1 KVA 240 v transformer output 6v,8v,12 v AC how to test if it can supply 83 amps?

[rob]

Hi

I am a total novice in electrics and am not sure were to post this
question?.. i hope some one can help me...I need a transformer that
can supply aprrox 6 Volts Dc at 75 amps..
I have just purchased a 240 v ac input 1 KVA rated transformer its AC
output is 6v ,8v and 12 V..Now i understand that i need to rectify
the current and have just got off ebay a dozen MBRP40030CTL 400A 30V
schottky diodes with low Vf.. i assume that these will be able to
supply a 75 amp current.. at 6 v?..I got them because there rated at
400 amps..and were very cheap!
http://www.onsemi.com/pub/Collateral/MBRP40030CTL-D.PDF

My other question is that there is label on side of the transformer
which is hand written saying that it is rated at 83 amps?..
Label pic
http://www.saspyro.pwp.blueyonder.co.uk/label.JPG

yet when i looked inside i found a large fuse holder with 63 amps at
550v printed on it.?
http://www.saspyro.pwp.blueyonder.co.uk/fuse.JPG

yet the fuse thats attached to it says 40 amps at 550VAC..


http://www.saspyro.pwp.blueyonder.co.uk/Fuse2

Now i am confused ? if the fuse is 40 amps does that mean i have only
transformer that can supply 40 max?..

Inside the transformer there is a board with 1KVA printed on it

http://www.saspyro.pwp.blueyonder.co.uk/inside.JPG

i hope this makes some sense?

Is there any way i can test to see if it can supply 75amps ?

Thanks
Rob

 

[John Fields]

Hi

I am a total novice in electrics and am not sure were to post this
question?.. i hope some one can help me...I need a transformer that
can supply aprrox 6 Volts Dc at 75 amps..
I have just purchased a 240 v ac input 1 KVA rated transformer its AC
output is 6v ,8v and 12 V..Now i understand that i need to rectify
the current and have just got off ebay a dozen MBRP40030CTL 400A 30V
schottky diodes with low Vf.. i assume that these will be able to
supply a 75 amp current.. at 6 v?..I got them because there rated at
400 amps..and were very cheap!
http://www.onsemi.com/pub/Collateral/MBRP40030CTL-D.PDF

My other question is that there is label on side of the transformer
which is hand written saying that it is rated at 83 amps?..
Label pic
http://www.saspyro.pwp.blueyonder.co.uk/label.JPG

yet when i looked inside i found a large fuse holder with 63 amps at
550v printed on it.?
http://www.saspyro.pwp.blueyonder.co.uk/fuse.JPG

yet the fuse thats attached to it says 40 amps at 550VAC..


http://www.saspyro.pwp.blueyonder.co.uk/Fuse2

Now i am confused ? if the fuse is 40 amps does that mean i have only
transformer that can supply 40 max?..

Inside the transformer there is a board with 1KVA printed on it

http://www.saspyro.pwp.blueyonder.co.uk/inside.JPG

i hope this makes some sense?

Is there any way i can test to see if it can supply 75amps ?

First things first...

1. If the transformer is rated at 1kVA with a 240V input, then that
means it's designed to operate with 240V on its primary, and should be
able to supply a total of 1kVA to a load connected to the secondary.
From your photos it appears that the transformer has a single primary
and a single, tapped secondary, so at 1kVa the currents which should
be available from the secondary at the various taps will be:

VA 1000
I = ---- = ------ = 166.6A for the 6V case,
E 6

1000VA/8V = 125A for the 8V case, and 1000VA/12V = 83.3A for the 12V
case, so it looks like the secondary was wound with wire designed to
supply a maximum of 83A out of the secondary, no matter which tap you
use, and that's what that "83" on the label means.

2. The voltage and current written on the fuseholder is what the
fuseholder is rated for, which is no more than 660V across it (at
least it _looks_ like 660V to me) with no fuse in it, and no more
than 63 amps through it with a fuse installed. That means you
shouldn't put anything bigger than a 63 (60) amp fuse in it.

