Obtaining 460v from a "480 -> 240" ACME Transformer

IIRC, step up / step down transformers do have a few tricks to make them more efficient going one direction. I belive there was some mention of eddy currents and such because one winding was next to the core and the other farther away or something like that.

This was a recent subject of discussion.

I doubt it is more than 10% going the wrong way. Monitor the temperature of the transformer, for your intermittant duty plasma cutter, I'd not be too worried if the numbers work out.

Btw, I've seen 440, 460, 480. Looks like 10% variation isn't a big deal.

As far as starting the motor, I bet it works. We use a lot of transformers to step down

480v to 240v to run cnc equipment. I don't think we are padding the KVA requirement too much.

Fuse the thing and give it a whirl.

Wes

adequate primary power. I don't think 10 KVA is going to do much for a 1500 ampere welder but it might give it a functional test at least.

Be very careful as you are dealing with not only a lethal voltage but also enough power to make things exciting if something goes wrong.

Don Young

Hi Iggy;

Buck boosts run fine either way, "Buck only" won't. Look for a "270 or

277" voltage among the outputs. If it's there, there is a good chance that the high voltage winding has a tap for 277 common with the enclosure and introducing power through the low volt windings will cause a dead short.

If it does have a 277 tap you'll need to cut the internal wires that attach to the case (sometimes as many as 4) and ground only on the input or just run it ungrounded (lots of folks do).

Matt

As you are about to venture into the area of high voltage equipment, if you don't have previous experience in this area, you should be prepared and equipped with the proper supplies and equipment.

It would be a good time to contemplate your earlier concern related to those high voltage capacitors you had at one time.

You should be prepared by going to an electrical supply (not a consumer store hardware department) for properly rated wire and all termination parts.. this includes properly rated switches and fuses too.

A transformer like the one referred to won't shut itself off just because there happens to be a piece of meat between/across the terminals.

Ordinary household wire isn't suitable or safe for use at higher voltages. Jackets on romex-type jackets are marked with a 600V rating, but that only applies as the design/manufacturing specifications rating when the wire is used properly up into the 200+VAC range (not bent, twisted in a knot, tightly kinked or otherwise abused).

A nick in the insulation of a conductor carrying 400+VAC is very likely to cause an arc-over if it gets near another conductor or grounded metal part. This hazardous event might not be so spectacular, but someone may react by flinching/jumping/falling onto or into an object that could cause injury. An observer looking under the hood of a car, someone blows the horn, the observer reacts and bumps head on car hood, sort of scenario.

I'm fairly certain that no one would like to experience a surprise corona taking place in their home or shop.

It would be very wise to not trust the integrity of a cheap DMM or other test/measurement equipment on high votage equipment. High voltages require properly designed test instruments in good operating condition (not de-rated by by improvised repairs). The user needs to understand and respect the capabilities of the test equipment designed for high voltages. Designed for the purpose, not made in an alley with a lot of inferior components, made to look like a real instrument.

I've personally seen poor quality meters that have had internal flash-overs when used on high voltages. I could easily see the outline of the users' fingers on the meter cases and probes.

I don't know what might be more hazardous than having a corona take place in one hand with the other hand holding a probe on a high voltage terminal inside a cabinet full of high voltage connections, with the exceptions of playing with guns or explosives.

Current passing thru the heart is often deadly.

It may make 460 volts, but unless its a 3 phase transformer with 3 phases in and out, the motor likely won't work. Most motors of that size are 3 phase and simply buzz, overhat and burn out when fed single phase.

Grumpy, I was thinking of setting it up as a phase converter.

Wild Bill, all good points, especially about special wire on the 480v side. I will only use wire rated at 600v AC.

i

Most electrical equipment is good for plus or minus 10% minimum. Therefore your

460v equipment should work just f> I have a plasma cutter that is slightly broken (Hypertherm 600 with

Transformers do work in revers just fine.

