Mechanical Rectifiers.

Gallery opened March 2004

Updated: 19 Apr 2016
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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.


MECHANICAL RECTIFIERS FOR HIGH VOLTAGE

Historical rectifier technology had limits on the voltage it could handle as well as the current. When really high voltages were needed, mechanical recitification was again resorted to. A classic application was energising electrostatic precipitators, which removed fine particles from power station exhaust gases and other chemical processes.

Left: Lodge-Sturtevant rotary rectifier system

Described in the original caption as a 'later development'.

Picture courtesy of Malcolm Richards

I am very grateful to Malcolm Richards for the following information:

"With regard to mechanical rectifiers, you might be interested to know that they were still used in power stations right up to the 1980ís to produce high voltage DC for electrostatic precipitators. Each rectifier had an insulating disc several feet in diameter with contacts on the periphery, fed from a high voltage transformer and driven by a synchronous motor. Currents were in the high milliamp range with voltages up to 60kV. Perhaps surprisingly, contact wear was not a serious problem. They have now, as far as I know, all been replaced by solid state rectifiers. Ozone production was something of a problem, and the rectifier rooms were thoroughly ventilated with electric fans."

"I recall that the rectifiers were fed from HV transformers via induction regulators, and the control circuits stepped up the voltage in increments until flashover occurred somewhere within the precipitator, which was detected and the voltage was backed off a little until the arc was broken. This process was repeated after a brief interval, so that the voltage was maintained at just below the flashover point. which depended, in part at least, on the varying composition of the flue gases. The later electronic controls were much faster acting and more sensitive, and once this had been demonstrated, the mechanical rectifiers were doomed. I worked at Ironbridge power station, and I think the last ones went about 1982. They were very impressive to watch in action, with a strong smell of ozone and the crackling of arcs. The makers of the ones at ironbridge were Lodge-Cottrell of Birmingham, the Lodge of course being Sir Oliver Lodge."

I can report that Lodge-Cottrell are still in the electrostatic precipitation business.


OTHER MECHANICAL RECTIFIERS

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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