3. The 40 amp fuse that's in there means that the last person who used
the transformer fused it for 40 amps. If you want to fuse it for a
different current you'll need to change the fuse, but if you want to
run it at 75A you'll need to change the fuseholder as well.

4. Since you're allowed 83 amps out of any tap on the secondary you'll
be able to take 75 amps, no problem, but if you need 6VDC there might
be a problem if you use the 6V tap, since you'll be dropping some of
the voltage available from the secondary in the diodes when you
rectify the AC to get DC. On the other hand, if you used the 8V tap
you could still get 75A out of it, but if you wanted 6V out you'd have
to get rid of the extra volt or so with about a 100 watt regulator.

Then there's the question of the filter capacitor. That is, whether
you need one or not. What's your application?


5. Finally, to test it at 75 amps, what you'll need to do is put a
resistance across the 6V tap which will allow 75 amps to flow through
it with 6V across it. That resistor will need to be:

E 6V
R = --- = ----- = 0.08 ohms
I 75A

and it'll dissipate:


P = IE = 75A * 6V = 450 watts while it's connected, and you
should measure the current and the voltage out of the transformer _at_
the transformer while it's being exercised.

 

[Rob]

Hi

I am a total novice in electrics and am not sure were to post this
question?.. i hope some one can help me...I need a transformer that
can supply aprrox 6 Volts Dc at 75 amps..
I have just purchased a 240 v ac input 1 KVA rated transformer its AC
output is 6v ,8v and 12 V..Now i understand that i need to rectify
the current and have just got off ebay a dozen MBRP40030CTL 400A 30V
schottky diodes with low Vf.. i assume that these will be able to
supply a 75 amp current.. at 6 v?..I got them because there rated at
400 amps..and were very cheap!
http://www.onsemi.com/pub/Collateral/MBRP40030CTL-D.PDF

My other question is that there is label on side of the transformer
which is hand written saying that it is rated at 83 amps?..
Label pic
http://www.saspyro.pwp.blueyonder.co.uk/label.JPG

yet when i looked inside i found a large fuse holder with 63 amps at
550v printed on it.?
http://www.saspyro.pwp.blueyonder.co.uk/fuse.JPG

yet the fuse thats attached to it says 40 amps at 550VAC..


http://www.saspyro.pwp.blueyonder.co.uk/Fuse2

Now i am confused ? if the fuse is 40 amps does that mean i have only
transformer that can supply 40 max?..

Inside the transformer there is a board with 1KVA printed on it

http://www.saspyro.pwp.blueyonder.co.uk/inside.JPG

i hope this makes some sense?

Is there any way i can test to see if it can supply 75amps ?

Thanks
Rob

Your transformer is rated at 1000kVA, so at 12V this gives
1000/12=83A, which fits with the label. As this transformer is second
hand, the original owner only needed 40A and so installed a 40A fuse.
After all, your AC mains outlet may be rated at 13A, but that doesn't
stop you fitting a 1A fuse in your table lamp. Remember to mount your
rectifier diodes on a heatsink.

 

[John Fields]

Your transformer is rated at 1000kVA, so at 12V this gives
1000/12=83A, which fits with the label. As this transformer is second
hand, the original owner only needed 40A and so installed a 40A fuse.
After all, your AC mains outlet may be rated at 13A, but that doesn't
stop you fitting a 1A fuse in your table lamp. Remember to mount your
rectifier diodes on a heatsink.

---
Why? take a look at the data sheet and find the curves for Vf VS If.
If he's trying to eke out that last little bit of voltage from the 6V
secondary tap he might want to let the diodes run warm.

BTW, if you had bothered to read the thread before you posted, you
might have noticed that you were only echoing what had been posted
previously.

