What"s the excitation/magnetizing current one can expect from a large 120-240V stepup transformer?

[SoulFetish]

I just did a PT swap in a Mesa Mark V for a customer moving to the EU. In order to test the amp, we needed to power it up on using a step-up to give us the proper mains voltage. The first one we had, had developed an internal short from the primary to the secondary, which we saw immediately when powering it up on the variac. We ended up finding another one in the basement, which was a huge EI core transformer. I want to say the stack was somewhere around 10"L X 12"H X 5"W (something like that).
Now, it was grubby and looked old, but seemed to work fine. The only thing was that under no load, the idle current seemed to climb up as the voltage across the primary reached 80V RMS. When settled at 120V, the idle current rested at about 1.5A under no load. Other than what seemed to me like an excessive idle current, the voltages and currents with a loaded secondary worked as expected and the testing was completed with no issues.
What do you guys make of this?

[vintagekiki]

When settled at 120V, the idle current rested at about 1.5A under no load.

When the variac is set to 120V, measure its idle current.
1.5A idle current no load it is 180W idle power. It indicates that something is happening with the transformer. It is either defective or wrong connected.
The idle power at transformers is usually 5-10% of the rated power

[J M Fahey]

True, but that is a MONSTER of a transformer,so all things considered ....

And 80/90% of that current is reactive, so not that much dissipative/heating power going on there.

IF really curious,leave it ON, unloaded , and check temperature with your trusty finger-o-meter every 15 minutes or so.

I expect it to get warm at best.

Monster indeed: those core dimensions give me a center leg/core surface of: 10cm*12.5cm so 125 sq cm, which give me a *conservative* rating of over 15kW

Being an autotransformer: twice as much.

No regular consumer stuff, that´s what I´d expect to power a small machine shop or similar which by chance got wrong voltage (imported?) machinery.

Here Electrical Inspectors would not even speak of "watts" or "VA" but straight of "installed horsepower" , it would feed 20HP of electrical motors with adequate safety margin (starting demands, stalling motors,etc.)

[vintagekiki]

We ended up finding another one in the basement, which was a huge EI core transformer.
The only thing was that under no load, the idle current seemed to climb up as the voltage across the primary reached 80V RMS. When settled at 120V, the idle current rested at about 1.5A under no load.

A transformer who spent XY years in the basement under unknown conditions.
The basement opens a lot of questions, in what condition is the wire insulation, interlayer insulation, whether is damaged the electrolytic insulation of sheets core ...
It is quite risky for any professional use.

[nosaj]

i would propose for the next time you run into this . Just wiring up a 220 outlet in your shop. So next time you've just got a 220v line. If time is money .... and who knows you may already have a 220 line in your shop you just didn't notice.
nosaj

[SoulFetish]

i would propose for the next time you run into this . Just wiring up a 220 outlet in your shop. So next time you've just got a 220v line. If time is money .... and who knows you may already have a 220 line in your shop you just didn't notice.
nosaj

Problem is I would want to have use of my variac for this kind of testing. Running it off 220 would saturate the core

[g1]

If JM is right with the 15Kw guestimate, and VK is right with the 5 to 10% idle current, it seems to be running cool.

[Helmholtz]

Transformer magnetizing current is not related to real idle power.

Without knowing the power factor (cos phi) it is not possible to calculate idle power from the idle current.