Continuous load current of single phase to single phase cyclo converter Inductive Load
Continuous load current of single phase to single phase cyclo converter Inductive Load:
Fig: Input (a) and output (b) voltage, and current (c, d) waveforms for a cyclo-converter with continuous load current.
- The load current is discontinuous, as the inductance of the load is low. If the inductance is increased, the current will be continuous.
- To repeat, non-circulating mode of operation is used, i.e., only one of the bridges − #1 (positive), or #2 (negative), conducts at a time, but two bridges do not conduct at the same time, as this will result in a short circuit.
- Also, the ripple frequency in the voltage and current waveforms remains same at 100 Hz. The output frequency is one-fourth of input frequency (50 Hz), i.e., 12.5 Hz. So, for each half-cycle of output voltage waveform, four half cycles of input supply are required.
- Taking bridge 1, and assuming the top point of the ac supply as positive, in the positive half cycle of ac input, the odd-numbered thyristor pair, P1 & P3, is triggered after phase delay (θ = ωt = α1) such that current starts flowing the inductive load in this half cycle.
- The current flows for about one complete half cycle, i.e., up to the angle, (π α1) or (π α2), whichever is higher, even after the input voltage has reversed, due to the high value of load inductance.
- In the next (negative) half cycle, the other thyristor pair (even-numbered), P2 & P4, is triggered at (π α2). At that time, reverse voltage is applied across each of the conducting thyristors, P1/P3, and the thyristors turn off.
- The current flows through the load in the same direction, with the output voltage also remaining positive. Also, the current flows for about one complete half cycle, i.e., up to the angle, (π α2) or (π α3), whichever is higher.
- This procedure continues for the next two half cycles, making a total of four positive half cycles. From these four waveforms, one combined positive half cycle of output voltage is produced across the inductive load. The firing angle (α) of the converter is first decreased, in this case for second half cycle only, kept nearly same in the third one, and finally increased in the last (fourth) one.
- To obtain negative output voltage, in the next four half cycles of output voltage, bridge 2 is used. If the bottom point of the ac supply is taken as positive in the negative half of ac input, the odd-numbered thyristor pair, N1 & N3 conducts, by triggering them after phase delay (θ = 4 π α1).
- The current flows now in the opposite (negative) direction through the inductive load, with the output voltage being also negative. The current flows for about one complete half cycle, i.e., up to the angle, (5 π α1) or (5 π α2) whichever is higher, as the load is inductive.
- Similarly, the even-numbered thyristor pair, N2 & N4 conducts in the next half cycle, after they are triggered at (5 π α2). Both the conducting thyristors turn off, as reverse voltage is applied across each of them. Both the output voltage and current are now negative. Also, the current flows for about one complete half cycle, i.e. up to the angle, (5 π α2) or (5 π α3) whichever is higher.
- The above process also continues for two more half cycles of input voltage, making a total of four. From these four waveforms, one combined negative half cycle of output voltage is produced with same output frequency of 12.5 Hz.
- The pattern of firing angle − first decreasing and then increasing, is also followed in the negative half cycle. It may be observed that the load (output) current is continuous as also load (output) voltage. One positive half cycle, along with one negative half cycle, constitute one complete cycle of output (load) voltage waveform.