Voltage and amp loss reverse feeding a transformer
Close
Login to Your Account
Results 1 to 10 of 10
  1. #1
    Join Date
    Sep 2016
    Country
    UNITED STATES
    State/Province
    Missouri
    Posts
    863
    Post Thanks / Like
    Likes (Given)
    765
    Likes (Received)
    262

    Default Voltage and amp loss reverse feeding a transformer

    The tag below is on a transformer available for purchase. I think it may be too small for my purpose. My only 480 machine is a Fosdick jig bore. I'm not sure what the amperage is on the jig bore motors. I cant get that information today. I don't think any of the motors are over 3hp.
    I have had the jig bore for years. I know jig bores are obsolete. Its only reason for not being up and running is it's 480 voltage.


    What I'm looking for is what amperage output I could expect out of this transformer reverse feeding It ?
    Ill be feeding it with a 10 h.p. RPC with line voltage 120/120/105. The 105 being the manufactured leg.
    I also remember there can be a voltage spike at start up when reverse feeding.
    What do you do about that ?
    Is tis t the wrong transformer?

    Sorry about the photo rotation

    transformer.jpg
    don't click the attachment
    Attachment 299556

  2. #2
    Join Date
    Jun 2001
    Location
    St Louis
    Posts
    18,573
    Post Thanks / Like
    Likes (Given)
    2094
    Likes (Received)
    3277

    Default

    It is a 480:240 3 Phase transformer. it is connected in a not totally conventional way, but it is 3 phase and 9kVA.

    At that power level it will have little or no compensation, so the reverse connection should be close to ideal in voltage transformation.

    At about 1000 to 1500 VA per HP, you should have enough capability for the machine.

    Your voltage quoted from the RPC is odd, I expect you are measuring to ground, where you should be measuring line-to-line.

  3. Likes Yan Wo liked this post
  4. #3
    Join Date
    Sep 2016
    Country
    UNITED STATES
    State/Province
    Missouri
    Posts
    863
    Post Thanks / Like
    Likes (Given)
    765
    Likes (Received)
    262

    Default

    I now see that 9 KVA = 23.53 amps so I'm no longer worried about the size. I don't think the jig bore has a motor that draws over 12 amps. Ill check that tomorrow.

    I still don't know how too deal with a voltage spike that can exceed 10 x the voltage at start up. That's what I've read anyway. I'm searching an that answer.
    With all the available transformers online I may be better off finding a step up transformer. They seem too be fewer available.
    Buying this one from these friends may cost more. Reality check of true value with them. I need a comparison price.
    Thanks

  5. #4
    Join Date
    Jun 2001
    Location
    St Louis
    Posts
    18,573
    Post Thanks / Like
    Likes (Given)
    2094
    Likes (Received)
    3277

    Default

    The voltage spike thing is maybe a bit overblown. Most devices are set up to withstand several times normal voltage, with a margin. So up to 10x does not seem like an amount that is very worrisome for motors and transformers. There could be implications for VFDs, and other electronics, mostly due to the potential presence of MOVs on the input to the VFD..

    There may be a current spike at turn-on, because the secondary is being used as the primary. The secondary is wound farther out from the core than the intended primary, and there is a "coupling efficiency" difference vs a winding that is close to the core (leakage inductance, for the EE types). The voltage spike may be from that.

    In any case, while the NEC now forbids the connection unless the manufacturer marked the transformer as suitable for that use, at the power level you are discussing, it normally works acceptably. Have done it at higher power than that without problems.

    The voltage compensation is normally an issue only at lower VA ratings than your 9 kVA.

  6. Likes Yan Wo, mllud22 liked this post
  7. #5
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,680
    Post Thanks / Like
    Likes (Given)
    1970
    Likes (Received)
    3680

    Default

    I don't understand this voltage spike on turn on. There is a current spike on connection which can be none up to very large. It is controlled by the time in the AC cycle that the power was disconnected last time and the point when it is reconnected. The magnetic flux stored in the core drops down when power is disconnected, but even a good core, properly annealed and all that can store a remarkable amount of flux. You have to match the starting point within a few percent to completely avoid a surge. Mostly, don't worry about it, turn the power on, let the lights dim for a moment, and keep on truckin'.

    When you disconnect the power there can be a voltage spike from the field collapse. Again, it can be none to thousands of volts. Transformers are designed with it in mind.

    Bill

  8. Likes mllud22 liked this post
  9. #6
    Join Date
    Jul 2004
    Location
    Houston, TX USA
    Posts
    31,205
    Post Thanks / Like

    Default

    My 3KVA bought new 17 years back has run "backwards" without issue all that time

  10. #7
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    California
    Posts
    2,772
    Post Thanks / Like
    Likes (Given)
    28
    Likes (Received)
    669

    Default

    9100 is right, it's a CURRENT spike on energizing a transformer, not a voltage spike, and it will only last a fraction of a second, usually too fast for fuses or circuit breakers to react (but not guaranteed). Don't worry about it though.

  11. Likes mllud22 liked this post
  12. #8
    Join Date
    Sep 2016
    Country
    UNITED STATES
    State/Province
    Missouri
    Posts
    863
    Post Thanks / Like
    Likes (Given)
    765
    Likes (Received)
    262

    Default

    JST
    Thank you for your thorough explanation. Ill have to ask my son about where we got those line voltage numbers. He's a HVAC guy and my go to guy when making connections. I have a fair knowledge in the electrical end on machinery but always go over it with him. He was familiar with reverse feeding transformers but not in practice.
    The issue with the voltage spike is only what I got from reading about reverse feeding transformers.
    Your explanation and Bills clears that issue up.

    Bill your post puts me at ease and helps me understand. It seems to be the opposite of when you start a motor that draws high amperage on start up but doesn't kick the breaker because the current draw is short enough to be within the breakers delay. Dumb statement?
    When I google reverse feeding a transformer the voltage spike is mentioned but not explained thoroughly.
    Consequence of the spike. You both explained.

    John
    That puts me at ease about the reverse feeding. The jig bore is my only 480 machine. Its been used to stack boxes on because of the 480 issue. I want to use it.

    Now if I can get the transformer at a decent price. Other than a sticky shop film it looks good. It was wired too a mill
    .
    Thanks mike

  13. #9
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    15,124
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    10450

    Default

    Quote Originally Posted by mllud22 View Post
    Consequence of the spike. You both explained.
    It wont bug anything in the machinery world. It "probably" won't bug a PC or peripheral, either.

    But they used to be more fragile. Spikes could cause a "reset"!

    As said, computers are better protected long-since, but still.

    Shout us up if that strikes. Your gear is easy to tame if even it needs any taming at all.

  14. #10
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,680
    Post Thanks / Like
    Likes (Given)
    1970
    Likes (Received)
    3680

    Default

    Fuses and breakers are made in countless delays and other characteristics. A complete set of specs on a breaker covers delay, which depends on the percentage overload, maximum peak current, maximum current it will break without internal damage like fried contacts, make and break times, and on and on.

    Then there was the kid hired to clean up who sprayed a water based cleaner on a 240 three phase breaker. He got some burns on his arms and trashed a $200+ breaker.

    Bill


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •