Operation of an AC-AC converter with resistive load
Operation of an AC-AC converter with Resistive load:
Fig: Operation of a Phase Angle Controlled AC-AC converter with a resistive load & The rms output voltage and the most important harmonics versus triggering angle α.
- The device(s) is triggered at a phase-angle 'α' in each cycle. The current follows the voltage wave shape in each half and extinguishes itself at the zero crossings of the supply voltage.
- In the two-SCR topology, one SCR is positively biased in each half of the supply voltage. There is no scope for conduction overlap of the devices. A single pulse is sufficient to trigger the controlled devices with a resistive load.
- In the diode-SCR topology, two diodes are forward biased in each half. The SCR always receives a DC voltage and does not distinguish the polarity of the supply. It is thus always forward biased.
- The bi-directional TRIAC is also forward biased for both polarities of the supply voltage. The rms voltage Vrms decides the power supplied to the load. It can be computed as :
- As is evident from the current waveforms, the PAC introduces significant harmonics both into the load and the supply. This is one of the main reasons why such controllers are today not acceptable.
- The ideal waveform is half wave symmetric. However it is to be achieved by the trigger circuits. The controller in trigger circuit ensures this for the TRIAC based circuit.
- While the TRIAC has a differing characteristic for the two polarities of biasing with the 32V DIAC - a two terminal device- triggering is affected when the capacitor voltage reaches 32 V. This ensures elimination of DC and even components in the output voltage.
- For the SCR based controllers, identical comparators for the two halves of the AC supply, which generates pulses for the two SCRs ensures DC and even harmonic free operation. The PAC operates with a resistive load for all values of α ranging from 00
- The fundamental current, if can be represented as