03-07-2012, 04:30 PM
AC to AC Voltage Converters
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Introduction
In the last lesson − first one in the second half of this module, firstly, the basic principle of operation of the cyclo-converter circuits has been presented. This followed by the discussion of the circuit, and the operation of the single-phase to single-phase cyclo-converter circuit with both resistive and inductive loads, in detail. Two full-wave bridge converters (rectifiers) connected back to back, with four thyristors as power switching device in each bridge, are used. Also described are the advantages and disadvantages of the cyclo-converter. The dc link converter is introduced briefly, along with its advantages and disadvantages.
In this lesson − the second one in the second half, firstly, the three-phase to single-phase cyclo-converter circuit, using two three-phase full-wave thyristorised bridge converters, is presented. Then, the operation of the above cyclo-converter circuit, with both resistive and inductive loads, is described in detail, along with voltage waveforms. The mode of operation used is the non-circulating current one. The following are discussed in brief −.the circulating current mode of operation for the above, and also the cyclo-converter circuit, using two three-phase half-wave converters.
Keywords: Three-phase to single-phase cyclo-converter, Voltage waveforms, Non-circulating current, and Circulating current modes of operation, Three-phase full-wave bridge, and half-wave converters.
Three-phase to Single-phase Cyclo-converter
The circuit of a three-phase to single-phase cyclo-converter is shown in Fig. 30.1. Two three-phase full-wave (six-pulse) bridge converters (rectifier) connected back to back, with six thyristors for each bridge, are used. The ripple frequency here is 300 Hz, six times the input frequency of 50 Hz. So, low value of load inductance is needed to make the current continuous, as compared to one using single-phase bridge converters described in the previous lesson (#4.4) with ripple frequency of 100 Hz. Also, the non-circulating current mode of operation is used, where only one converter − bridge 1 (positive) or bridge 2 (negative), conducts at a time, but both converters do not conduct at the same time. It may be noted that each thyristor conducts for about )3/(120π°, i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 1& 2 conduct for about )6/(60π°, i.e., one-sixth of a cycle. The thyristors conduct in pairs as stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, which will result in short circuit at the output terminals. The sequence of conduction of the thyristors is 1 & 6, 1 & 2, 3 & 2, and so on.
Circulating Current Mode of Operation
In all the cases described earlier (Lesson 29 and current one (#30)), both for single-phase to single-phase and for single-phase to three-phase cyclo-converters, circulating current-free or non-circulating current mode of operation was described, wherein only one of two bridge converters conducts at a time, but not both, in which case the converters would be short-circuited. The positive (P) converter conducts, when the current is in the positive half of the cycle, whereas the negative one conducts with the current flowing in the negative half. But, in this case, i.e. circulating current mode of operation of the cyclo-converter, both the converters would conduct at a time, with an inter-group reactor (IGR) between the positive and negative groups as shown in Fig. 30.4. It may be noted that, though the output voltages of two converters in the same phase have the same average value, but their output voltage waveforms as a function of time are, however, different, and as a result, there is a net potential difference (voltage) across two converters.
Cyclo-converter, using two three-phase half-wave converters
A three-phase to single-phase cyclo-converter, using two three-phase half-wave converters, is shown in Fig. 30.5. The principle of operation is same here as described earlier. Each thyristor conducts for around )3/2(120π⋅°, with the thyristors in each converter triggered in sequence, i.e. 1, 2 & 3, whether it is P-type or N-type. It may be noted that the thyristors, 1, 2 & 3 are connected to the phases, A, B & C, respectively, in series with the load impedance, as shown in Fig. 30.5. The ripple frequency is 150 Hz, three times the input frequency of 50 Hz, as this converter is a three-pulse one. So, the inductance in the inductive (R-L) load must be high, as compared to one used in the earlier case, to make the current continuous. This inductance acts as the filter for the output (load) current. The mode of operation here is non-circulating current one. It may be noted that harmonic content, both in the output voltage and current waveforms, is higher than those present in the earlier case using two three-phase full-wave bridge converters.