Step up transformer load calculation.

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John1T

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We have a client that needs to step up voltage from 208 to 480, the compressor fla is 78 amps. I calculate that the run current on the primary would be about 185 amps. What would the start up current be? Thanks John
 
Be proportional with the start up current of the motor which can vary according to motor design.
 
If available, try to get the literature/specs on the motors for the compressor. Those will often have locked-rotor amps of the motor which you can use as the starting amps. If the compressor is existing, the locked-rotor amps may be listed on the motor nameplate. Rule of thumb is six times full-load amps but can be less depending on the motor.
 
John1T said:
If I fused the primary side at 150% of the fla, do you think that I would be ok?
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450.3 would not allow 150% OCP on the primary without secondary protection.
My first suggestion is along the line of hbendillo's post.
Get all the info you can from the compressor manufacturer.. good possibility they have already "invented this wheel" and have a transformer recommendation.
If not, give their info of FLA and RLA to a transformer supplier/manufacturer and let their engineers earn their keep.
Keep in mind you must comply with 450.3, 240.21(C), 250.30 and possibly 250.21(C),
When you are dealing with transformers & motors, especially compressors, it's best not to shoot from the hip.
 
John1T said:
We have a client that needs to step up voltage from 208 to 480, the compressor fla is 78 amps. I calculate that the run current on the primary would be about 185 amps. What would the start up current be? Thanks John
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LRA? Plan on 7x the FLA for NEMA design B but you also must consider inrush which may be 13x but that often varies because of the motor design where energy efficient motors could be quite high.
 
John1T said:
We have a client that needs to step up voltage from 208 to 480, the compressor fla is 78 amps. I calculate that the run current on the primary would be about 185 amps. What would the start up current be? Thanks John
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You already have pretty good answers on your specific question. Nothing else I could add..

Following is based on the xfm will be feeding only the motor - dedicated xfm.

And there are issues with transformers dedicated to a single motor load. primarily transformer energization inrush current, motor inrush and LRC, Voltage drop at the motor during starting.

What size transformer are you looking at? 480V, 3ph, 78A is 65kva. If you are looking at a 75kva, consider the voltage drop. At locked rotor the VD could easily be 30+%. If this is a package unit with a non-nema motor that could easily be a problem. As already suggested, definitely get the mfg motor inrush and LRC data.

Consider a 150kVA, 208D/480Y. This will cut the voltage drop during motor starting in half - The motor will like that.

For a 150KVA, primary (208) FLA is 416A, xfm inrush is as much as 10X, say 4000A.

I don't know the capacity of the 208V system, but you definitely want the transformer on all the time - minimize xfm energizatons. I highly suspect xfm energization will severly blink the lights - and everything else.

With the information we have, I'd select the primary CB at 125% FLA, next size up, 600A, conductors just over 500A.

Without mfg data, consider the motor LRA at 6X, as already suggested - say 1000A on the primary. This will also blink the lights but not nearly as bad as the xfm energization. Delay the motor start for at least a few seconds after xfm energization.

Consider sizing the secondary conductors at 125% motor FLA. Terminate in a listed combination starter suitable for the motor. If it is a package unit, size the conductors to the CB, or MFG direction.

Just some random thoughts - hope it helped.

ice
 
Got the specs on the compressor, Ingersol Rand. They call for a max fusing of 150 amps at 460 volts, the transformer company, Acme, called out a 112.5 kva transformer, with a reverse hookup, which IR approved. This gives me 400 amp protection on the 208 side, I believe. Does this sound about right? Thanks for all the input. John
 
John1T said:
Got the specs on the compressor, Ingersol Rand. They call for a max fusing of 150 amps at 460 volts, the transformer company, Acme, called out a 112.5 kva transformer, with a reverse hookup, which IR approved. This gives me 400 amp protection on the 208 side, I believe. Does this sound about right? Thanks for all the input. John
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Just skimmed through the thread but sounds ok.
 
Before I purchased, I think I'd check with IR and Acme. With a reverse hook up you are going to end up with a 480 delta supply to your compressor. This would mean either ungrounded or corner grounded 480, perhaps not best suited for the compressor..
I might consider ACME TP793714S which I believe will give you a 208 primary with a 480/277 secondary which might be better suited and will simplify grounding.
 