 

[Fritz Schlunder]

Err... I know you are smarter than this Johnnie boy. At 75A DC output the
diodes will start smoking very quickly without adequate heatsinking. At
around 75A instantaneous forward current the voltage drop at maximum
junction temp. is somewhere around 0.25V, or around 19W per element. Add to
this something vaguely around say 2W for reverse leakage current, and a
heatsink is definitely required.

BTW, if you had bothered to read the thread before you posted, you
might have noticed that you were only echoing what had been posted
previously.

It appears Rob posted from Google Groups which has a large delay between
when people post and when messages appear. It is possible he never even saw
your post before he posted his own message. Cut him a little slack.

 

[Fritz Schlunder]

Hi

I am a total novice in electrics and am not sure were to post this
question?.. i hope some one can help me...I need a transformer that
can supply aprrox 6 Volts Dc at 75 amps..
I have just purchased a 240 v ac input 1 KVA rated transformer its AC
output is 6v ,8v and 12 V..Now i understand that i need to rectify
the current and have just got off ebay a dozen MBRP40030CTL 400A 30V
schottky diodes with low Vf.. i assume that these will be able to
supply a 75 amp current.. at 6 v?..I got them because there rated at
400 amps..and were very cheap!
http://www.onsemi.com/pub/Collateral/MBRP40030CTL-D.PDF

These diodes are well suited to the task at hand.

Is there any way i can test to see if it can supply 75amps ?

If you don't need any kind of filtering and are perfectly content with
really lumpy DC power direct from the bridge rectifier, then yes this
transformer should be adequately capable of producing your 6V(ish) at 75A
DC. Naturally you would have to remove or replace the fuse with something
greater than 75A. Fusing on the primary side is somewhat easier and cheaper
since 250V fuses at a few amps are much cheaper and more common than giant
75A+ fuses (although at your low voltages 30V automotive blade fuses should
suffice). On the other hand a fuse on the primary side would need to have
greater overrating to avoid nuisance fusing since it must contend with
transformer saturation surge currents at initial power up.

If you need 6V DC at 75A DC well filtered, then the situation changes
significantly. The transformer outputs vaguely 6V AC, but when filtered
yields a peak voltage of roughly 8.5V less two schottky diode drops. When
well filtered with a large capacitor, the power factor will be substantially
reduced. Capacitive input filters fed from full wave rectified sinusoidal
sources can have typical power factors of around 0.6. In other words, if
your load still draws 75A DC (at around or a bit less than 8V DC with the 6V
AC tap) with the capacitive filter, the effective heating in the transformer
secondary will be more than what a 6V 75A RMS AC load would produce.

Suppose your load draws 75A DC at 8V but with a powerfactor of 0.6 (due to
input filter capacitor). This means your load uses 75*8 = 600W, but
600W/0.6 = 1000VA which is the maximum rating of your transformer. It isn't
totally clear if your transformer is rated for 1000VA load from any of the
taps, or if it is rated for 1000VA from the 12V tap only, but somewhat less
on the lower voltage taps. The 6V tap has half the turns of the 12V tap, so
if one assumes the length and size of the wire used for the 6V tap is half
that of the 12V tap, then 1000VA at half the voltage is twice the current.
However, P=I^2*R, so for half the resistance (half the length of wire for
the full 12V tap), but twice the current, the power dissipated is double.
On the other hand the 6V winding is probably located closer to the center of
the transformer where the diameter of each loop of wire is smaller, so the
6V tap probably has somewhat less than half the resistance of the 12V tap,
so the heating situation isn't quite so bad.

All and all, it is hard to say for certain if 75A DC is overdoing it a
little bit if you use a capacitive input filter. Without the filter a load
current of 75A DC is fine, but with the filter 75A might be pushing it
slightly over its limits. Long term reliability may be somewhat
compromised, so for peace of mind improved cooling using forced air and/or
reducing the load current to something a bit less than 75A might be
advisable.

What exactly is your application?

 

[John Fields]

These diodes are well suited to the task at hand.




