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Thread: Complaints of Delta Supplied Three Phase

  1. #1

    Complaints of Delta Supplied Three Phase

    We recently designed an installation in which a 40 HP Submersable motor was served by a two pot transfomer bank... Each bank was 25 KVA resulting in a total of 50 KVA. Acknowledging that the 40 HP can be assumed equivalent to a 30 KW load (PF assumed to be unity) and the 50 KVA two pot bank derated to 86.6%, should the customer have any complaints. He is complaining about not having true three phase, and unless Im missing something, he's mistaken. He claims that had he know he was to be served from a two pot bank he would have set his equipment up differently, but now the current set up will cause him problems

    Let me know your thoughts

  2. #2
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    I don't know why he would have to change anything different between an open delta and a full delta, the voltages are the same, unless he was expecting a wye connected bank, you didn't say whether this was a 240 volt or 480 volt system, if 240, maybe his controls are set up for 208 instead of 240.

  3. #3
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    Quote Originally Posted by altarengineering View Post
    We recently designed an installation in which a 40 HP Submersable motor was served by a two pot transfomer bank... Each bank was 25 KVA resulting in a total of 50 KVA. Acknowledging that the 40 HP can be assumed equivalent to a 30 KW load (PF assumed to be unity) and the 50 KVA two pot bank derated to 86.6%, should the customer have any complaints. He is complaining about not having true three phase, and unless Im missing something, he's mistaken. He claims that had he know he was to be served from a two pot bank he would have set his equipment up differently, but now the current set up will cause him problems

    Let me know your thoughts
    Your customer does have 'true' three phase.

    There is nothing inherent in an open-delta that would require him to set up his equipment differently from a closed-delta. Of course there is an issue if he had been expecting a wye connection.

    But, how is your system grounded? A center-tapped delta is not directly interchangeable with a corner grounded one.

    Your transformer bank output is 43.3kVA 3-phase, personally, I think this is a little small for staring a 40HP submersible pump.

    Many people wrongly assume that an open-delta is synonymous with a wild-leg/high-leg/center-tapped system, especially when the voltage is 240V line-line.
    Last edited by jim dungar; 05-14-12 at 02:52 PM.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post

    Many people wrongly assume that an open-delta is synonymous with a wild-leg/high-leg/center-tapped system, especially when the voltage is 240V line-line.
    Am I misunderstanding you? Most open deltas are wild leg/high-leg/center-tapped systems, at least all of the open deltas I've seen any way, may be different in industrial areas or up north.

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    Quote Originally Posted by hillbilly1 View Post
    Most open deltas are wild leg/high-leg/center-tapped systems, at least all of the open deltas I've seen any way, may be different in industrial areas or up north.
    Most does not mean all.

    While I cannot remember getting an open-delta utility service that was not 'center-tapped' for 240/120 3Ph4W, I can tell you I have designed many customer owned banks that were not (I think my largest was +1000A from 480V up to 600V to feed an assembly line).
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    ...
    Your transformer bank output is 43.3kVA 3-phase, personally, I think this is a little small for staring a 40HP submersible pump.
    That jumped right out at me too!

    altarengineering,
    Even if you are using a VFD for this pump you are a little too small; assuming 480V, a 45kVA transformer is 54A and you only have 43.3kVA so that makes it 52A, a 40HP motor is 52A FLC. Any voltage drop down the well and you are sunk but if you are starting it with anything other than a VFD, fuggedaboudit. It likely won't even finish accelerating.
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    Since Jraef, a most learned motor guru, didn't pick up on this, I'm a bit hesitant.
    It seems this was discussed here previously but I can't located the thread. A few years back, I had a customer with serious problems on current inbalance and loss of submersible pumps. After a lengthy back-and-forth with the utility, TVA sent engineers to the site. There specifics were way over my head, but the conclusion was that submersible pumps, more than most other motors, were subject to give more problems on an open-delta supply than a 3 pot delta or wye supply.
    I can testify that once the services were changed from open delta, the problems disappeared.
    I can put you in touch with the POCO engineer involved for specifics if you wish. Just PM me.
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    Quote Originally Posted by augie47 View Post
    Since Jraef, a most learned motor guru, didn't pick up on this, I'm a bit hesitant.
    It seems this was discussed here previously but I can't located the thread. A few years back, I had a customer with serious problems on current inbalance and loss of submersible pumps. After a lengthy back-and-forth with the utility, TVA sent engineers to the site. There specifics were way over my head, but the conclusion was that submersible pumps, more than most other motors, were subject to give more problems on an open-delta supply than a 3 pot delta or wye supply.
    I can testify that once the services were changed from open delta, the problems disappeared.
    I can put you in touch with the POCO engineer involved for specifics if you wish. Just PM me.
    That may be the issue, since open deltas are used to save the expense of a third transformer, they probably undersized the transformer to begin with to save money, on the application that you had experience with, they probably just threw another transformer of the same size up there instead of changing the whole bank, which greatly increased the capacity. Just a thought.

  9. #9
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    As long as the power delivered starts the motor and there is no other problems the undersizing of the transformer is likely POCO's problem. Many of them do like to size them close to load and even a little under the load at times - especially if the load is not continuous, and by that I am not talking 3 hours like NEC calls continuous, 8 hours on and 16 off is something they do consider for time for things to cool down.

    Open delta still has all voltage possibilities on the secondary as a full delta. It can be corner grounded, mid point of one phase grounded or ungrounded.

    Most farm loads I run into if the total HP is greater than 25 or 30 the POCO usually prefers to supply with full delta or wye transformer bank. Occasionally they do have times where some distance is involved to get all three phases to the site and they may settle for an open delta. I have run into open delta services supplying 60 and 75 HP motors, but is a little rare. On all of those I am aware of with that big of a motor there is not primary line with all three phases supplying the installation so full delta or wye is not an option unless rebuilding the primary to carry all three phases.

  10. #10
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    As others post, the transformer capacity is marginal for the pump size if used with a VFD, and almost certainly to small without.

    An open delta supply is not ideal for motors that are large in relation to the service capacity/transformer size.
    OFF LOAD the voltage between the 3 phases should be equal.
    However when a load is applied the voltage will drop, and NOT EQUALLY on the three phases.
    The voltage drop on the "fake" phase will be greater than on the other two, remembering that the current passes through two transformer windings, with voltage drop in each.

    Most induction motors will tolerate a supply of from 90% up to 110% of the nominal or nameplate rating. A much closer tolerance is however required as to voltage difference between phases.
    Some motors will only tolerate 1% or 2% difference in voltage between phases.
    That is, a supply of 90% of nominal, but the same on each phase, is OK, but A supply of 90% nominal on one phase and 96% of nominal on another phase is not acceptable.

    Presuming that the motor will run, I would suggest measuring the on load voltage of each phase and confirming that they are not only within tolerance, but also within 2% of each other.
    Also measure the on load current in each phase and compare with the nameplate rating.

    I suspect that you will find find either low voltage in general, or an excessive difference between phases, or both.
    In case of a deep well pump you can not readily measure the voltage at the motor, measure elswhere and calculate/estimate the actual voltage at the motor from the length and size of the cable.

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