Polyphase rectifiers
Three-Phase Half-Wave Rectifier:
Three phase half wave rectifier
Figure illustrates a circuit for a 3-phase half-wave rectifier. The three diodes are connected to the three phases of a 3-phase transformer star-connected secondary. Neutral point N of the secondary forms the negative terminal for the load and is earthed, as illustrated in figure. Input and output waveforms are also shown in the figure. The effect of transformer leakage reactance and resistance are neglected as it is the anode forward voltage drop.
Each diode conducts during one-third of the cycle, as obvious from figure. When one diode conducts, the other two remain inactive because then their cathodes become positive with respect to their anodes. This process repeats itself during each ensuing cycle. The dc voltage (between cathode and neutral point) fluctuates between the peak value of alternating voltage per phase VSmax and half of this value i.e. — VSmax (neglecting diode voltage drops).
The above arrangement is very useful if 3-phase supply is available. Even with no smoothing arrangement there is no point at which the rectified voltage falls to zero, as it does acts in frequency of the ac supply. Smoothing, if desirable, is much more easily achieved.
However, dc saturation of transformer core is caused due to flow of direct current of each diode in the transformer secondary phase windings, but it can be avoided bv using zig-zag secondary or three phase full-wave rectifier is shown in figure. Here each diode conducts only for one-sixth of the period i.e. n/3. Output The disadvantages of using a large number
shorter time per cycle and construction number is 6, since if more than 6 diodes out a comparative increase in the output of the rectifier have seen that only one diode operates at phase of the transformer secondary’s and With a transformer having delta-connected line-to-line.
For any polyphase rectifier, the secondary windings of the main supply transformer must be connected so as to provide a neutral point, for this constitutes the negative terminal of the dc output circuit.
Interphase-Transformer
Use of Interphase Transformer:
The factors to govern the number of phases for which a rectifier is to be designed are
(1). low harmonic generation in the output circuit
(2). better transformer utilisation factor
(3). low voltage regulation and
(4). high power factor.
The condition 1 requires that the number of phases should be high, but to fulfil conditions 2, 3 and 4 it is necessary that the number of phases be kept low power factor and low voltage regulation and advantage of 6-phase or 12-phase rectifier of 12-diode rectifier. In this modified scheme m diodes for a rectifier are split into a number diode rectifier. Each diode group has its own individual secondary winding, the star points of the various groups, instead of being directly connected together, are connected through output circuit.
The function of the interphase transformer in the above scheme is to equalise potential effectively working in parallel. Thus, although the output dc voltage waveform has the characteristic of a 6-phase rectifier with its low harmonic content of twice the supply frequency, the load is actually shared by two 3-phase systems operating in parallel at a terminal voltage which is the mean of the terminal voltages of the phases operating together. Each phase of the transformer operates for one-third of the cycle instead of one-sixth and mean output voltage is the same as that of a rectifier with three diodes instead of six. The mean dc voltage obtained is 1.17 Vs rms or 0.827 Vs max less the diode voltage drop which is identical with that obtained from a 3-phase diode rectifier
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