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Opinions on pushing an 120v PT to 240v

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redruM69

Fri Feb 03 2017, 08:22PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
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Posts: 58

Does anybody have an experience running a potential transformer beyond, or even double its rated voltage? I have this beast in my possession. Its 175:1 21kv. 120v output. I have no doubt the secondary can handle 42kv, as the BIL is rated up to 150kv, but what about the primary @ 240v instead of 120v? I know these things are grossly overbuilt, but they ARE designed for measurement purposes.

Who has pushed the limits on these things?

Hydron

Sat Feb 04 2017, 11:27AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Could just try it i guess? Crank it up on the variac while monitoring the current (preferably with an oscilloscope), see if it begins to saturate as the voltage goes up. 2x rating is quite a bit though!

Mads Barnkob

Mon Mar 06 2017, 09:24AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I agree with hydron, you will have to test this by measurements, we can only guess on the core size and thus we can not calculate the flux density in its normal operation.

Doubling the input voltage will double the flux density.

Dr. Slack

Mon Mar 06 2017, 10:59AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The ratings of a transformer bite in different ways. For instance, you can over-current any transformer as long as you keep the temperature down.

Coil voltage has two limits. There's a fairly hard limit where the core goes into saturation. If this was a power transformer, I could categorically say that no, you cannot over-volt by a factor of 2, as the core will have been designed to run quite close to the saturation limit, for economy. There is also a limit where the insulation is at risk of breakdown.

This is a PT, and might have been designed to run the core at very low flux, perhaps for accuracy, perhaps for power consumption. So it's worth a try. Run the 120v winding from a variac and a series filament bulb as a current limiter and current indicator. Crank it up and measure the current. There will be a knee where the magnetising current will increase rapidly with increasing voltage, choose what magnetising current is acceptable to you.

From the BIL, it looks like you'll have no problem with only a factor of 2 overvoltage.

Hazmatt_(The Underdog)

Fri Mar 10 2017, 02:46AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

A good solid transformer is going to handle 125% of its nameplate rating, and possibly 150% for short duration, but you're asking for 200% and I wouldn't go there.

The low voltage winding is the secondary and its going to get very hot very quickly. The losses are going to increase dramatically as the core saturates, and doubling the primary current is going to cause a lot of heating due to IIR.

Another trouble is if you melt one of the low voltage connections from the excessive current draw, it's impossible to repair without de-potting, and why ruin such a nice transformer?

It's 2KVA, asking for 4KVA is not going to happen, the core won't support it, nor will with windings, it's not built for it.

The low voltage winding is probably 12 AWG, and you're asking for it to handle possibly 40A into a .5 ohm winding, I don't think that's reasonable.

Dr. Slack

Fri Mar 10 2017, 09:23AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

@Hazmatt You seem to be confusing over voltage with over current. Testing a transformer for what voltage it will handle, with a filament bulb in series for over-current limiting and indication, is only an insulation over-voltage breakdown hazard, not an overcurrent links melting hazard. As this is a PT with a huge BIL, we can dismiss the likelyhood of the first with only 200% voltage rated voltage. When the core saturates, the lamp will light, nothing more exciting than that.

The plated VA rating, 2kVA at 30C, will be made up of two components, core loss due to the applied voltage, and copper loss due to the rated current. Once the saturation voltage is has been measured, then the consumption off load will need to be measured at both the rated and the higher voltage to see whether there's still any margin between the increased core loss and heat generation it can handle.

Hazmatt_(The Underdog)

Fri Mar 10 2017, 04:51PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

He wants to put 240V into a 120V winding = blown winding.

That's what he said in the first post, I'm not confused about that.

Go ahead and do it, blow out the winding, and make a really nice doorstop. Probably cost over $2K to replace it too.

Just remember I said it was a bad idea, so its not my fault.

For my PT, 100:1 1500VA transformer I have a step-down transformer 5kw 240:120. I can't really show it off though because I have a 400 lb. Lathe in the way right now and its a total bitch to move by myself.

If I didn't have a step-down I would use a "ballast" and I don't see anywhere he mentions a ballast, he's just gonna put 240 right in and blow out the transformer. there was no mention of sliding inductors, fixed inductors, capacitors, lamps, stove heating elements, or power resistors in the first post as a ballast, just 240 right in.

Dr. Slack

Fri Mar 10 2017, 04:56PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

@redruM69 looks like Hazmatt and I are saying almost the same thing.

He says don't slam 240v into a winding rated at 120v, no way, no sir, no how.

I say don't slam 240v into a winding rated at 120v, wind it up carefully with a Variac and a filament lamp as a series current limiting resistor measuring the magnetisation current to see what the saturation voltage is. You may be lucky, but if not, nothing will go bang, and you will know what voltage you can go to.

You choose.

Sulaiman

Fri Mar 10 2017, 06:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Just connect directly to 240 Vac via a 240 V filament lamp and measure the voltages.
OK other than the e.h.t. danger.

Hazmatt_(The Underdog)

Fri Mar 10 2017, 07:48PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

My prediction with a 240V lamp in series with the transformer:

24V across the transformer, the remainder voltage across the bulb.

I'd like to know what is measured, I am curious.

The transformer will look like a dead short on the low side.

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