Maximum transformer output

[RobPhill33]

I am just wondering, how close to the max VA rating can you run a transformer continuously?

[dhaen]

Depends on the "headroom" put in by the manufacturer.
Any reasonable tx should be able to run at max forever at room temperature. Above that, you have to start to de-rate.

You have to determine what "max VA" is:
Running into any rectifier stresses the transformer far above the DC watts you get out.

[jackinnj]

You heat the core. P=R*I^2 by the square of the current drawn -- if you heat enough the insulation may collapse, shorting the transfomre. The transformer will always be "heating" so this is the limiting factor.

The other limiting factor is potential core saturation -- when the core saturates the permeability collapses so you have a potential catastrophic fault situation.

[Pjotr]

A transformer can run at its maximum VA with a resistive load continuously. But manufacturers also usually specify the temperature rise for the VA rating. A temperature rise of 60 deg.C is not uncommon. So if this temperature rise is not allowable you have to de-rate the stated VA rating.

What counts is the RMS current drawn from the transformer. That mainly causes the heat (P = I^2 x R). So if you hook-up a rectifier + large buffer capacitor the RMS current will be much higher than the average current. The average DC current can usually be no more than 1/2 to 1/3 of the current derived simply from the VA rating in such cases.

[Circlotron]

Originally posted by Pjotr
So if you hook-up a rectifier + large buffer capacitor the RMS current will be much higher than the average current. The average DC current can usually be no more than 1/2 to 1/3 of the current derived simply from the VA rating in such cases.

That's a pretty conservative figure. From actual measurements I have made, a cap input filter present a power factor of about 0.6, so that means for a 100VA tranny with a single winding feeding a bridge rectifier you can draw about 60 watts dc, neglecting any loss in the diodes. An accurate way to measure transformer winding temperature rise is to measure the cold resistance and then the hot resistance of the winding. A fan blowing air onto a transformer is a good idea if you want to explore the outer limits of overclocking. Better to hit the magnetic limit first rather than the thermal limit for reasons of insulation life and therefore safety.

/Circlotron - wears both belt and braces when playing with electricity.