9.5. Rectifier Circuits—Multiphase
A widely used rectifier circuit, especially for low-voltage, high-current rectifiers, is shown in Fig. 9.4 at the left. In the days before availability of efficient silicon rectifiers and controlled rectifiers, this was the customary circuit. It is called a delta six-phase double-wye, USAS Circuit 45. An interphase transformer is used to allow 120° conduction in the rectifiers. Only a single rectifier is in the series circuit between each transformer winding and the return from the load; hence, the argument goes, it is more efficient than the bridge circuit at the right, a delta six-phase wye, double-wye, USAS Circuit 23, that has two rectifiers in series. But things are really not quite so simple. The rectifier losses are indeed cut in half. However, the circuit at left has transformer secondary windings that conduct half-wave currents and will probably have higher eddy current losses than the transformer at right. The interphase transformer introduces still more losses. Also, the capital cost is increased by the lower kVA efficiency of the secondary windings and the need for an interphase transformer.
Magnetics costs generally outweigh semiconductor costs by a considerable degree, so the circuit selection based on the lower lifetime costs must consider the long-term cost of capital as well as the operational cost of losses. However, this may be a hard sell to many users who have been buying rectifiers for years, because the myth of superiority of the interphase circuit is well established in the industry.
As a compounding factor, the transformer vendors seldom will warrant the efficiency of their transformers with anything but sinusoidal currents, so the additional losses from half-wave conduction and the added eddy currents seldom appear in quotations from them.
9.6. Commutation
Three-phase rectifiers transfer the load current from phase to phase in sequence. The current cannot be transferred instantaneously, however, because of inductance in the supply. The process of driving the current out of one phase and into another is termed commutation, and it always results in a loss of output voltage to the load. Figure 9.5 illustrates the process for a three-phase bridge rectifier. VA, VB, and VC are the line-to-neutral voltages. At time 1, the current is flowing from phase A through the positive bus to the load and returning through the negative bus to phase C. The positive bus voltage is the phase-A voltage. At time 2, the positive phase-B voltage is becoming greater than the positive phase-A voltage, and the current begins to transfer from phase A to phase B. The voltage driving the current transfer is the line-to-line voltage VBA. During this period, the positive bus voltage is the average of phase A and phase B voltages. At time 3, the current transfer into phase B is completed, and the positive bus voltage is now the phase-B voltage.
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The same process of current transfer takes place on the negative bus as well, and the output voltage of the bridge is always the difference between the positive bus voltage and the negative bus voltage. A commutation is taking place every 60°.
Commutations are driven by the voltage difference between the outgoing and incoming phases. Initially, the difference voltage is zero in a rectifier, and the di/dt is also zero. The commutating current can be visualized as a current that circulates from the incoming to the outgoing phase. It is equal to the time integral of the voltage difference divided by the sum of the source inductances in each of the two phases. The circulating current is the current in the incoming phase, and it subtracts from the current in the outgoing phase. Commutation is completed when this current is equal to the load current.
UNIT 4. POWER SUPPLYING