Topaz Iso Trannie Was Running Hot

[jpk9876]

Hi,
I've been a tourist on this site and just registered. It's reassuring to know there's others that run into the same situations as I
I recently acquired a 5kva Topaz for my system and it ran warmer than I liked.
Wired for 120, about 1.5 amp load, 70F room, temp rise to 120F maybe a little more, definitely uncomfortable to keep ones hand on it! And not enough to cook on
I found this thread; Xentek 5kva isolation transformer runs hot! (now I can see the pics!)
and wiring it for 220 it runs as cool (maybe cooler) as my 2.4kva Topaz and a 1kva Xentek. Thanks lms!
I do have a couple of questions, Bob91343 talked about testing for shorts with an inductance bridge, I have a B&K LCR meter with trannie in 120 mode I get 17.7mh across pri. when I short sec. I get 2.5mh. Since its been on all day and cool, I'm probably OK, but would like to verify? In 220 mode I get 65.5mh pri, short sec. changes to 5.1mh.
Last since it's in balanced mode, where do I connect the shield. Normally it's on the N, should I move it to ground? Should I ground the center-tap?
Thanks,
Joe

[Speedskater]

"jpk" is your Midwest near my Midwest?

[jpk9876]

Hi Kevin,
I'm about 550 miles west of you, just outside the "Gateway To The West" I've always thought of the area as Midwest, Even though I'm west of the gateway? If any of this makes any sense?
Joe

[johansen]

Can you hook it up to a watt meter and read the no load loss?


also, the inductance you're reading is not remotely accurate, a 17mH inductor would pull 18 amps from the line... 6.4 ohms.

If you have a varac, about the only way to measure these things is to short the secondary and read the voltage at the primary required to get full load current. You'll then have to measure the phase angle to figure out the leakage inductance.

[Speedskater]

550 miles is a bit far to drive to measure my transformer.

[jpk9876]

Thanks for the reply.
I don't have a watt meter per se, but could probably fake it w/ amp and volt meters? Will an 8-10 amp variac work?
But first what's the difference between an inductance bridge and a LCR DMM?
I just used my LCR DMM to measure a 2.5kva Topaz wired for 120 and get 23.9mh w/ sec open and 1.3mh sec shorted. I rechecked the meter zero and it's ok.
Reading the manual at the mh ranges it uses 1ma @ 1khz.
I just measured a 640 watt toroid for an amp project, 2.3H pri, short second it goes way down to 140uh. A much better ratio?
I went back and remeasured the 2.5kva; 8H open and 1.2mh shorted sec. back to the original 5kva 2h open ,3.3mh w/shorted sec.
How do these numbers sound? I have used the meter for C and R and it appears the contacts for L were flaky from not using, much better after rotating selector sw several times! I'm not sure I totally trust the meter yet for L, I feel better w/ these readings! Although a little frustrated as I did rotate the switch a few times when I first started.
What equation were you using to calculate current draw?
Thanks,
Joe

[johansen]

What your measurement tools show is that without a doubt there are no shorted turns. The toroidal transformer's core has much lower losses, especially at 1khz, which is why it shows 100 times the inductance as does the 40 pound(?) isolation transformer As far as the current goes, if you were to assume the transformer represents an ideal inductor shunted across an ideal transformer, than the no load current would simply be Vac/ (2*pi*60hz*mH) I used the example of a 17mH inductor to show that in real life, the effective value is much higher than that, at least by an order of magnitude. Note that the primary inductance of a transformer doesn't really matter, what matters is the actual power draw in watts, which you will need a watt meter to measure.

[jpk9876]

Thanks johansen,

The calculated power draw for the 2.5 kva in 120 mode, open secondary = 170 watts. (121v x 1.4a) yet this unit stays relatively cool in use?

For the 5kva that started this thread, in 220 mode = 20.5 watts (121v x 0.17a)
In 120 mode 150 watts (121v x 1.21a) this of course rises to 120F over several hours. It is rated for (up to) 900 BTU/hr. About 250 watts.

Why such a difference of current between 120 and 220 on the 5kva?
I'm also wondering why the 2.5kva unit doesn't heat up like the 5kva?

The 2.5kva weighs 58lbs and the 5kva weighs 89lbs

Thanks,
Joe

[johansen]

If what you mean by calculated power draw is inductive reactance from your inductance measurements, then that is reactive power and not "real" this should help you out: Lessons In Electric Circuits -- Volume II (AC) - Chapter 3

[jpk9876]

Thanks for the link.
What I meant by calculated power, since I don't have a power meter, is I measured the no load current draw and multiplied it by the line voltage. I used two DMMs simultaneously.
JK