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Thread: Anyone experienced with transformers? Adding a 440V machine to 220V 3 phase

  1. #16
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    Hi Phillip, that's an isolation transformer, not an auto-transformer.

    You need a Wye secondary (480 volt) with the neutral grounded, or an auto-transformer which is smaller and less expensive......Rod.

  2. #17
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    A single voltage motor may very well have internal wiring for a lower voltage. It just means the maker did not bring those connections out to the junction box. They have to open it up and fish around looking for the splices and bring new leads out to the junction box. Of course they may be deeply buried and not possible to get too or they may not exist inside there at all.
    Bill D

  3. #18
    Well, just to update this thread a bit - after inquiring with several electric motor shops in the surrounding area, many of which said something to the effect of “uhh, no we don’t have anything like that...”, I was referred to an electrical supply house in Knoxville, TN that sells surplus and refurbished transformers.

    The closest they had to my needs was a 7.5 kVa Acme 3 phase dry auto transformer that is 460v primary to 230v secondary for $100 + about the same amount in freight shipping. I’m of the understanding that I can backfeed it to go from 230v up to 460v.

    The bummer of it is that earlier that day I was in Knoxville picking up the drill press in question and simply didn’t know about them at that point in the process and could have just gone by there and picked it up in person. ~ $200 for a tested 3 phase transformer is way cheaper than I’ve been able to find anything online in the last couple weeks and a lot of what I’ve seen is untested.

    I will update the thread once I have the transformer and get it all wired up. Hopefully this will be what I need to run the drill press.

    Here are a few photos of the drill in question - should be a sweet setup when it’s up and running. 8 Speeds from 100-2200 RPM, 1.5 hp motor, MT3 spindle, Power down feed option with 4 different speeds, 360 degree production table that can adjust all the way down the column. The quill depth is about 5 1/4”. The head also has up and down travel of about 10-12”, overall weight is 220kg (485#) but is more compact than many much smaller capacity belt drive models.
    Still waters run deep.

  4. #19
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    Phillip, I have done what you're trying to do. I concur with the advice for the 7.5kva dry pack.

    In my shop, I have a 400hp 480VAC generator that feeds an 800 amp 480 three phase panel board. This panel board also supplies a dry pack transformer that I use as a step down transformer to feed a 400A 240 three phase panel board via a transfer switch.

    I also have a 30 hp rotary phase converter that feeds the same transfer switch as the dry pack. This setup allows me to feed the 240 three phase panel from either the RPC or the generator (via the step-down dry pack transformer). I am able to use the RPC for most of my 240 three phase loads, minimizing generator run time.

    I have 15 hp 480V sawmill edger that I also run off of the RPC. There is a breaker in the 240V three phase panel that supplies a small 240 to 480 dry pack that is slaved to the edger.

    So, the RPC powers a 240 3 phase panel board. The 240 panel board has a breaker that supplies a 240/480 3 phase dry pack transformer, which then feeds a 15hp 3 phase motor. The setup works fine.

  5. #20
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    Quote Originally Posted by Phillip Mitchell View Post
    Well, just to update this thread a bit - after inquiring with several electric motor shops in the surrounding area, many of which said something to the effect of ďuhh, no we donít have anything like that...Ē, I was referred to an electrical supply house in Knoxville, TN that sells surplus and refurbished transformers.

    The closest they had to my needs was a 7.5 kVa Acme 3 phase dry auto transformer that is 460v primary to 230v secondary for $100 + about the same amount in freight shipping. Iím of the understanding that I can backfeed it to go from 230v up to 460v.

    The bummer of it is that earlier that day I was in Knoxville picking up the drill press in question and simply didnít know about them at that point in the process and could have just gone by there and picked it up in person. ~ $200 for a tested 3 phase transformer is way cheaper than Iíve been able to find anything online in the last couple weeks and a lot of what Iíve seen is untested.

    I will update the thread once I have the transformer and get it all wired up. Hopefully this will be what I need to run the drill press.

    Here are a few photos of the drill in question - should be a sweet setup when itís up and running. 8 Speeds from 100-2200 RPM, 1.5 hp motor, MT3 spindle, Power down feed option with 4 different speeds, 360 degree production table that can adjust all the way down the column. The quill depth is about 5 1/4Ē. The head also has up and down travel of about 10-12Ē, overall weight is 220kg (485#) but is more compact than many much smaller capacity belt drive models.
    That's a great price for a transformer that size! I'm glad you found what you needed!

    I live in Knoxville, Tn and I'm curious what shop you found it at? Small world!

  6. #21
    Quote Originally Posted by Eric Arnsdorff View Post
    That's a great price for a transformer that size! I'm glad you found what you needed!

    I live in Knoxville, Tn and I'm curious what shop you found it at? Small world!
    Eric, the place is called Volunteer Equipment and Supply in Knoxville. It sounded like they have quite an inventory on surplus transformers of a wide range of sizes.
    Still waters run deep.