ActionDave said:
Take on off the shelf delta to wye step down transformer and use the other way, float the xo and use the high side of the transformer to run your load.
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I'm sure you are correct. Useless of me to have asked to confirm. But, it did surprise me that a transformer company that offers a 208D/480Y would recommend a reverse wired stepdown. I was hoping the ACME reps were not such dim bulbs.

As augie noted - not a real bright idea. In addition to augie's comments there is an issue of severely increased inrush over a 208D/480Y.

If it was a case of leadtime, there are a dozen other mfgs available.

ice
 
How much of an increase in the inrush? I depended on Acme to spec in the best possible transformer. A large increase in the inrush would be a problem. Not to mention grounding.
 
John1T said:
How much of an increase in the inrush? ...
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I recall an older (1990s) ge paper discussing reverse wired Y primary -D secondary transformers. As I remember, they discussed forwarded wired (D-Y) having 10X inrush 1 out of 6 energizations, and reverse wired (Y-D) having 20X inrush.

Since then I found a newer (2013?)paper citing tests on a ge 75kva, 480D/208Y. The results were 11X forward wired and 37X reverse wired

John1T said:
... I depended on Acme to spec in the best possible transformer. .
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Don't know what to say about that. Still, the following conversation comes to mind:

Me: I have a GE paper in hand that shows reverse wired D-Y transformers have really high inrush - like maybe 37X.

ACME: Oh that's just GE poor design. Our's don't do that.

Me: So no need for me to be concerned about upsizing the xfm feeder?

ACME: Nope you're fine

Me: oookkaayyyy:happysad:

I'm polking fun here. My experience is ACME makes as good a transformer as GE, SQD, Hammond, .....

However, my customers would have a freaking fit if they had a trip on energization and they could still remember the last time it had happened. And I'll be the one receiving the fit - not the xfm seller.

John1T said:
... Not to mention grounding.
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Augie is dead on about the grounding. You will either have a corner grounded 480D, or ungrounded 480D with ground detectors. I don't like corner grounded Delta. It looks really weird the see a 480V disconnect with one of the poles slugged - freaks out the electricians too:eek:

Ground detectors are not difficult. Three 480V, transformered incandescent pilot lights will work. But now you have a failure to check every day.

If you have time, I'd wait for the correct transformer.

ice
 
Have you gotten the specifications on the motor itself?

Is it a 460V motor, or a multiple voltage motor _connected_ for 460V???

In particular, could the motor be re-connected for 230V, and then wired with a buck-boost transformer (much smaller for the same load KVA)...or is the motor perhaps rated for 200-230V with no need for a transformer at all?

I agree with the others that should a transformer be needed, you will be better served by a proper step up transformer than with a 'reverse connected' step down transformer.

The 'input' side of a transformer is the one with the different voltage taps, which lets you adjust for the actual supply voltage present.

The 'output' side of a transformer is the one with the configuration (wye or delta) that makes it easy to get your desired grounding setup.

The winding layout will be designed so that you have lower inrush when feeding the 'input' coils.

-Jon
 
winnie said:
...The 'input' side of a transformer is the one with the different voltage taps, which lets you adjust for the actual supply voltage present.

The 'output' side of a transformer is the one with the configuration (wye or delta) that makes it easy to get your desired grounding setup.

The winding layout will be designed so that you have lower inrush when feeding the 'input' coils.
Click to expand...

Jon - you're on a roll.:thumbsup:
Add a short piece on "Compensated windings".

ice
 
If you put the transformer on the line side of the motor starter, you pay for the unloaded losses in the transformer 24/7.

If you put the transformer on the load side of the motor starter, you have no unloaded losses, but you probably need a bigger starter.

Time for a quick engineering economics study. You will need an estimate of the compressor's duty cycle.
 
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fmtjfw said:
If you put the transformer on the line side of the motor starter, you pay for the unloaded losses in the transformer 24/7.

If you put the transformer on the load side of the motor starter, you have no unloaded losses, but you probably need a bigger starter. ....
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Size 4 on the 480V side, size 5 on the 208V side.

3000A to 12,000A inrush (depending on the xfm) every time the xfm is energized

1100A LRC when the motor is energized.

Hummm .... I suspect I would lean toward putting the starter on the 480V side.

ice
 
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