Transformer for 480 volts to 380 volts

[pick]

We are installing some European built equipment in a juice plant and need a 100 kva transformer to step
480 volt three phase down to 380 volt three phase. Transformers of this rating usually have a long lead time.
Would there be anything wrong with using a three phase transformer with a 600 volt primary and a 480 volt
secondary? With 480 on the primary, the secondary should produce 384 volts. Would the lower excitation
current cause any undesirable changes in performance?

 

[GoldDigger]

The efficiency will be lower since the ratio of iron to copper losses will not be the same as with the design voltage applied. But that will be a small effect as long as you do not undersize the transformer. The actual KVA will be less than the nominal, and that will affect your size choice.

 

[mike_kilroy]

Transformers of this rating usually have a long lead time.

Would there be anything wrong with using a three phase transformer with a 600 volt primary and a 480 volt
secondary?

As a xfmr supplier, I often here that; but it is all relative.... What does 'long lead time mean to you? 2 days? 2 weeks? 20 weeks? What can you live with?

If your juice shop is in your garage as a hobby, then I'd say go for it.

If not, I wouldn't touch that with a 10 foot pole! Now for the short time you wait for the real McCoy proper xfmr to come in, maybe... who is going to take on the responcibility for supplying 100kva incorrectly 2 years from now?

 

[jim dungar]

In the old days we used standard 480:120V transformers connected in a buck-boost arrangement. But something in the back of my head says this voltage change may no longer be NEC compliant.

Look at Page 10 in this app guide.
http://static.schneider-electric.us...mers/Energy Efficient-NEMA TP1/7400HO9501.pdf

I believe there are some 240 x 480:24/48 buck-boost connections that might get you to 400Y/231, but not in the 100kVA range.

 

[Jraef]

Why are you transforming in the first place? If it is just for AC motors, DO NOT use the transformer!

Motors work correctly (by which I mean producing torque) based on the design ratio of voltage and frequency, plus or minus 5-10%. A motor designed for 380V 50Hz, as commonly used in the EU, is therefore designed for a V/Hz ratio of 7.6:1. If you supply it with 480V (which is what a 460V motor is designed for), the ratio is 8:1, virtually the same. the motor will spin 20% faster because of the difference in frequency, but that can't be helped, you bought 50Hz equipment.

If however you transform down to 380V, that is now 380V 60Hz, so the V/Hz ratio is now 6.33:1, well OUTSIDE of the tolerance band of the motor design! Your motors will have a loss of peak torque equal to about 30% of rated torque, so they will work harder, draw more current and over heat.

If there are transformers inside, similar issues will be in play regarding the V/Hz ratios. My suggestion is to get hold of an internal wiring diagram and investigate EACH part of the machine system BEFORE deciding how to approach it.

 

[kwired]

Why are you transforming in the first place? If it is just for AC motors, DO NOT use the transformer!

Motors work correctly (by which I mean producing torque) based on the design ratio of voltage and frequency, plus or minus 5-10%. A motor designed for 380V 50Hz, as commonly used in the EU, is therefore designed for a V/Hz ratio of 7.6:1. If you supply it with 480V (which is what a 460V motor is designed for), the ratio is 8:1, virtually the same. the motor will spin 20% faster because of the difference in frequency, but that can't be helped, you bought 50Hz equipment.

If however you transform down to 380V, that is now 380V 60Hz, so the V/Hz ratio is now 6.33:1, well OUTSIDE of the tolerance band of the motor design! Your motors will have a loss of peak torque equal to about 30% of rated torque, so they will work harder, draw more current and over heat.

If there are transformers inside, similar issues will be in play regarding the V/Hz ratios. My suggestion is to get hold of an internal wiring diagram and investigate EACH part of the machine system BEFORE deciding how to approach it.

And if you run it on say a VFD with 50hz 380 volt output the motor sees exactly what it is designed for:happyyes:

 

[Jraef]

And if you run it on say a VFD with 50hz 380 volt output the motor sees exactly what it is designed for:happyyes:

Yes, of course, but then again if there are other loads that are NOT motors, a VFD is not the solution for those!

 

[Besoeker]

the motor will spin 20% faster because of the difference in frequency, but that can't be helped, you bought 50Hz equipment.