If you don't need any kind of filtering and are perfectly content with
really lumpy DC power direct from the bridge rectifier, then yes this
transformer should be adequately capable of producing your 6V(ish) at 75A
DC.

 

[Fritz Schlunder]

At 75A DC output the
diodes will start smoking very quickly without adequate heatsinking. At
around 75A instantaneous forward current the voltage drop at maximum
junction temp. is somewhere around 0.25V, or around 19W per element. Add to
this something vaguely around say 2W for reverse leakage current, and a
heatsink is definitely required.

Before someone calls me on this oversight, I should call myself on it. In a
full wave bridge rectifier each diode only conducts the load current part of
the time, and only blocks voltage part of the time, so the above theoretical
dissipation figures would probably be closer to half that stated or
somewhere around 10W total. 10W per device is still probably too much for
use without proper heatsinking.

 

[rob]

From: John Fields <[email protected]>

Thanks John for that fantastic reply..I am much more confident now
that i have more information on my PSU...

I am using the PSU to power an electrochemical cell for making
Chlorate/Perchlorate.. Unsmoothed Dc current will work ok for this
purpose....The voltage is not critical ..

You mention that i will need a 0.08 Ohms resistor to produce a 75 amp
load...
I have purchased some 18 gauge Nikrothal 80 wire with a .040"
diameter, 0.4062 ohms per foot..

Am i correct in thinking that i can divide 0.4062 (foot ) by 12 to
get the Ohms per inch?.. which = 0.03385 Ohms (per inch) so i would
need 2.37 inches ?...I intend to use this as a variable resistor to
increase/decrease the current flowing in to the cell..

Thanks

Rob

 

[Lord Garth]

Thanks John for that fantastic reply..I am much more confident now
that i have more information on my PSU...

I am using the PSU to power an electrochemical cell for making
Chlorate/Perchlorate.. Unsmoothed Dc current will work ok for this
purpose....The voltage is not critical ..
<snip?
Rob

Take care with this substance...it is a contact explosive.

 

[Rich Grise]

Sure. Just put a 0.08 ohm, 450 watt resistor on the secondary, and meter
the volts and amps. Or 75 ea. 6V @ 6W light bulbs. ;-)

Fuse the primary at NO MORE THAN 4 amps. At 75 amps on the output, it
should only be drawing - oh, I'll leave that arithmetic as an exercise
for the reader. ;-)

Cheers!
Rich


Cheers!
Rich

 

[Rich Grise]

It appears Rob posted from Google Groups which has a large delay between
when people post and when messages appear. It is possible he never even
saw your post before he posted his own message. Cut him a little slack.

People who post from google groups don't deserve slack.

The _only_ thing google beta has going for it is posts show up right away.
But it's still the person's responsibility to read the thread, and copy/
paste context material.

What they really _should_ do, of course, is use a real newsreader and
their ISP's news server. I've never heard of an ISP that doesn't have
a news server.

Thanks,
Rich

 

[Rich Grise]

Thanks John for that fantastic reply..I am much more confident now
that i have more information on my PSU...

I am using the PSU to power an electrochemical cell for making
Chlorate/Perchlorate.. Unsmoothed Dc current will work ok for this
purpose....The voltage is not critical ..

You mention that i will need a 0.08 Ohms resistor to produce a 75 amp
load...
I have purchased some 18 gauge Nikrothal 80 wire with a .040"
diameter, 0.4062 ohms per foot..

Am i correct in thinking that i can divide 0.4062 (foot ) by 12 to
get the Ohms per inch?.. which = 0.03385 Ohms (per inch) so i would
need 2.37 inches ?...I intend to use this as a variable resistor to
increase/decrease the current flowing in to the cell..

Yeah, use 2 3/8" of wire if you want to start a fire.

6V * 75A = 450 watts. Your wire will disappear, and splash molten
Nikrothal all over the bench and your lap and your face.

