Circuits
Single-phase mercury-arc rectifiers were infrequently used because every time the AC voltage dropped to zero the arc would be extinguished. The direct current produced by a single-phase rectifier contains a varying component (ripple) at twice the power supply frequency, which was undesirable in many applications for DC. The solution was to use 2, 3 or even 6 phase AC power supplies so that the rectified current would maintain a more constant voltage level. Polyphase rectifiers also balanced the load on the phases of a polyphase supply system, which is desirable for reasons of system performance and economy.
Most applications of mercury-arc valves for rectifiers used half-wave rectification with two or more phases sharing a common cathode, and with separate anodes for each phase.
The cathode is connected to the DC load, which in turn is connected to the center tap of an AC transformer, which always remains at zero potential. For each AC phase, a wire from each of the two end taps is connected to an anode "arm" on the mercury-arc rectifier. As the voltage on each anode goes positive, it will begin to conduct through the mercury vapor to the cathode. As the anodes of each AC phase are fed from opposite ends of the transformer winding, one will be positive, and the other negative, and thus a current will always be maintained from one or more positive anodes to the cathode.
For rectification of a single-phase alternating current, two anodes were normally used, each connected to the outer ends of a centre-tapped transformer secondary winding. The resulting circuit is effectively a two-phase, half-wave rectifier.
With three-phase alternating current, three or six anodes were used to provide a smoother direct current. Six-phase operation can improve the efficiency of the transformer as well as providing smoother DC current by enabling two anodes to conduct simultaneously. During operation, the arc transfers to the anodes at the highest positive potential (with respect to the cathode).
In HVDC applications, a full-wave three-phase bridge rectifier or Graetz-bridge circuit was usually used, each valve accommodated in a single tank.
Read more about this topic: Mercury-arc Valve
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