208v transformer with 238v input voltage

I have the opportunity to get a 5 ton hoist for my bridge crane, currently has a 3 ton, but the bridge is rated for 5 tons in its original setting, and my set-up has the bridge spanning 10 feet less.
Anyway, this 5 ton hoist is a 480 volt only motor, so have on hand a 9 kva transformer, but it is listed as 120/208 low side voltage and 480 high side voltage. Am I nuts to attempt to run this transformer with 238v low side? This hoist is never used on a continuous basis, if it runs for 5 minutes at a time would be a maximum.
So what say ye?

Dale Nelson

swellwelder said:

I have the opportunity to get a 5 ton hoist for my bridge crane, currently has a 3 ton, but the bridge is rated for 5 tons in its original setting, and my set-up has the bridge spanning 10 feet less.
Anyway, this 5 ton hoist is a 480 volt only motor, so have on hand a 9 kva transformer, but it is listed as 120/208 low side voltage and 480 high side voltage. Am I nuts to attempt to run this transformer with 238v low side? This hoist is never used on a continuous basis, if it runs for 5 minutes at a time would be a maximum.
So what say ye?

Dale Nelson

Click to expand...

If there is still space between the coils on the 3 leg core, you can wind appropriate sized wire around those legs to add to the number of turns on the secondary. -this might only require 20 feet of wire for about 10 turns on each leg.

The proper thing to do would be to use high temperature magnet wire and appropriately thermally couple it to the existing coil, (meaning several coats of epoxy, polyurethane, etc, etc) but because your application is of exceptionally low duty cycle, you should not have a problem. (Add a few thermocouples if you are curious and if needed add a fan to the transformer case)

Anyhow shoving 238 into a 208 transformer gets you 549 volts on the output.

Your other option is using three 12/24 or 16/32v transformers to subtract 24 or 36 volts from your 238 volt three phase feed. (which as you know is 240v nominal)

swellwelder said:

I have the opportunity to get a 5 ton hoist for my bridge crane, currently has a 3 ton, but the bridge is rated for 5 tons in its original setting, and my set-up has the bridge spanning 10 feet less.
Anyway, this 5 ton hoist is a 480 volt only motor,
....
have on hand a 9 kva transformer, but it is listed as 120/208 low side voltage and 480 high side voltage.

Click to expand...

More to it than just the wrong voltage.

Wrong "type".

AFAICS, when you say 237 VAC, a 230 or 240 VAC "nominal" utility mains service, you do NOT have a 208 Wye 3-P source. RPC's are not Wye. VFD's are not Wye. Phase-Perfect are not Wye. All are Delta.

What you need is not at all unusual or exotic.

Vanilla 2:1 220/230/240 VAC primary, 440/460/480 VAC secondary, Delta-to-Delta step-up-transformer. Preferably with adjusting taps.

A Delta primary, Wye secondary to "re-derive" a Neutral, rather than have a "high leg" with respect to Protective Earth would be my personal pref - I don't need the 120 VAC to Neutral part - but the common ones are 416 VAC Wye, not 480 VAC, so..

Figure out the KVA the hoist motor, plus other, future, 480 VAC loads might need, and seek a nominal 240:480 Delta-Delta transformer.

DONE. Don't screw around with "wishing" the one you have it something it is not, nor Frankenkludging it to pretend it is.

if the OP only has single phase, i would look into wiring that 480v only motor for 277 delta and running it from a 240v vfd.

or use a single phase transformer to feed a 480v 3 phase induction motor as an rpc. you can cancel out most of the reactive current with capacitors and your transformer only need to be rated for the same hp as the motors to be driven.

johansen said:

if the OP only has single phase, i would look into wiring that 480v only motor for 277 delta and running it from a 240v vfd.

or use a single phase transformer to feed a 480v 3 phase induction motor as an rpc. you can cancel out most of the reactive current with capacitors and your transformer only need to be rated for the same hp as the motors to be driven.