Get some fat wire with a R of about 0.01 ohms per foot, and use 8 feet
of it.

Or shop around for a 6V, 450 watt electric heating element.

And fuse the primary at about 2 amps, for the 75 amp test.

Good Luck!
Rich

 

[Rich Grise]

Take care with this substance...it is a contact explosive.

According to a current thread on you don't even
need an electrolytic cell - with the right mixture of reagants, the KClO4
precipitates out of solution on its own.

And, on its own, it's not explosive. It's a very strong oxidizer, but it
still needs fuel. You might be thinking of ammonium perchlorate - that's
pretty nasty stuff.

Good Luck!
Rich

 

[John Fields]

Thanks John for that fantastic reply..I am much more confident now
that i have more information on my PSU...

I am using the PSU to power an electrochemical cell for making
Chlorate/Perchlorate.. Unsmoothed Dc current will work ok for this
purpose....The voltage is not critical ..

You mention that i will need a 0.08 Ohms resistor to produce a 75 amp
load...
I have purchased some 18 gauge Nikrothal 80 wire with a .040"
diameter, 0.4062 ohms per foot..

Am i correct in thinking that i can divide 0.4062 (foot ) by 12 to
get the Ohms per inch?.. which = 0.03385 Ohms (per inch) so i would
need 2.37 inches ?...I intend to use this as a variable resistor to
increase/decrease the current flowing in to the cell..

---
I mentioned using a 0.08 ohm load to test your transformer because if
you place a 0.08 ohm load across a 6 volt source, from Ohm's law
you'll get:

E 6V
I = --- = ------- = 75 amperes
R 0.08R

flowing through the resistance if the source is capable of supplying
that current. Also, I mentioned that the resistor will be dissipating
450 watts, which is a HUGE amount of power for a 2.37 inch long piece
of Nichrome to handle.

There's also another problem, and that's that if you're planning on
using the Nichrome wire as a current limiting resistor, it'll be in
series with the cell, so if you want to get 75 amps into the cell the
sum of the resistances of the cell _and_ the resistor will have to be
equal to 0.08 ohms, which will make the resistance of the resistor
substantially less that 0.08 ohms.

Here's your proposed circuit:

XFMR BRIDGE CELL
+------+ +------+ +------+
MAINS-----|P S|--[FUSE]--|~ +|---[R1]<--|--+ |
| | | | | | |
| | | | | [R2] |
| | | | | | |
MAINS>----|P S|----------|~ -|----------|--+ |
+------+ +------+ +------+


Notice that if the output of the bridge is 6VRMS, in order for 75 amps
to flow through the cell the _sum_ of R1 (the nichrome resistor) and
R2 (the cell itself) must be 0.08 ohms. That means that you'll
probably be dealing with _very_ short lengths of white-hot Nichrome
wire to get 75A into the cell. Not a good idea, for two reasons: The
first is that white-hot Nichrome is dangerous, and the second is that
as Nichrome heats, its resistance increases. You could make things
better by getting longer lengths of Nichrome and connecting them in
parallel, but that's still a messy way way to do it, and if you need
to be changing resistances in the middle of a run it's going to turn
into a nightmare.

What I _strongly_ suggest you do is get a Variac (240V 50Hz in, 0 to
240V out at 5 or 10 amps. Five would work and would be less expensive
than ten, but ten wouldn't have to work as hard.) and use it to
control the voltage _into_ the transformer. Neat, clean, and you can
monitor the input current and get a pretty good idea of what's going
into the cell, although mneasuring the output current would still be
best. Also, using the Variac would allow you to use the 8V tap (or
even the 12V tap) and get away from too low a voltage out of the
bridge because of the diode drops and too low a voltage into the cell
because of wiring.

 

[Roger Johansson]

What they really _should_ do, of course, is use a real newsreader and
their ISP's news server. I've never heard of an ISP that doesn't have
a news server.

You would have been right 10 years ago, but today very few ISP's have a
newsserver, and even fewer run it in a good way, if they have one.

