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Early issues with rectification off AC mains

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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0 davidwalker2 over 5 years ago

I have some experience with mercury arc rectifiers, having restored a pair of 1930’s rectifiers with a colleague, and which are now on display operating at Kempton Steam Museum on steaming weekends ( http://www.kemptonsteam.org/history/arc-rectifiers/ ). With appropriate configuration of the supply transformer Mercury arc rectifiers need not generate as much interference as you might think.

My experience is with 6-phase rectifiers. The 6-phase supply is normally derived from a 3-phase transformer with delta connected primary and two star connected secondarys in antiphase with each other. Connecting the star points together gives a 6-phase supply to the 6 arms of the rectifier, the star points being connected to the rectifier cathode. Each rectifier anode conducts in turn, carrying the full load current. Unfortunately this results in a relatively high third harmonic content in the primary current.

To overcome this the two star points are connected via a transformer similar to a centre tapped autotransformer, each star point connected to an end of the winding, the rectifier cathode to the midpoint. The transformer is magnetized by the current flowing in the individual secondary windings, producing corresponding voltages which tend to equalise the voltage between the two groups of phases. The effect is that the voltage at each of the anodes is the instantaneous average of the two secondary groups. The result is that at any time, the voltage at two of the anodes is similar (having been averaged) and thus two anodes conduct at any time, carrying half the load current. Thus the third harmonic is not generated.
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0 davidwalker2 over 5 years ago

I have some experience with mercury arc rectifiers, having restored a pair of 1930’s rectifiers with a colleague, and which are now on display operating at Kempton Steam Museum on steaming weekends ( http://www.kemptonsteam.org/history/arc-rectifiers/ ). With appropriate configuration of the supply transformer Mercury arc rectifiers need not generate as much interference as you might think.

My experience is with 6-phase rectifiers. The 6-phase supply is normally derived from a 3-phase transformer with delta connected primary and two star connected secondarys in antiphase with each other. Connecting the star points together gives a 6-phase supply to the 6 arms of the rectifier, the star points being connected to the rectifier cathode. Each rectifier anode conducts in turn, carrying the full load current. Unfortunately this results in a relatively high third harmonic content in the primary current.

To overcome this the two star points are connected via a transformer similar to a centre tapped autotransformer, each star point connected to an end of the winding, the rectifier cathode to the midpoint. The transformer is magnetized by the current flowing in the individual secondary windings, producing corresponding voltages which tend to equalise the voltage between the two groups of phases. The effect is that the voltage at each of the anodes is the instantaneous average of the two secondary groups. The result is that at any time, the voltage at two of the anodes is similar (having been averaged) and thus two anodes conduct at any time, carrying half the load current. Thus the third harmonic is not generated.
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Early issues with rectification off AC mains

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