Page 178 - Power Electronics Handbook

170 Static switches

harmonic is approximately equal to the chopper frequency and can be
filtered out relatively easily for high chopping frequencies.
A.C. chopper regulation has two basic advantages over phase-control
techniques:
(i) The speed of response is faster. This is so since a chopper can be
turned on or off at any instant in the a.c. cycle, whereas in
phase-controlled circuits once the power switches have been turned
on they will go off only when the supply voltage has reversed.
(ii) The harmonic content is lower, which is evident when comparing the
load current waveforms given in Figure 8.13 with those in Figure 8.3.
For low voltages the phase-control circuit produces a series of short
current pulses, spaced 180" apart, whereas for chopper regulation the
waveform can still be made to approximate to a sine wave, provided
the chopping frequency is relatively high. Figure 8.15 shows the plot

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0 0.2 0.4 0.6 0.8 1 .o
Duty cycle
F@m 8.15 Power factor curves
of effective power factor, assuming a resistive load, against duty
cycle, which is fLj(rc+ro) for chopper regulators and (n-a)/n for
phase-controlled circuits. This graph shows that the chopper
regulator has a higher power factor below 50% duty cycle, whilst
above this value the phase-controlled waveform is superior. The
difference at these higher voltage settings is not large, so that overall
a chopper regulator gives a better performance.
The disadvantages of chopper regulators are principally:
(i) Higher cost due to more elaborate control and power circuitry.
(ii) Greater radio frequency interference generation, since the chopping
is performed several times per cycle.