240v Single phase - 3 phase transformer - 480v single phase - RPC & Compressor

I'm interested in running my rpc on 480v in an effort to keep voltage drop on the 75' of input lines to a minimum. I'm considering transforming to 480v before the RPC with a 150kva 3 phase transformer in the 240v single phase supply line. My thought is by placing the transformer near the beginning of the supply line and upping the supply voltage to 480v from 240v I will half the current on the 75'of #2 copper to the RPC.
Right now im running #2 copper about 75' to my 30HP RPC at 240v and that powers a 240v 15HP 3 phase compressor.

I realize that using single phase 240v into 3 phase transformer wont work perfect but will it be close enough if I chose the right tap from the transformer?

A three phase transformer is actually three single phase transformers in a common enclosure. You will just be using one of the three for the setup you described.

Illinoyance said:

A three phase transformer is actually three single phase transformers in a common enclosure. You will just be using one of the three for the setup you described.

Click to expand...

Not so. Most are wound on common 'iron', so they no longer act as three independent 1-P would do. Furthermore, the KVA ratings do not remain the same when only a portion is utilized in that manner. Serious upsizing would be needed.

A proper 1-P step-up is needed instead. High-voltage idler, then 3-P step-down better perhaps than 1-P step down ahead of a lower-voltage idler.

Much better to convert to 3-P with an RPC placed at the head end, use 3-P step up for transmission, then 3-P step down at the load-end.

The two 3-P transformers used as-designed, plus the impedance of the wire, can then actually assist with smoothing and stabilizing the generated leg.

Add a smaller supplementary idler, load end, caps for mid-usage range, and it is about as good as RPC'ed power gets.

But only 75 feet?

Probably far less costly to:

A) JF live with some drop,

B) Place the compressor at the head end, run 75' of large-bore air tubing. Average shop can easily run that footage in addition, anyway

C) pull larger wire, or

D) pull a second circuit, use two RPC's, dedicate only one to the compressor, split the other loads accordingly.

Transformers plus the freight for the heavy buggers are not as cheap as any of those four alternatives.

How much money to keep a 150kva xformer lit, versus buying bigger wire ?

And is the 150kva xformer free ?

I wouldn't think voltage drop on 75' of #2 copper wire would be very significant with that size load... probably <1%? Is your voltage too low at the compressor?

What you propose sounds like a lot of expense and effort for minimal return.

240v RPC - 240v 3P XFMR 480v - 480v XFMR 240v - 240v Compressor

Original thread starter here ...
First thank you for the responses...

Ive learned in my research that a 3 phase transformer used on single phase will not perform as expected (lower output voltage) due to the reduced magnetic flux in the common core, from the lack of the 3rd phase.

Monarchist said:

Not so. Most are wound on common 'iron', so they no longer act as three independent 1-P would do. Furthermore, the KVA ratings do not remain the same when only a portion is utilized in that manner. Serious upsizing would be needed.

A proper 1-P step-up is needed instead. High-voltage idler, then 3-P step-down better perhaps than 1-P step down ahead of a lower-voltage idler.

Much better to convert to 3-P with an RPC placed at the head end, use 3-P step up for transmission, then 3-P step down at the load-end.

The two 3-P transformers used as-designed, plus the impedance of the wire, can then actually assist with smoothing and stabilizing the generated leg.

Add a smaller supplementary idler, load end, caps for mid-usage range, and it is about as good as RPC'ed power gets.

But only 75 feet?

Probably far less costly to:

A) JF live with some drop,

B) Place the compressor at the head end, run 75' of large-bore air tubing. Average shop can easily run that footage in addition, anyway

C) pull larger wire, or

D) pull a second circuit, use two RPC's, dedicate only one to the compressor, split the other loads accordingly.

Transformers plus the freight for the heavy buggers are not as cheap as any of those four alternatives.

Click to expand...

Monarchist,
Thanks for the fresh eyes on this. I like the idea of the 2 XMFR idea. It may possibly allow me to work with what I have on hand. My original thoughts of running the compressor on 480v had me redoing all the breakers, magnetic contact heaters, capacitors and motor winding configurations.
With only running 480v on the longest wire run and XMFR back to 240v at the compressor I get accomplished my main goal.
Heres the "possibly" part .. my second transformer



I dont know if this will work for either of my transformers. I believe it to be large enough (those are 3 - 400A fuses left of the 7up can, on the input to the transformer) but my knowledge of transformers is still limited. This XMFR pictured is from a Robicon VFD and I only have there part number to go by. Looking it over I notice the 3rd winding from the left is slight smaller and without all the taps the other cores have.

Any help in identifying the type of XMFR I have?

Also any links to some good reading on transformer theory and design would be much appreciated.

