Single Phase Half Bridge Cycloconverter in Non Circulating Current Mode (With R Load)
- When operated in the noncirculating current mode, only one converter (either P or N) is ON at any given instant of time. Thyristors from both the converters won’t conduct simultaneously.
- The load voltage waveforms for a single phase cycloconverter with resistive load are as shown in Figure B.
- As can be seen, the SCRs from bridge 1 are turned on to produce positive voltage across the load while the SCRs from bridge 2 are turned on to produce a negative voltage.
- Each bridge conducts for three half cycles of input supply thereby dividing the input frequency by 3 . The sequence of triggering the SCRs for various output frequencies is as shown in Figure C.
- In the noncirculating current mode, SCRs from only one bridge are triggered at any given instant. The firing pulses to the SCRs from the other bridge are “blocked”.
 |
| Figure A |
 |
| Figure B |
 |
| Figure C |
- The waveforms shown in Figure b and c are for the noncirculating current mode.
- The bridge-1 in Figure A is also called as the P-converter because thyristors from this converter is produced a positive instantaneous voltage across the load.
- Similarly bridge-2 in Figure A is called N – converter because SCRs from this converter produce a negative instantaneous voltage across the load.
Gating sequence :
- At a time to thyristors from either P-converter and N-converter conduct simultaneously as son in Figure B and C.
- For f0
= 1/3 fin the gating sequence of Syristos is shown in Table A.
Table A : Gating sequence of thyristors for f0 = 1/3 fin
|
Half |
1 |
2 |
3 |
4 |
5 |
6 |
|
Conducting |
1,2 |
3,4 |
1,2 |
7,8 |
5,6 |
7,8 |
|
Converter |
P |
P |
P |
N |
N |
N |
Output frequency (fout) :
- As seen from Figure B one cycle of output voltage waveform of the coconverter consist of three positive and three negative half cycles.
- Therefore the one cycle period of the output is three times more than that of the ac input. Therefore the output frequency fout is one third of the ac supply frequency
fout
= 1/3 fin
= 1/3 fin
- In general if the input frequency is to be divided by N” (N is an integer) then “N” positive and “N” negative half cycles should be produced across the load by triggering the SCRs in a proper sequence.
- The output voltage waveforms to divide the frequency by 2 and 4 are as shown in Figure C.
- From the waveforms shown in Figure B and C it is clear that the waveform at the output of the cycloconverter is not sinusoidal in nature. In fact with reduction in the output frequency, it tends to be a square wave.
Gating sequence :
- The gating sequences of SCRs for f0 = 1/2 and f0 = 1/4 fin are as shown in Table B and C respectively.
Table B : Gating Sequence for thyristors for f0 = 1/4 fin
|
Half |
1 |
2 |
3 |
4 |
5 |
6 |
|
Conducting |
1,2 |
3,4 |
5,6 |
7,8 |
1,2 |
3,4 |
|
Converter |
P |
P |
N |
N |
P |
P |
Table C : Gating Sequence for thyristors for f0 = 1/2 fin
|
Half |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
1 |
|
Conducting |
1,2 |
3,4 |
1,2 |
3,4 |
5,6 |
7,8 |
5,6 |
7,8 |
1,2 |
|
Converter |
P |
P |
P |
P |
N |
N |
N |
N |
P |
Reduction in output harmonics :
- We can observe from the output voltage waveforms of Figure B and C, that the cycloconverter output voltage contains harmonics. The expression of PF is as follows :
PF
= cos θ [1/ π (π – α + sin 2 α/2)]1/2
- This shows that the PF of a cycloconverter circuit is poor for large values of a i.e. for low output voltages.
- The output voltage of cycloconverter is basically made up of segments of input voltages and the average value of a segment depends on the value of firing angle a for that segment.
- If we vary the firing angle of segments is varied in such a way that the average values of segments corresponds very closely to the sinusoidal variation of output voltage, then it is possible to reduce the harmonic contents in the output voltage.
- Because the average output voltage is a cosine function of the firing angle, we can generate the firing angles for segments by comparing the cosine wave at line frequency with an ideal sinusoidal reference voltage at the output frequency which can produce the output voltage waveform of Figure D.
- Note that due to progressive change in firing angle we get an output voltage which is very close to the sinewave.
Output voltage control :
- The ac output voltage of the cycloconverter can be changed by varying the firing angle of the thyristors.
- With the resistive load each SCR conducts for a duration of (π – α) and turns off naturally at the end of each half cycle.
Why Is Inductance L used ?
- By mistake if two SCRs from the two converters are turned on simultaneously (eg. S1 and S6 or S2 and S5) then the supply voltage will be short circuited.
- The inductance L is introduced in the circuit to avoid this possible short circuit.
Figure D
