Total VA Available on Multiple Tap secondary?

[megajocke]

Originally posted by AndrewT
Hi Mega,
your reasoning has lost me.
this statement seems to be at odds with
I thought there was a flux limit beyond which the core became inefficient. More primary current equals more flux due to the extra {current*Turns} product. If the flux limit sets an {I*T} limit then Current I becomes limited as well. That then determines the limiting input VA.
Or am I completely wrong?

No, you are not completely wrong. Yes, you are right in that the primary current increases. But so does the secondary current, and they flow in opposite directions and cancel each other out. For a resitive load, current flows out of the positive secondary end but primary current into the positive primary end. These wires go in the same direction so the currents cancel each other out.

There are some side effects though, the increased voltage drop over the primary resistance causes the magnetizing voltage to drop, decreasing flux a little. This is what prevents catastrophic flux walking in push-pull converters used in for example car amplifiers. I'm not sure in which way leakage inductance changes the flux,but none of these effects should be very big in a normal transformer.

One way of calculating saturation point is to calculate at which current-turn product the core saturates, this is useful for designing chokes and such. The ampere-turn limit of a core increases when you introduce an air gap. The saturation current of a transformer is usually not much bigger than the peak magnetization current. This causes lots of fun stuff, like toroids popping your breakers when power is turned on at the zero crossing as the flux will grow to double the usual value.

Another way of calculating saturation point is to calculate the volt-seconds per turn for saturation. This product does not change when the core is gapped actually. At the same voltage current increases when you gap because of the decreased inductance.