Mechanical Rectifiers.

Updated: 11 Mar 2004
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Before the invention of semiconductors, rectification at high currents involved serious losses.

There were various vacuum/gas devices, such as the mercury-arc rectifiers, thyratrons, ignitrons, and Vacuum diodes. Solid-state technology was in its infancy, represented by selenium and copper oxide rectifiers. All of these gave excessive forward voltage-drop at high currents.

One answer was mechanically opening and closing contacts, if you could do it fast and cleanly enough. The wonderful machine shown below was designed by Read and Gimson et al, at BTH Rugby in the early 1950s. It is a three-phase mechanical rectifier working at 220V and 15,000 Amps, and its application was the powering of huge banks of electrolysis cells.

The central shaft was rotated by synchronous motor, driving an eccentric with a throw of about 2mm. (0.077 inch) Push-rods from this operated the contacts. The timing was critical, and was adjusted by rotating the position of the eccentric on its shaft, and by sliding wedges between the eccentric and push-rods.

Crucial to this system were the commutating reactors, inductors that ensured the contacts closed when the voltage across them was small, and opened when the current was small. Without these, contact wear would have intolerably heavy.

This machinery was undoubtedly successful; its efficiency was determined to be 97.25%. Contact life was never fully determined but considerably exceeded 2000 hours. However, the rapid development of the silicon diode made it ultimately a technological dead-end.

The Dynamo.
Any dynamo contains what is effectively a mechanical rectifier in the shape of its commutator. These are too well known to need describing here.

The Synchronous Vibrator.
In more innocent times, the word "vibrator" meant only one thing to the radio engineer. It was a way of powering valve car-radios from a car battery.
A vibrator was a small electromechanical device rather like an electric bell, that chopped up the 12V DC so it could be applied to a transformer and stepped up to a suitable HT voltage. More sophisticated types, known as synchronous vibrators, had an extra pair of contacts that rectified the power.

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