Transformer Help

[Little Bill]

I need to take 240V 3Ø to 480V 3Ø on a hermetic compressor. The load is 25A. My understanding of auto transformers (buck/boost) is they are for a small change, up or down. However, I saw one that is listed as a auto transformer that says either 480V/240V or 240/480V. My questions are:
Should I use a step-up transformer or auto?
Using a on-line calculator, I came up with needing a 25 kVA, does that sound right?
Also, would the same kVA rating be for both step-up and auto?

 

[winnie]

Autotransformers become less beneficial the larger the voltage change, however there is still some material usage benefit even for large changes.

25A at 480V 3 phase works out to about 21kVA, which probably gets rounded to a standard size. You might need a transformer of higher rating than the continuous load to deal with startup currents.

Autotransformers are often rated by the delivered kVA, which is what you care about. The delivered kVA will be greater (possibly much greater) than the kVA being transferred magnetically between windings; this is the whole reason for using an autotransformer. Imagine for a moment that you have a 100V to 10V transformer with 100A secondary current rating (1kVA); if you connect this transformer as an autotransformer to get 110V output, you _still_ have the capability of 100A secondary current, so the _delivered_ kVA is 11kVA, and the autotransformer rating of this setup would be 11kVA. However there is still only 1kVA flowing magnetically from the 100V coil to the 10V coil.

With 3 phase autotransformer arrangements you need to be careful about what happens the output phase voltage balance relative to ground. For some applications this doesn't matter (eg. driving a motor with no ground reference) and for some it does (feeding a VFD with ground reference MOVs for spike protection).

-Jon

 

[GoldDigger]

With 3 phase autotransformer arrangements you need to be careful about what happens the output phase voltage balance relative to ground. For some applications this doesn't matter (eg. driving a motor with no ground reference) and for some it does (feeding a VFD with ground reference MOVs for spike protection).

-Jon

Stating it another way, a wye autotransformer installation will give you a nice balanced system indistinguishable from a 480 wye service or a 240 to 480 isolation transformer. The potential problems with absolute phase and voltage to ground will come if you try to use a single core delta autotransformer or three autotransformers in a delta configuration. You have serious problems if you try to use two single phase autotransformers to try to create, in effect, an open delta output.

 

[Little Bill]

I said in my OP that I came up with a 25kVA transformer. The actual calculation came to 20.8kVA. First I rounded up to 25 because I figured that was the next size up, then figured the extra would help with in-rush. Is that about right, or do I need a larger one to account for in-rush?

 

[mike_kilroy]

Autotransformers become less beneficial the larger the voltage change, however there is still some material usage benefit even for large changes.

1) 25A at 480V 3 phase works out to about 21kVA, which probably gets rounded to a standard size.

2) Autotransformers are often rated by the delivered kVA, which is what you care about. The delivered kVA will be greater (possibly much greater) than the kVA being transferred magnetically between windings; this is the whole reason for using an autotransformer. Imagine for a moment that you have a 100V to 10V transformer with 100A secondary current rating (1kVA); if you connect this transformer as an autotransformer to get 110V output, you _still_ have the capability of 100A secondary current, so the _delivered_ kVA is 11kVA, and the autotransformer rating of this setup would be 11kVA. However there is still only 1kVA flowing magnetically from the 100V coil to the 10V coil.

-Jon

Interesting look at it from a different perspective:

1) This 21kva isolation transformer will actually onlyl need to be 10.5kva physical size built as an autotransformer, thus saving cost.

2) I am not sure how others build this 1kva autotransformer, but we would never build it as an 11kva physical size unit. We would use wire size rated for the 100 amps for the output windings and only wire size rated 1kva/100v= 1amp for the input side wire and weld them together at the center. Hence it would be rated 1 kva & not capable of 11kva at all. It thus willl be muich smaller and less expensive and built to do the 100v 1amp input, 10v 100 amp output. This is also how we would build it as an isolation transformer, so if someone hooked both windings together to turn it into an autotransformer, it still would only be capable of 1kva rating.

2a) If someone needed 110v @ 100 amps, and had only 100v to work with, then for sure, this autotransformer would be built using 100 amp wire for both windings & wouuld be rated 11kva.

 

[Little Bill]

Interesting look at it from a different perspective:

1) This 21kva isolation transformer will actually onlyl need to be 10.5kva physical size built as an autotransformer, thus saving cost.

2) I am not sure how others build this 1kva autotransformer, but we would never build it as an 11kva physical size unit. We would use wire size rated for the 100 amps for the output windings and only wire size rated 1kva/100v= 1amp for the input side wire and weld them together at the center. Hence it would be rated 1 kva & not capable of 11kva at all. It thus willl be muich smaller and less expensive and built to do the 100v 1amp input, 10v 100 amp output. This is also how we would build it as an isolation transformer, so if someone hooked both windings together to turn it into an autotransformer, it still would only be capable of 1kva rating.

2a) If someone needed 110v @ 100 amps, and had only 100v to work with, then for sure, this autotransformer would be built using 100 amp wire for both windings & wouuld be rated 11kva.