Today most participators in usenet have to find newsservers on their own.

Up till a few weeks ago the individual net university server in Berlin
was the best, but they decided to start taking a small fee, so a lot of
people have to find new free newsservers, or pay.

More and more people are forced to use some web interface, because they
do not know where to find a free newsserver. And many youngsters have no
idea that there is a thing called usenet to begin with. They think
newsgroups is the same as web forums.

 

[The Phantom]

Hi

I am a total novice in electrics and am not sure were to post this
question?.. i hope some one can help me...I need a transformer that
can supply aprrox 6 Volts Dc at 75 amps..

Rob,
Do you have any instruments that can measure currents of ~75 amps
AC? Of 5 amps AC?

Do you have an AC voltmeter?

The tests that others have proposed so far require you to dissipate
power on the order of .5 kw.

The standard method for testing the capability of a transformer
without actually dissipating a power equivalent to the rating of the
transformer is the short circuit test.

You need a variac for this. You dead short the 6 volt output of the
transfomrer. You connect a variac capable of about 5 amps output to
the power line. Turn the output of the variac all the way down to
zero output (THIS IS VERY IMPORTANT). Connect the variac output to
the 240 volt winding of the transformer with an ammeter in series to
monitor the current. SLOWLY turn up the variac until the ammeter
reads the rated current for the 240 volt winding (about 4 amps AC in
this case). You may need to put a (power) resistor of a few ohms in
series with the 240 volt winding of the transformer if the current
comes up too fast (which it undoubtedly will) when you SLOWLY turn up
the variac. (If you don't have any suitable power resistors, use an
electric heater or some light bulbs as resistors). (An even better
alternative would be to follow the variac with a small step-down
transformer. Something with about 5 amps output at 6 or 12 volts.
The variac would power the line-voltage winding and the low voltage
winding would power the 240 volt winding of the 1kva transformer you
are testing.)

When you have the current into the 240 volt winding set to 4 amps AC,
you just wait around and see how hot the transfomer gets. The current
in the shorted 6 volt winding should be about 160 amps. Ideally you
should short the 6 volt winding into a suitably rated shunt and then
you could directly measure the current there.

Without knowing the details of the transformer insulation system, I
can't say what the maximum allowable temperature rise is, but if the
transformer doesn't start stinking too much, you're probably ok. So
far, we would be testing the transformer at its full rated power.

Since you only want 75 amps from the 6 volt winding, you could set the
primary current in this test to 1.875 amps AC. This would correspond
to 75 amps in the 6 volt secondary. Just let it run for a few hours
while monitoring the temperature rise. This will provide a test of
the transformer at the actual current which you will be using.

Rectifying the output of the transformer will cause the current to be
non-sinusoidal which will increase the copper (I^2*R) losses.

You could also connect your rectifiers in whatever configuration you
will use, and short the output from the rectifiers. This will give
you a method of testing the heatsinking of the rectifiers. As before,
turn the variac which is supplying the 240 volt winding ALL THE WAY
DOWN before you start the test. Turn up the variac SLOWLY while
monitoring the current in the 240 volt winding until you reach 1.875
amps AC. You should use a true RMS ammeter when the rectifiers are in
circuit for best results. I wouldn't turn the current up to 1.875
amps right away with the rectifiers in circuit; turn it up gradually
while monitoring the temperature of the rectifiers. If the rectifiers
sizzle when touched with a wet finger, you probably should use some
more heat sinking.

The advantage of the short circuit test method is that you only
dissipate a power equal to the losses in the transformer which should
be under 100 watts for a transformer of this size. (light bulbs or
electric heaters extra) It does not include the core loss, but that
shoud be a negligible part of total losses at nearly full power out of
the transformer.

 

[Lord Garth]

According to a current thread on you don't even
need an electrolytic cell - with the right mixture of reagants, the KClO4
precipitates out of solution on its own.

