PAC as a static switch & AC Chopper
Fig: A complete transistorized AC-AC chopper topolgy Fig: Load voltage and current waveforms for an inductive load
- The AC-AC converter has to be augmented with two additional controlled devices clamping the load. A large capacitor across the supply terminals is also to be inserted. These devices which are mostly transistors of the same variety as used for the chopper are necessary to clamp the voltages generated by the switching-off of the current carrying inductors in the load while the input capacitor takes care of the line inductances.
- The harmonics in the line current and load voltage waveforms are significantly different from those with the PACs. Mostly switching frequency harmonics are present in both the waveforms. The output voltage is shown to be about 50% for a 0.5 Duty Ratio chopping.
PAC as a static switch:
Fig: Load voltage and current control with a two-stage sequence control Fig: PAC as a static switch
- Both single phase and three phase PACs are often used as static switches for applications like switching on of highly inductive loads without transients or for regulating output AC voltages by switching in tapings of a transformer. Such sequence control PACs while controlling the output voltage also permit improvement of the power factor as seen by the source.
- Sequence control can be two or multiple phase depending upon the application. The outer TRIACs connected to the higher voltage leads of the input transformer are triggered at the desired angle α, to realize the required load voltage. Obviously this voltage is greater than that available at the low voltage terminal of the transformer.
- This device continues conduction into the next half of the supply voltage till the load current falls to zero. The inner TR2 starts conduction subsequently, requiring a wide pulse or a train of pulses. TR1 can be however triggered by a single pulse.