Quote:
Originally Posted by trobbins
Referring to just "phase voltage" can be a bit ambiguous. For clarity, a delta input with 415Vac refers to the phase-to-phase voltage, and it is that voltage across each primary winding on each core leg (limb).
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Agree! To be absolutely clear, I always ask, for instance, the "3-phase 200V supply" is that 200V line-line, or line-neutral? It is so easy to get things wrong by a factor of √3.
Quote:
Originally Posted by woodchips
Both primary and secondary are three wire Wye connected, so each primary is 230V, each secondary is about 14V.
The transformer is the normal three leg three phase type, each limb with its own primary and secondary. There is no connection to the star points. Each limb has a full wave rectifier, so six rectifiers in total.
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That slightly worries me - with no connection to star point, either primary or secondary, there's nothing to 'fix' the voltage at the star point.
Connected to a rectifier as load (or indeed, any load where the currents don't sum to zero at every instant), the star point gets 'pulled' one way or another so the winding voltages don't stay at 230V each. Some transformers have a set of isolated identical windings, connected in delta, to prevent this. Others, with either pri or sec in delta will self-balance. And of course if the primary star point can be connected to supply neutral, everything is happy (and unbalance currents flow through this star-point connection).
The conundrum, why is there no voltage on the third winding? Think of it by symmetry. Two identical windings on the straight legs of a stadium-shaped core, one going one way, one going the other, energised in series. Everything happy. Voltage divides equally.
Now add an iron 'bridge' with a winding on it, between the bare, curved parts of the stadium. Assume this winding has a voltage induced in it. What will its polarity be? Relative to one winding, you'd get one polarity. Relative to the other, you'd get the other polarity. The only way to reconcile this, is that both are correct, so the induced voltage must be zero.
No flux travels through the bridge, or the third limb, in this scenario.