safe way to test a hi current low voltage transformer

[[email protected]]

I picked up a very large used stepdown transformer at a local flea
market, 120 v primary
looks like 12 ga wire so it will draw a lot of current, It has several
primary side taps to give
8-10-14-18-15 volts @ 100amp output.
Other than just pluggin it in and hoping it dosent blow the wiring is
there a way to check the
transformer with limiting the current? I thought of using a small
isolation transformer hooked
to so i could check the voltages & figure out the taps without blowing
something up.

 

[John Popelish]

I picked up a very large used stepdown transformer at a local flea
market, 120 v primary
looks like 12 ga wire so it will draw a lot of current, It has several
primary side taps to give
8-10-14-18-15 volts @ 100amp output.
Other than just pluggin it in and hoping it dosent blow the wiring is
there a way to check the
transformer with limiting the current? I thought of using a small
isolation transformer hooked
to so i could check the voltages & figure out the taps without blowing
something up.

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.

 

[tomw]

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.

 

[[email protected]]

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.

 

[John Popelish]

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.

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.

 

[Al]

I picked up a very large used stepdown transformer at a local flea
market, 120 v primary
looks like 12 ga wire so it will draw a lot of current, It has several
primary side taps to give
8-10-14-18-15 volts @ 100amp output.
Other than just pluggin it in and hoping it dosent blow the wiring is
there a way to check the
transformer with limiting the current? I thought of using a small
isolation transformer hooked
to so i could check the voltages & figure out the taps without blowing
something up.

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

 

[Al]

In articl

I picked up a very large used stepdown transformer at a local fle
market, 120 v primar
looks like 12 ga wire so it will draw a lot of current, It ha severa
primary side taps to giv
8-10-14-18-15 volts @ 100amp output
Other than just pluggin it in and hoping it dosent blow the wirin i
there a way to check th
transformer with limiting the current? I thought of using a smal
isolation transformer hooke
to so i could check the voltages & figure out the taps withou blowin
something up

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 primar
and
the output voltage of the transformer. I slowly crank up the voltag
and
and watch what happens. I then apply a load, suitable for the observe

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

A

 

[Rich Grise]

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.

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