Page 266 - Power Electronics Handbook

256 Forced commutation techniques
The circuit of Figure ll.ll(a) can be analysed using the six comparison
points, as before:
(i) The on time is fiied by the resonant time in the circuit, so that only
variable-frequency operation is possible.
(ii) The on time, caused by resonance, is equal to that given by
(11.10)
where Le is the effective inductance of the series combination of the
load and of L1. This resonant time determines the on period of the
chopper circuit shown. After the main thyristor has turned off
capacitor C commences to discharge through resistor R1 and the main
thyristor cannot be turned on again, to commence the load cycle,
until this voltage has fallen to a sufficiently low value. Otherwise
capacitor C will not charge to a high enough voltage for commutation
and the magnitude of the load current pulse will be reduced.
Therefore the minimum off time of the circuit can be quite large
compared to the on time, giving a low maximum output voltage.
(iii) The current flowing in the load and in inductor L1 both boost the
voltage on the commutation capacitor, just prior to the start of a
commutation cycle, therefore the commutation voltage is increased in
proportion to the load. However, because the voltage on the
commutation capacitor is always reset to as close to zero as possible
before the start of the next cycle, this voltage boosting can be a
nuisance in requiring the capacitor to discharge by a greater amount
during the off periods of THl.
(iv) A commutation failure would generally occur if R1 is of a low enough
value to provide current in excess of the holding current of "HI, and
THI does not turn off after a half cycle, possibly due to C not being
sufficiently discharged from a previous cycle. Under these conditions
THI remains on continuously, supplying load current via R1 and
commutation is not re-attempted. An alternative failure mechanism
is when R1 is large and the capacitor is unable to discharge to a low
enough voltage between the periods when TH1 is off, so that when
the thyristor is fired it can only deliver very small pulses of load
current before its current falls below the holding value, turning it off.
(v) The rating of the main thyristor is not increased by the reset current
of the commutation capacitor, which occurs through the resistor R1.
(vi) There is no low-impedance short-circuit current path across the d.c.
supply in the event of a commutation failure.
The circuit of Figure ll.ll(a) has a very limited maximum output
voltage range, as described in (ii), and is also relatively inefficient since the
commutation reset power is dissipated in resistor R1. Figure ll.ll(b)
shows a modified circuit in which the load is connected across the
commutation capacitor. Once again the capacitor charge circuit must be
underdamped to provide an effective reverse voltage to commutate
thyristor TH1, but now when this thyristor turns off, at the end of its
resonant half cycle, the capacitor is reset by discharge through the load,
rather than an external resistor, so the efficiency of the system is much
higher.