But if there are pumps involved, you could be in the brown sticky stuff.

My suggestion is to get hold of an internal wiring diagram and investigate EACH part of the machine system BEFORE deciding how to approach it.

Totally agree. There is no one size fits all solution when you have different frequencies and voltages.

 

[pick]

In my original post I forgot to say that a neutral is also required on the secondary, I should have said 380/ 220 volts.

The need for 380/ 220 is based on the prints provided by the contractor who is installing the machine. I am presently discussing alternatives with them. The machine in question has over a dozen motors, some of which are already on VFDs, and some across the line, as well as some heater loads.

As the only electrician in a plant that employs seven mechanics, I am just now working on what I should have gotten done a week ago. This is "early due diligence" but in a belated fashion. I will try today to go over the machine circuits with them.

I am told that this line will start up in two weeks, however a transformer with the exact specs given to me by the contractor is 30 days ARO.

The efficiency/ size trade off for the 600/ 480 is about what I thought. I would need to upsize to 150 kva with "Y" secondary, then confirm local stock. I would prefer to purchase exactly what is requested. This may be possible, as I have not exhausted all possible sources.

Thank you all for your input.

 

[mike_kilroy]

Might be worth asking who the drives and vfds are mfgred by; if servos or spindles and made by Siemens, you may have a strict input transformer design requirement that is never met by the off the shelf generic distribution transformers....

 

[kwired]

sounds like you not only need right voltage but most likely need correct frequency - that will not be done by a transformer

If this is new equipment someone dropped the ball when ordering it - as they likely could have gotten you something designed for a 480 volt 60Hz system, especially if they do much for export equipment.

 

[petersonra]

You might want to have an engineer take a look at the equipment and decide what is appropriate. The heaters won't care about the frequency change. The motors that are on VFDs or other drives probably won't either, but someone is going to have to run it down to be sure.

The across the line starters could potentially be replaced by VFDs.

My guess is just dropping the voltage down is not a complete answer. Someone with some familiarity with this kind of thing should probably be employed to look at the whole situation very closely.

 

[pick]

I'm not aware of any Siemens servos or spindle drives, just motors across the line and with VFDs, as well as heater loads. Looking at the drawings, it looks like there are a lot of single phase loads that use 230 volts from phase to neutral.

I found a 150 kva 600 volt primary, 480/ 277 volts "Y" secondary in stock at a local transformer vendor. According to their application engineer, using a 150 kva for a 100 kva at 80 percent voltage reduction is enough of a derate and reasonable performance can be expected. In his words it is all about power, and derating should follow the square of the voltage change. I feel that this is somewhat oversimplification, but I am comfortable with this upsizing.

I have made management and the contractor aware of all the ramifications and feel that I have done my due diligence.

Everyone's input is appreciated, some very good ideas were presented.

 

[pick]

A couple more posts came in while I was replying, I definitely agree that 380/ 230 volts is an oddball configuration. Ultimately it is up to the contractor to make this stuff run, and they say they are happy with 380/230 at 60 HZ.

By the way, this is used equipment, I figure it to be around 20 years old. And this happens all the time: management buys stuff and thinks about the infrastructure to support it later. The electrician always finds out at the last minute.

 

[petersonra]

I have made management and the contractor aware of all the ramifications and feel that I have done my due diligence.

So if there is a problem it is on you.

 

[Besoeker]

A couple more posts came in while I was replying, I definitely agree that 380/ 230 volts is an oddball configuration.

Oddball for you I'm sure. But not for the EU zone.
Actually it nominally 400/230 and pretty standard for domestic, commercial, and light industrial. The usual arrangement is 11kV delta to 400V star with the neutral brought out to give 230V line to neutral.

Ultimately it is up to the contractor to make this stuff run, and they say they are happy with 380/230 at 60 HZ.

They might be happy. Not so sure that the across the motors that powered across the lines will be. Or the driven machinery. The motors will run 20% faster and with less available torque.

By the way, this is used equipment, I figure it to be around 20 years old. And this happens all the time: management buys stuff and thinks about the infrastructure to support it later. The electrician always finds out at the last minute.

Yep. Tail end charlies.
I wish you luck with the project.