And, on its own, it's not explosive. It's a very strong oxidizer, but it
still needs fuel. You might be thinking of ammonium perchlorate - that's
pretty nasty stuff.

Good Luck!
Rich

You're right Rich, I was thinking of ammonium perchlorate. Sorry!

 

[Fritz Schlunder]

The advantage of the short circuit test method is that you only
dissipate a power equal to the losses in the transformer which should
be under 100 watts for a transformer of this size. (light bulbs or
electric heaters extra) It does not include the core loss, but that
shoud be a negligible part of total losses at nearly full power out of
the transformer.

Are you sure? When designing a power transformer you get allot of variables
to play with to try to optimize the design. It has been my understanding
that often the most optimum design (for size/weight) makes the core losses
approximately equal to the copper losses while operated under full load.

 

[John Fields]

Rob,
Do you have any instruments that can measure currents of ~75 amps
AC? Of 5 amps AC?

Do you have an AC voltmeter?

The tests that others have proposed so far require you to dissipate
power on the order of .5 kw.

The standard method for testing the capability of a transformer
without actually dissipating a power equivalent to the rating of the
transformer is the short circuit test.

---
AFAIK, the short test is used to measure the copper losses, not the
"capability" of a transformer, whatever that means.
---

You need a variac for this. You dead short the 6 volt output of the
transfomrer.

---
NO!!! If you do you will exceed the current rating of the secondary
and, possibly, damage the transformer. The transformer is rated for
1kVA out of the _entire_ secondary which, at 12VRMS out comes to the
83.3 amps noted on the transformer's faceplate. That is, the
transformer secondary is wound with wire which is designed to carry
83.3 amps no matter which voltage tap is used.
---

You connect a variac capable of about 5 amps output to
the power line. Turn the output of the variac all the way down to
zero output (THIS IS VERY IMPORTANT). Connect the variac output to
the 240 volt winding of the transformer with an ammeter in series to
monitor the current. SLOWLY turn up the variac until the ammeter
reads the rated current for the 240 volt winding (about 4 amps AC in
this case). You may need to put a (power) resistor of a few ohms in
series with the 240 volt winding of the transformer if the current
comes up too fast (which it undoubtedly will) when you SLOWLY turn up
the variac. (If you don't have any suitable power resistors, use an
electric heater or some light bulbs as resistors). (An even better
alternative would be to follow the variac with a small step-down
transformer. Something with about 5 amps output at 6 or 12 volts.
The variac would power the line-voltage winding and the low voltage
winding would power the 240 volt winding of the 1kva transformer you
are testing.)

When you have the current into the 240 volt winding set to 4 amps AC,
you just wait around and see how hot the transfomer gets.

---
That's not really a valid criterion, since the core losses are only
going to be a fraction of what they would normally be with 240VAC on
the primary. Moreover, if you're going to do it properly you need to
monitor the temperature rise of the transformer over ambient and make
sure it doesn't exceed the spec.
---

The current in the shorted 6 volt winding should be about 160 amps.
Ideally you should short the 6 volt winding into a suitably
rated shunt and then you could directly measure the current there.

---
The shunt's an OK idea, but it needs to short out the _entire_
secondary, for the reason given earlier.
---

Without knowing the details of the transformer insulation system, I
can't say what the maximum allowable temperature rise is, but if the
transformer doesn't start stinking too much, you're probably ok. So
far, we would be testing the transformer at its full rated power.

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No, you'd only be exciting the copper, not the core. To determine the
core losses, you'd need to do the open circuit test.
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Since you only want 75 amps from the 6 volt winding, you could set the
primary current in this test to 1.875 amps AC. This would correspond
to 75 amps in the 6 volt secondary. Just let it run for a few hours
while monitoring the temperature rise. This will provide a test of
the transformer at the actual current which you will be using.

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This isn't a good test either, because the core losses will be even
less than they were previously and won't reflect the conditions under
which the transformer will be expected to function in real life.