Click to expand...

Lots of options, surely.

But this one is a slam-dunk for 240 <=> 480 common-enough 2:1 transformer.

I could even DONATE a 15 KVA I no longer need, save that shipping ~400 lbs Avoir Sterling/Dulles VA to South Dakota would blow any savings clear out of the water.

You don't say whether you have three phase, although it seems likely. If so, a 120/208 rating implies wye connection, not a problem. You don't need a neutral for that application. I usually design a transformer for 15% overvoltage. Running it on 238 would be right up against the tolerance, a poor idea. If you know someone with a 120/208 service, hook to it and measure the voltage on a turn or two wrapped on one of the coils as Johansen suggests. That will tell you how many turns to add. Remember that if the added turns are in series with wye connected coils, the added voltage will be 1.414 times what you measure. For that sort of intermittent service I would just wrap some THHN wire around the coils and be done with it. I have the equipment to vacuum impregnate transformers but I wouldn't bother.

Bll

9100 said:

For that sort of intermittent service I would just wrap some THHN wire around the coils and be done with it.

Click to expand...

That is exactly what I would do. and a bit more turns than necessary to get the no load power loss down into the range of "forget about it"

and the lower voltage on the crane motors means less jerky starts. the stops will be just as fast as the brake, but the lower starting torque might be nice.

Sell the unit you have , buy the one you need.

'couldn't be $100 between them..

Anything other is pissing up a tree.

240 to 480V Transformer

Standard transformers are normally designed for +- 10% voltage variation
At around 15% your going to be driving the core iron into magnetic saturation.
Saturation means excessive heat and wasted energy loss, along with the excessively high output voltage.

208 x 1.1 = 229 Max Input Voltage
208 x 1.15 = 239 Saturation Voltage

238V is at the saturation point as Bill pointed out.
A crane is a tool that you want to keep energized at all times, so it is at the ready.
Leaving the transformer on at saturation voltage isn't good.
The transformer ratio is 480/208 = 2.3
Your output voltage would be 238 x 2.3 = 549
That's pretty high for the crane controls, again 15% high.

Modifying the transformer, by adding turns as suggested, probably won't be possible.
Most 9KVA units are potted wall mount units.

As suggested a 2:1 delta/delta unit is the standard solution, that would leave you with a corner grounded 480V delta, if the output is properly grounded, as is required, for a isolation transformer output.

If the crane has electro-mechanical controls, that probably wont be a problem.
If the crane has a VFD drive, that could be a problem.

Another less common solution, could be some 1Φ transformers, connected in open delta.
Can be cheaper, depending on what is available local to you.
This setup would also give you a corner grounded output, when the output is properly grounded.
It could also be center tap grounded, giving you a wild leg 480 output, not much better, than corner ground.
2) 5KVA units would yield 8.6KVA, similar to your 9KVA existing unit.
Could be smaller possibly, once you figure out what your load really is.

You might be able to mount these 2 units, right on the bridge control panel.
Then you could leave your existing 240V building supply wiring and crane power rails,
depending on size. What you have versus what you need, for the upgrade.

Food for thought...

SAF Ω

Another problem not mentioned... If you have a 240V service, you can't assume your line voltage will always be 238V. The power co. could make line changes that leave you with 260V, and still be within their +\- 10%. My service here is relatively stable at 253V, but drops at night.

Dennis

Thanks for all the info. I have decided to do it the right(easy?) way and just get the proper transformer My sparky says to get a 240 v delta to 480 v delta and things should go a lot better

Dale

swellwelder said:

Thanks for all the info. I have decided to do it the right(easy?) way and just get the proper transformer My sparky says to get a 240 v delta to 480 v delta and things should go a lot better

Dale

Click to expand...

But again, if your crane has a VFD, delta is a risk to the drive. Best would be a 480/277 wye secondary. They exist, just not common.