home-made neutral

[ericmark]

Think from memory called a reactor? Basicly an auto transformer, three windings connected to a star point is normal, however one centre tapped winding across any two phases can also be used to generate a neutral following the USA hot wire system.

The problem as I see it is how to work out which has been used? The easy way is likely to have a look, fact I know of two, means there may be more, can see how a simple ohm meter could show between two I know of, but there may be others.

Many years ago we had some portable traffic lights the Peak radar heads were replaced with Mulard and I was scratching my head, as in the workshop worked A1, but in the field the radar would not work, it turned out Mulard were using the centre tap earth and used the 55 volt supply, the generator did not have a centre tap so heads would not work.

We were supplied with a centre tapped winding, which had to be mounted inside the traffic light head or a fault could cause head to become live, while waiting for official units I tried using two lamps in series but found really strange voltages, 70 volt per leg but only 110 volt total, some thing inside the radar head was doing strange things.

I was about 19 then 67 now, so can't remember all that went on, but it did introduce me to phase shift. Since then seen it many times, mainly in small generators, capacitors, inductance, used to control output with some odd results.

I have seen enough weird and wonderful designs not to be supprised any more, but also know not to jump in and assume.

 

[AdrianUK]

Think from memory called a reactor? Basicly an auto transformer, three windings connected to a star point is normal, however one centre tapped winding across any two phases can also be used to generate a neutral following the USA hot wire system.

Thinking of the US high-leg delta? The problem with that scheme is that the voltages from the "neutral" to the phases are not all the same. The opposite ends of the winding which have been taped will be 120V w.r.t 'neutral' (for a nominal 240V winding) but the other leg, ie the connection point of the other two delta windings will be 208V w.r.t. 'neutral'.

A zig-zag earthing transformer does have a 'star point' but the windings are a little different from an auto transformer. Each limb has a winding from 1/2 of one phase on it, plus 1/2 a winding from another phase but connected in anti-phase.

 

[SimonH2]

The 3P converter (Transwave IIRC) I have in the garage is an auto-transformer to get 415V from the 240V input, plus some caps to get the third phase. As it's wired, neither of the two lines coming off the autotransformer are directly connected to either the L or N going in - the 240V is connected to a section of the winding "in the middle" - with the result that the output (referenced to input N) is a bit like the 55-0-165V Ericmark describes above.
But since two of the phases are hard linked (via the transformer) to the supply, if you synthesised a neutral then it would in fact have a direct and fixed relationship to the supply neutral - in this case, I'd expect it to be something around the 120V mark but it depends on the exact wiring of the transformer. it would also vary depending on the loading connected since the third phase is only generated by some caps and will wander around depending on the load.