safe way to test a hi current low voltage transformer

I always use a nice big heater to limit the current of things that could go bang. You could also use a multimeter to check that none of the windings are shorted to the metal core of the transformer or are open circuit.

ok i understand that the lamp or heater would limit current safely but will i be able to check the voltages ? I thought of using a lamp in series, but i thought the voltage drop across the lamp should be close to line voltage not leaving any for the transformer.

Connect a large incandescent lamp (say, a 250 watt brood lamp) in series with the primary. It will both limit the current, if the transformer is defective, or you misunderstand its retings, and indicate about what the line current is by its brightness.

Measure the primary voltage at the transformer and the secondary voltages. If nothing draws big current, you can expect those ratios to hold when full line voltage is applied. For instance if the primary ends up with 103 volts after the lamp (or toaster) and the secondary shows 14.3 volts, unloaded, you can assume that at full 120 line volts, the secondary will produce 14.3*120/103 = 16.7 volts, unloaded.

You can still check the voltages. With a lamp in series with the inductance of the transformer, there will be a phase shift, but I don't think you'd need to worry about that for preliminary tests like this.

I assume you have an AC voltmeter.

Put the lamp in series with the primary, and the line across them. The lamp shouldn't light, at least not very bright at all - with no load, the transformer is a big inductor, which should have considerable reactance.

Anyway, while that's sitting there, take your voltmeter and measure the primary voltage - that's at the primary windings themselves, the "hot" lead of the voltmeter should go to the junction between the lamp and the primary, and the "neutral", of course should go to neutral. I ass-u-me that you've put the lamp on the hot side of the line.

Record that voltage. That's the primary voltage for these tests.

Now, just meter the voltage at any secondaries, and the turns ratio is left as an exercise for the reader. Gaw! I LOVE saying that!!!!

Cheers! Rich

I use variacs for this purpose. I connect the output of the variac to the input of the transformer. I monitor the current into the primary and the output voltage of the transformer. I slowly crank up the voltage and and watch what happens. I then apply a load, suitable for the observed output voltage, and repeat. Do this for all output windings.

Al

at a local flea

a lot of current, It has several

volts @ 100amp output.

the wiring is

current? I thought of using a small

could check the voltages & figure out the taps without blowing

I use variacs for this purpose. I connect the output of the variac to the input of the transformer. I monitor the current into the primary and the output voltage of the transformer. I slowly crank up the voltage and and watch what happens. I then apply a load, suitable for the observed

output voltage, and repeat. Do this for all output windings.

Al