Probably, but don't miswire it. I would be generous with the fuses, and do the initial tryout using a variac to drive the 240-volt input, creeping up and measuring voltages to see that things are following the script before applying full power.

At that voltage level, you are not allowed to use directly activated switches, and must instead use contactors in grounded metal boxes, and so on. I'm sure we will hear from the electricians on the group about that.

It's possible, if you can get past the startup surge. A pony motor may be needed to spin the big guy up, to reduce the surge.

You can also use a 480-volt VFD to make the three phase from single-phase 480 volt power.

Joe Gwinn

Technically not high voltage. But high voltage relative to what you are probably used to.

WHile this is correct, your corner hardware store will probably have properly rated "termination parts " and wire. May or may not have properly rated switches, fuses and breakers

Untrue. If the wire is marked with 600V rating, then it is suitable for use at all voltage up to 600V, period. Using it "bent, twisted in a knot, tightly kinked or otherwise abused" at 120V and you still have a problem. [And only the last two of those are really a problem anyway]

Not significantly more so that 120/208 or 240,, slightly more so yes. Given the lashup he is contemplating, the same nick at 240 is likely to be significantly more spectacular when it goes up, due to the much larger available fault current. taking place in their home or shop.

MANY of the cheap ones are rated 300V only.

Damn straight

That (finger outline) is unlikely for an internal flash over itself. THat takes an external flash over, usually generated when the user pulls the test leads away from the voltage source, while the internal arc still exists.

I do! It is generating an arc under those same conditions. Even more dangerous is doing that without the proper PPE.

[Corona is a different animal entirely, and is almost nonexistent at 600V and below. It is even unusual at 4160. Common however at 7200 and up. And the more UP you get the more common it becomes.]

jk

There's no prohibition on manually operated switches up to at least

600VAC that I'm aware of, as long as the switch is properly rated. 600V switches are widely available.

I wanted to use switches on the 240v input side.

Completely not the case.

You can use switches up to any level, yes,direct manually actuated ones.

They do get BIG as the voltage gets higher, Size of a hallway closet in the case of 12 and 21 KV. jk

And do not stand in front of the switch when you turn it off. If it is running full load to a reactive device, the arc may blow the cover open.

Paul

I didn't qualify it enough. I'm not talking about substation switches. I heard this from an electrician in Rhode Island.

Another poster mentioned arcs blowing the cover off so one should stand off to one side. Sounds like a job for a contactor in a steel box.

Joe Gwinn

Perhaps he was talking about some requirement other than the NEC. For example, I built equipment for GE's steam turbine division and they had some in-house rules that were considerably stricter than the NEC. The one that comes to mind was a limit of 240V in flexible cords.

I've seen it happen on a 480V motor starter. Pretty exciting.

k

There is no difference form the code point of view. A switch is acceptable for use at all voltages up to it's rated voltage.

2 Issues here, 1 is if it does that it is being used on a circuit where the available fault is greater than it is rated for. [ the kAIC rating] And 2: Contactors fail that way too and much more frequently. [Because they see a much larger number of operations]

jim

jk

The electrician probably had local industrial practice and/or code in mind. He was very definite on the subject, but didn't mention where the rule came from. The conversation was in an old mill building in Pawtucket, RI, that houses a number of small manufacturers.

Hope the cover was screwed down tightly.

This may be the reason for such a local rule.

I've read that there is a big difference between 240 volts and 480 volts

- with the lower voltage, arcs over common insulators will usually self-extinguish, while at 480 they will not extinguish until power is removed.

Joe Gwinn

In that environment they could have been following JIC/NFPA79 guidelines for machinery, which specify that control voltage should be

115VAC. The intent is that switches on control panels, limit switches, contactor coils, etc., operate at 115V. This does not preclude higher voltage at disconnects and manual power switches. These are voluntary standards and are not intended to be applied to premises wiring. If I have a choice, I use 24VDC controls on the equipment I build.

It was before the bang, it was across the room after.