) for various delay angles, vpk being the peak
line voltage, as before.

(14.14)

for 0 < a < 60"
4 1
PL - sin 2or + - cos 2a (14.15)

for 60" < a < 90"

1
1
+ -sin2a - a (14.16)
4
for 90" < a < 150"

Similar equations can be derived for a half-controlled bridge, controlling
a star-connected load, and these are given by equations (14.17) to (14.19),
the delay angles at which the various equations applying now being
different.

(14.17)

for 0 < a < 90"

(14.18)

for 90" < a < 120"

(14.19)

for 120" < a < 210"
Induction heaters are usually operated at a high frequency, in the region
of l-lOkHz, and they present a predominantly inductive load. They are
usually driven from d.c. link inverters, of the type described in Chapter 13.
Because of their high inductance a capacitor can be chosen which, when
connected in series with the heater load, will cause resonance at the
inverter frequency, as shown in Figure 13.18. Not only does this ensure
that the inverter thyristors are commutated by series capacitor techniques,
but a sine wave is also applied to the load.