Page 419 - Power Electronics Handbook

408 Power semiconductor circuit applications

Electronic excitation circuits are small, robust and have fast response
due to the low system lag. If there is a fall in the output voltage of an
alternator, say due to the loss across the machine sub-transient reactance
when an induction motor is switched on, it is necessary to increase the field
current rapidly and therefore raise the output voltage again. This calls for a
field current above the normal excitation level. Power electronic control
circuits are well suited to provide this field-forcing action and the ratio of
maximum to full-load excitation field voltage, called the forcing margin, is
usually three.

14.4 Heating and lighting
Heating and lighting applications are frequently considered together since
they both involve predominantly resistive loads. Several power electronic
circuits can be used, Figure 8.1 already having introduced three variants of
single-phase thyristor controllers, with waveforms for phase angle control.
If the a.c. input supply has an r.m.s. value of V,, and a peak of vpk then
the power PL supplied to the load for a delay angle of a is given by equation
(14.12).

V2
= 2 (n + sin 2a - a) (14.12)

Phase angle control results in a lagging power factor and harmonic
generation, so for high current loads its use is usually not allowed by supply
authorities. If the thermal time constant of the load is relatively high
compared to the input a.c. period, then integral half cycle control can be
used to vary the power to the load, as illustrated in Figure 8.18, although
this technique cannot be used with incandescent lamps due their low
inertia. In this method, if the control switch is on for n half cycles in a total
period of m half cycles, then the power supplied to the load is given by
equation (14.13).

pL=-.- n (14.13)
v:k
2R m
Figure 14.59(a) shows a simple circuit which can be used for phase angle
control of a heating load. The capacitor begins to charge through the
variable resistor once the input supply passes through its zero voltage point
during each half cycle, and when this voltage exceeds the breakover
voltage of diac D1 it conducts and triggers triac TH1. The delay is
controlled by the charging of C and therefore by the value of resistor R.
An alternative circuit for a resistive single-phase load is shown in Figure
14.59(b) where a single thyristor is used within a bridge rectifier. The
capacitor charges through resistor R as before, but since the voltage across
it is d.c. only a silicon unilateral switch D1 is required, the thyristor turning
on when this breaks over. The range of control obtained with both the circuits
given in Figure 14.59 is limited, since the supply voltage has to rise to a