Ok, I'm getting somewhat confused here about whether an autotransformer should be used and if so, does it need to be sized smaller than a isolation transformer?
I did get more info on the refrigeration unit. It says it can be ordered with an autotransformer. This unit is used and didn't come with the transformer. So it sounds like an autotranformer can be used. My only other question is how to size it.
Primary voltage is 240V and needs to be 480V, both 3Ø. Amps is 25
With this info, I came up with 20.8kVA for an isolation transformer. How much lower can I go if using an autotransformer?

 

[mike_kilroy]

All the techno discussion here was to show details how the stuff works. Not important to your request. You simply need to go buy the 25kva autotransformer as you originally said.

 

[Little Bill]

All the techno discussion here was to show details how the stuff works. Not important to your request. You simply need to go buy the 25kva autotransformer as you originally said.

Thank You!

 

[electrofelon]

I think for an auto, you would be looking at something like this:

Hammond Y030QTCN3L0U

i assume you do not need a neutral on the 480 side? These always seem to be sold as "wye" configurations. I think you can skip the neutrals if you don't have or need them. I think what you don't want to do is skip the 240 neutral (which you won't have) if you needed a 480 neutral. IIRC you don't want to use an auto to derive a neutral.

 

[mike_kilroy]

Auto xfmrs are always sold as wye configuration because that is the only physical way to make one; cannot make one from a delta. So there is always a center neutral pass through - question of availability of it is in the part number... Some mfgrs don't bring the neutral out to a terminal for use while others do. It is there just may not be accessible, so Little Bill be sure to purchase the xfmr with it brought out to a terminal if you need it.

 

[synchro]

If the 240V 3Ø supply is a high-leg delta, then the voltages on the three phases are not symmetrical with respect to ground. So a 480V neutral will not be available from an autotransformer, and the center connection of the wye cannot be grounded. As Ethan said you can't use an autotransformer to derive a neutral. Even if the 3Ø supply was ungrounded, an autotransformer has a relatively high zero sequence impedance so it doesn't make a very good grounding transformer.

 

[Little Bill]

No neutral needed and yes, the supply voltage is a high leg Delta.

 

[electrofelon]

Auto xfmrs are always sold as wye configuration because that is the only physical way to make one; cannot make one from a delta. So there is always a center neutral pass through - question of availability of it is in the part number... Some mfgrs don't bring the neutral out to a terminal for use while others do. It is there just may not be accessible, so Little Bill be sure to purchase the xfmr with it brought out to a terminal if you need it.

I dont follow. Single phase autos ("buck boost transformers") are connected in open delta configurations all the time (Granted closed delta can cause phase shift and is not recommended). When I said "only seem to be available in wye configurations" I was referring to the larger three phase units, but I looked at hammond's connection diagrams and there are quite a few delta connections, so could you elaborate?

 

[winnie]

I said in my OP that I came up with a 25kVA transformer. The actual calculation came to 20.8kVA. First I rounded up to 25 because I figured that was the next size up, then figured the extra would help with in-rush. Is that about right, or do I need a larger one to account for in-rush?

I think that is about right unless you have special inrush considerations.

Ok, I'm getting somewhat confused here about whether an autotransformer should be used and if so, does it need to be sized smaller than a isolation transformer?​​​​​​

My apologies for being confusing.

The point that I was trying to make is that you need a transformer rated 25kVA as an autotransformer. The same transformer if it could be used as an isolation transformer would have a lower rating in that other use, but all you care about is the rating the way you are using it.

This is very common for single phase autotransformers. The manufacturer makes an isolation transformer with 120/240V primary and 16/32V secondary, which has a kVA rating as an isolation transformer. Then they provide a table of different connections each giving a different output voltage, each having a different kVA rating. The transformer itself doesn't change, but in different connections is capable of delivering different amounts of power to the load.

Jon

 

[mike_kilroy]

I dont follow. Single phase autos ("buck boost transformers") are connected in open delta configurations all the time (Granted closed delta can cause phase shift and is not recommended). When I said "only seem to be available in wye configurations" I was referring to the larger three phase units, but I looked at hammond's connection diagrams and there are quite a few delta connections, so could you elaborate?

Sorry, I did not mean to add confusion. Of course you can do lots of things with transformers. I believe you will not find a single core 3 phase delta autotransformer product anywhere - now that I type that, someone no doubt will! My point was all autotransformers I have worked with in my 45 years as an electrical engineer working in industry, selling & sometimes designing transformers, I have never run across a 3 phase delta autotransformer. It is not even physically possible in my mind. Now use two single phase buck boost as open delta and sure, but that is not a "3 phase delta autotransformer."

 

[Besoeker3]

I need to take 240V 3Ø to 480V 3Ø on a hermetic compressor. The load is 25A. My understanding of auto transformers (buck/boost) is they are for a small change, up or down. However, I saw one that is listed as a auto transformer that says either 480V/240V or 240/480V. My questions are:
Should I use a step-up transformer or auto?
Using a on-line calculator, I came up with needing a 25 kVA, does that sound right?
Also, would the same kVA rating be for both step-up and auto?

Do you need electrical isolation between the windings? If so, you won't be able to use an auto transformer.