Impractical said:

I wouldn't think voltage drop on 75' of #2 copper wire would be very significant with that size load... probably <1%? Is your voltage too low at the compressor?

What you propose sounds like a lot of expense and effort for minimal return.

Click to expand...

Impractical,
Your correct a lot of effort but I am also trying to use what I have on hand.
I am also exploring my power factor correction/voltage balance in an effort to minimize voltage drop. In the process of looking into this transformer idea and the knowledge gained along the way I learned while I thought my voltage balance was close enough, maybe its not.
Very little voltage imbalance in 3 phase motors can equal a large current difference. I have only recently got a clamp meter capable of measuring the current draw on the 3 phase legs. Measuring the actual current on the 3 phase legs on the compressor motor with it loaded will be my task before getting into wiring any transformers.

digger doug said:

How much money to keep a 150kva xformer lit, versus buying bigger wire ?

And is the 150kva xformer free ?

Click to expand...

Digger doug,
Also good points .. I'm not sure what the losses are in a 150kva transformer but I know they are more than in a 15kva transformer. Again my transformer knowledge is lacking. I do know copper wire larger than #2 THHN to go the 75' would have cost me more than what I spent on the 150kva XFMR, although not by much. In hind site the 150kva was a "more money than brains moment" where I took advantage of a opportunity thinking I could always sell it if I didnt use it.

dll6 said:

I took advantage of a opportunity thinking I could always sell it if I didnt use it.

Click to expand...

Sell it if you can do, do not duplicate it.

What hasn't been said yet is that large transformers introduce one SERIOUS starting inrush.

You would need at least a minor form of soft-start if you have 'the usual' residential / small shop power - as we must presume to be the case ELSE you'd have 3-Phase and not need an RPC.

Likewise, BIG RPC's can induce 'too much' of the harmonic distortion they ALL create for the comfort of other subscribers - perhaps even your own household - to not grumble to you - or even the powerco - about.

Don't create a monster.

It might be cheaper and more satisfactory all around over the long haul to trade the single, large, compressor for two smaller ones. Just 'stagger start' them, manifold their output. Need air, but less of it part of a day or week? Drop one offline.

Your dick won't get any smaller.

Your power bill, equipment spend, noise level, and Frankenmaintenance headaches should do.

Monarchist said:

Sell it if you can do, do not duplicate it.

What hasn't been said yet is that large transformers introduce one SERIOUS starting inrush.

You would need at least a minor form of soft-start if you have 'the usual' residential / small shop power - as we must presume to be the case ELSE you'd have 3-Phase and not need an RPC.

Likewise, BIG RPC's can induce 'too much' of the harmonic distortion they ALL create for the comfort of other subscribers - perhaps even your own household - to not grumble to you - or even the powerco - about.

Don't create a monster.

It might be cheaper and more satisfactory all around over the long haul to trade the single, large, compressor for two smaller ones. Just 'stagger start' them, manifold their output. Need air, but less of it part of a day or week? Drop one offline.

Your dick won't get any smaller.

Your power bill, equipment spend, noise level, and Frankenmaintenance headaches should do.

Click to expand...

I did not realize that the transformer would add to the inrush, that is info I will take note of and factor into my choices.
This "Monster" is on a 200A residential.
The RPC starts fast enough and seems to be balanced well enough to not notice any distortion but there is a noticeable harmonic distortion when the compressor starts, unloaded of course, that continues till its up to speed.
What fun would there be in downsizing .. I enjoy more satisfaction in putting it all together and making it work than in the finished product.

Anyway .. Who besides the women in our lives wouldnt want a bigger dick if they know they can build one in there shop/mancave, isnt that why were here?

Monarchist your points are valid. My neighbors dont deserve to feel any added effects to power quality by adding transformer inrush to it. Transformer project moved to the back burner in favor of voltage balancing my current setup.

Do you have suggestions on transformer reading material, I would still like to know what type transformer I have, and its possible uses?

dll6 said:

This "Monster" is on a 200A residential.

Click to expand...

Grocery-store Arithmetic, no corrections, etc, that's 200 X 240 = 48000 VA = 48 KVA.

A third of that, and my 15 KVA is as large as I am comfortable with. The refrigerator has to eat, also.

Next go, just use twice the HP as KVA, and you are good to go with a reserve such that the/a/ transformer will not be a barrier to starting a motor.

EG: 7.5 HP => 15 KVA, or one TENTH of the transformer you have now. 'Motor HP' to KVA is a far smaller number, but then you COULD have problems starting.

If copper-wound? Hide it until you can peddle it yourself.

If you're running 460 V, just make sure you're running 600 V wire after the transformer. Don't use the old 300 V stuff.