  7. #22
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    Contact Acme Transformer in Lumberton, NC http://localautomation.com/profiles/acme-electric.html I was the US Factory Electrical Service Engineer for a Czech Republic company, installing and servicing large machinery for them in the US and Caribbean, and had to have some special 3 phase transformers built. These guys did it at a very reasonable price and what they sold was UL accepted. You will be dealing directly with the guys who make them. If they have a standard product that will fill your need, they won't push you into a special either. Their sales force is very good at helping you pick out what is needed.

    Charley

  8. #23
    Can you guys elaborate on using a transformer in reverse? I got a 240v 3ph primary & 480v 3ph secondary transformer with a machine that I purchased recently. I have 240v 3ph in my shop already (no phase converter, real 3ph) so I donít need this transformer but I am looking at another machine that is 440v 3ph and I am wondering if I could use this same transformer in reverse as you guys described it?

  9. #24
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    Hi Bobby, yes a transformer can be reverse fed, except where prohibited by code (for example if the manufacturer labels one end primary, then that has to remain the primary).

    The issue with reverse feeding a standard Delta/Wye transformer is that now the Delta becomes the secondary, and you wind up with an ungrounded floating system, which can be very dangerous due to induced potential as well as the issue of detecting ground faults.

    You really need an auto-transformer for your application, or an isolation transformer of the correct configuration.............regards, Rod.

  10. #25
    The other approach is to get three single phase transformers of the correct voltage conversion and then you can hook they up any way you want. That is, you can hook them up delta or wye in the primary and wye in the secondary.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  11. #26
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    Quote Originally Posted by Rod Sheridan View Post
    Hi Bobby, yes a transformer can be reverse fed, except where prohibited by code (for example if the manufacturer labels one end primary, then that has to remain the primary).

    The issue with reverse feeding a standard Delta/Wye transformer is that now the Delta becomes the secondary, and you wind up with an ungrounded floating system, which can be very dangerous due to induced potential as well as the issue of detecting ground faults.

    You really need an auto-transformer for your application, or an isolation transformer of the correct configuration.............regards, Rod.
    You can ground one leg of a delta transformer output and it will be safe. This is a common and required practice. You will still have the rated output between the three phases, but the grounded phase will, of course, read zero voltage between it and ground. The output windings of a transformer are isolated from the input, so you need to provide a way of grounding this output as it is a derived source that is otherwise ungrounded. If the secondary is left ungrounded, any leakage through the insulation between the primary and secondary windings of the transformer could result in very high voltages being present between the secondary and ground. It's not so bad if the primary voltage is low, but a transformer with a 12,000 volt primary and 240 volt secondary that has no ground on the secondary could possibly have as high as 12,240 volts to ground on it. This is why grounding of the secondary is a "must" in almost all cases.

    With a Delta system, 3 single phase transformers can be used instead of a 3 phase transformer which is just 3 transformers packaged together. For grounding of the output, just ground one of the outputs as noted above. Do not do this if you go with an auto transformer system, since the output of an auto transformer is still grounded at the source. Only add the ground when an isolated secondary winding is in use. An auto transformer is a single winding with a tap at the desired output voltage, so the input is already grounded at it's source, and you are just using a higher voltage tap on this one winding for the output.

    Charley
    Last edited by Charles Lent; 04-21-2021 at 12:21 PM.

  12. #27
    I appreciate the continued commentary on the subject, though I must say that most of it is going over my head, which is fine.

    Let’s go back a few steps. I have found this tested and used 7.5 kVa dry auto transformer for a great price that has a listed primary of 460v and a listed output of 230V. I want to do the opposite and end up with 440/460v on the output side with 230V incoming. Can I (my electrician) safely backfeed this transformer and what do I need to know to do so safely?
    Still waters run deep.

  13. #28
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    Quote Originally Posted by Phillip Mitchell View Post
    I appreciate the continued commentary on the subject, though I must say that most of it is going over my head, which is fine.

    Letís go back a few steps. I have found this tested and used 7.5 kVa dry auto transformer for a great price that has a listed primary of 460v and a listed output of 230V. I want to do the opposite and end up with 440/460v on the output side with 230V incoming. Can I (my electrician) safely backfeed this transformer and what do I need to know to do so safely?
    Yes, IF the jurisdiction you reside in does not have a requirement that transformers with primary and secondary designations are correctly connected.

    If the auto-transformer is simply marked H1,H2,H3/ X1, X2, X3 or something similar then yes.

    If it clearly states primary and secondary you'll have to check with your electrician as to local rules.

    I don't expect that you'll have a problem...........Regards, Rod.

  14. #29
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    Philip,
    I don’t think you will have any problems. I’ve had two 480 primary- 240 secondary working in my shop for years. Incoming is 240 3 phase making 480 going out. Never an issue with these.

  15. #30
    Thank you Rod and Joe. I appreciate the replies...puts me at ease a bit. Hopefully I will receive the transformer tomorrow and begin the wiring soon.
    Still waters run deep.

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