208V transformer on 240V

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
234
I have an oddball custom-spec variac with a 208/120V winding.

From a practical standpoint, is there any reason I can't cheat by running it off of a 240V line?

Measured idle current at 208V was 3A. Measured idle current at 240V was 7A. It's roughly a 10kva unit.

This transformer is going to be housed inside of an enclosure with forced ventilation.

I know it is common practice for manufacturers to design motors which are capable of running on 208V or 240V, but I also understand that is generally accomplished by putting the motor into an under-excited state on 208V as opposed to over-exciting it on 240V as would be the analog here.

I really don't want to have to deal with a buck transformer just to run this thing.

Is the extra magnetizing current really going to cause that much trouble here? This transformer will not see continuous load. Probably 60 minutes maximum.
 
Last edited:

be80be

Joined Jul 5, 2008
2,069
That really don't add up it should be less current at 240 volts not more

And can's be 10KVA that would be more like 48 amps at 208 and 41 amps at 240 volts
The no load would be less for 240 volts then 208 volts any way it goes.

Now if it said it was 3 amp no load on 208 and your showing 7 amps on 240 you done shorted something out.
 
Last edited:

crutschow

Joined Mar 14, 2008
29,799
That really don't add up it should be less current at 240 volts not more
Nope.
A transformer's idle magnetizing current goes up with voltage (and way up if the core starts to saturate).
The magnetizing current is determined by the inductive reactance of the primary.
 

strantor

Joined Oct 3, 2010
5,967
I have a 30kVA 3ph transformer I use to get 480V from a 240V rotary phase converter. It's a 120/208V to 480V transformer, so feeding it 240V should result in an output of 554V, but luckily it has 8 taps on the high voltage side from 432V to 503V. I tapped it on 432V taps so the output is 498V, which is within the accepted deviation from 480V nominal.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
234
I gather if I just direct some fan-forced airflow across the two toroids I'll be fine, in light of the fact that this unit is rated for natural convection cooling?

Most datasheets I've seen for heat sinks suggest that forced air doubles the cooling of components compared to convection. I would assume roughly the same rule of thumb would translate to a transformer?

I've seen some big 1MVA convection oil-cooled transformers with 50% continuous overload ratings listed on their nameplates that take effect when optional/retrofit fan kits are installed on their radiators...

The air flow from a 50 watt squirrel-cage blower cooling a rectifier will discharge across the bottom core already in my project. I've got a second blower I could probably direct across the top core.

That ought to be a satisfactory solution, shouldn't it? The cores will simply waste a little extra power as heat that way. I don't imagine 15.3% over-excitation would greatly diminish their regulation characteristics/overheat the brushes or anything like that...
 
Last edited:
Top