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Good old Wikipedia scores again

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So if it weren't for Tesla's development of AC that could be trasmitted over long distances, we might all be watching gas-powered televisions by candlelight.

From Alan Belson:

The wikipedia entry above mentions mercury-arc rectifiers. This was the only successful commercially used method of converting ac to dc, [by non-mechanical means], before solid state devices became widely available in large power sizes. They are of course non-reversable. The larger rectifiers shown were mainly used for railroad and traction dc supply, dc being preferred for this purpose, but there were smaller all-glass rectifiers made for large-scale battery charging, as in telephony, small electric vehicles like milk floats, radio transmission, etc.

These devices use the "valve" phenomena of an arc struck in mercury vapor in a vacuum. Current flows predominantly in one direction and is curtailed in the other. It's not 100% rectified BTW, but good enough for traction and charging purposes. The units were usually supplied in pairs, for full wave rectification, and were usually 6-or-more phase designs. This not only smoothed the dc output somewhat, [ rippled dc, of course , ] but also kept the arcs alight as they swept round the chamber in sequence. Although the essential machine has no moving parts, it had to have pumps for the vacuum and for cooling. Plus the media itself, mercury, is highly toxic. Problems associated with harmonics also troubled the users of these machines, and complex resonant shunts were required. They tended toward higher voltages [ 1500v ] for traction as this increased efficiency, there being about a 25v loss across each arc.

Sealed Glass bulb units as low as 110v could be built, when efficiency might be 80% or so- the quoted efficiencies are over 95% at voltages above 550vdc, at full load. The efficiency drops off slightly at idle loads. Power factor was not significant, except as regards the actual transformers providing the supply voltage. The pics are from French Railway installations but these units were all built in the UK . I believe that 'Power Rectifiers Ltd' was a division of Westinghouse. The pics and diagram all date from the mid 1920's.

BTW, there was a weird machine, built and exhibited,in the 1920s, called a Transverter, which was shown at the British Empire Exhibition at Wembley in 1921. It was a "polyphase synchronous rectifier" with fixed windings and revolving brushgear, designed for long range dc transmission. This machine created 100,000 volts dc from an ac supply, but higher voltages would not have been a problem. At the other end of the dc line [ HVDC?] another Tranverter converted the dc back to ac, at whatever frequency was desired. It could thus link 50 and 60 hz generation plant, or indeed any frequency. I'll try and get some pics and data scanned, if anyone is interested.

Alan.