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Early issues with rectification off AC mains

David Bertenshaw
In 1949 a lighting dimmer was invented that used 3 phase, phase-controlled, half-wave rectification by means of 3 thyratrons. Its output was pulsed DC and was very effective in dimming incandescent lamps. It however was a prodigious generator of harmonics with damaging neutral currents, and had a poor power factor even at full output.

Nowadays such a product would be banned by supply authorities and product regulation, but things were much laxer then. However problems caused by large DC loads such as mercury arc rectifiers on AC supplies, would have been experienced by many practitioners in the 1950s. Can anyone point me to UK standards or custom and practice from that era that might have been known and gave guidance on the problems caused by rectifying AC loads?

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    David, the next public weekend at Kempton is on the 22nd September.  The engine is well worth seeing for it’s massive scale.

    I am an electronics broadcast engineer by career, my expertise on mercury arcs came from a number of sources, mostly textbooks from the 1920’s, handbooks from the Hewitic Company (Hewit was the inventor of the mercury arc rectifier, and his company was a major manufacturer of rectifiers, based in Walton on Thames), various internet sources on specialised things such as ultra violet radiation, but above all the experience of completely dismantling and refurbishing our recifiers.  There appear to be very few people around now that know anything about them.  I have been contacted several times for information from around the country and abroad.

    Incidentally, one curious feature of the interphase transformer is that at low current (say >3 amps, there is not enough flux in the transformer to be effective, so the device works as a 6-phase rectifier (dc out 1.35 times the rms secondary voltage).  At higher currents the transformer is in operation, the device works as a double 3-phase rectifier (dc out 1.17 times the rms voltage).  So as the load current increases the output voltage suddenly falls.

    You might also try posting on the IET Wiring and the Regulations site, https://www.theiet.org/forums/forum/categories.cfm?catid=205&entercat=y .  There are many experts both on regulations, standards, mains-borne interference.

    John, I suspect that the effect of television sets in the 50’s may not have been particularly noticeable. Many would have been powered from non-polarised two-pin plugs, and the line and neutral connections may not have been particularly consistent, balancing out the conduction cycle.  From memory though, many were fitted with valve or selenium rectifiers, and I think these had a higher forward resistance, so spreading the current over a longer conduction period.  I can recall 60’s televisions with the chassis at half mains, and that might imply they were bridge rectifiers.

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    David, the next public weekend at Kempton is on the 22nd September.  The engine is well worth seeing for it’s massive scale.

    I am an electronics broadcast engineer by career, my expertise on mercury arcs came from a number of sources, mostly textbooks from the 1920’s, handbooks from the Hewitic Company (Hewit was the inventor of the mercury arc rectifier, and his company was a major manufacturer of rectifiers, based in Walton on Thames), various internet sources on specialised things such as ultra violet radiation, but above all the experience of completely dismantling and refurbishing our recifiers.  There appear to be very few people around now that know anything about them.  I have been contacted several times for information from around the country and abroad.

    Incidentally, one curious feature of the interphase transformer is that at low current (say >3 amps, there is not enough flux in the transformer to be effective, so the device works as a 6-phase rectifier (dc out 1.35 times the rms secondary voltage).  At higher currents the transformer is in operation, the device works as a double 3-phase rectifier (dc out 1.17 times the rms voltage).  So as the load current increases the output voltage suddenly falls.

    You might also try posting on the IET Wiring and the Regulations site, https://www.theiet.org/forums/forum/categories.cfm?catid=205&entercat=y .  There are many experts both on regulations, standards, mains-borne interference.

    John, I suspect that the effect of television sets in the 50’s may not have been particularly noticeable. Many would have been powered from non-polarised two-pin plugs, and the line and neutral connections may not have been particularly consistent, balancing out the conduction cycle.  From memory though, many were fitted with valve or selenium rectifiers, and I think these had a higher forward resistance, so spreading the current over a longer conduction period.  I can recall 60’s televisions with the chassis at half mains, and that might imply they were bridge rectifiers.

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