Can dissimilar xfrmr secondaries be wired in parallel?

[leadbelly]

I do not think I have seen this information discussed.

Say you have a transformer with 2 dissimilar windings. Can these be wired in parallel? What is the result? One secondary is rated W VAC @ X A, the other Y VAC @ Z A.

[Sch3mat1c]

Depending on phase, you get a current equal to W + Y or W - Y volts across the total resistance of the windings. Current rating has no bearing here, as the only price paid in drawing excessive current from a transformer not rated for it is resistive heating in the wire and severe voltage droop (for the same reason).

(And yes that means you could make a 1kW transformer fit into a wall wart if you had enough liquid nitrogen and a few thousand turns of superconductor wire. The only dissipation would be that of the connecting wires and hysteresis/eddy current losses in the core!)

Tim

[leadbelly]

Shouldn't it be (W+Y)/2 or (W-Y)/2? How else would the case where W=Y (identical secondaries) result in a doubling of the current rating and no change in voltage rating?

[AudioFreak]

Originally posted by leadbelly
the case where W=Y (identical secondaries) result in a doubling of the current rating and no change in voltage rating?

For all intents and practical purposes, that is more or less correct.

[Sch3mat1c]

When W=Y, W-Y = 0 and there is no current flowing between the secondaries. Current ratings will add. If they are unequal, the higher voltage winding ("HV") will supply current to both the lower voltage winding ("LV") as well as the load, until the load becomes significant enough to draw HV's voltage down to LV's level. At this point a positive current will be drawn from LV (before, HV was pushing current into LV), but HV still dumps most of the load current.

I should clarify my earlier post by adding that's why it is a bad idea to parallel transformers. Remember that PT's are transformers as much as OPTs, in case you're paralleling those.

Tim

[tiroth]

The simple answer is no. Even in the case of identically rated secondaries, you can't parallel them unless their values were controlled at the factory to be identical. Otherwise you get enormous circulating currents through the low impedance secondaries.