Transformer question 480 corner ground delta to 240 high leg, only need 240 single
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    Default Transformer question 480 corner ground delta to 240 high leg, only need 240 single

    I need to install a transformer to get 240V. Supply is 480V corner ground delta. My question is how to properly ground X4. With X4 ungrounded, all three legs are 247 phase to phase, and 117V to ground. X4 shows 60V to ground. Is this just a floating voltage that will have no current if grounded?

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    The supply type is mostly irrelevant if you are installing a transformer to get your desired output.

    The secondary of a transformer is a "separately derived system" with no inherent relation to any grounding system. You need to re-establish the ground in order to comply with NEC if you are installing a "branch circuit" off that secondary.

    Pretty much any wire "can" be grounded to re-establish the ground on the secondary (to protect against the high voltage breaking through from the primary). With 240V, the two most popular ways to get it are:

    1) with a straight 240 L-L delta transformer, corner grounding. This takes a special distribution box, or replacing one fuse with a short. You cannot have a fuse in the grounded conductor (neutral).

    2) With a "high leg", "farm", or "lighting tap" transformer (all the same thing), one winding of the delta is center-tapped for grounding. That gives 240 V 3 phase, with the option of also getting 120-0-120V single phase from the tapped winding. The grounded point is a regular neutral for the 120/240 portion, but NOT a standard neutral for the 3 phase.

    There is not going to be much availability of a 240V L-L wye transformer, because the voltage to the neutral is not a useful one (around 139VAC).

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    Quote Originally Posted by JST View Post
    There is not going to be much availability of a 240V L-L wye transformer, because the voltage to the neutral is not a useful one (around 139VAC).
    If you want Wye @ more than 208, nominal? That ain't hard.

    Look for a "Drive Isolation" transformer. The Wye side is NOT 208/120.
    I do not have 4XX, so my one is an EGS/Hevi-Duty 27 KVA. 1:1 .... 2XX both sides.

    Sola/Hevi-Duty Products | Sola Drive Isolation Transformers

    ebay usually has several listed from the "usual suspects". Emerson // General Signal // EGS .. before Sola. And not-only.

    He has 4XX, where I do not, so:

    EGS Hevi Duty DT661H27S Drive Isolation Transformer 27kVA | eBay

    Nominaly 460 Delta => 230 / 133 Wye. There are others.
    Some have adjusting taps. Others, not.

    Given my mains are @ 246 VAC? Yes.. the "low Voltage" any leg to re-derived Neutral // Earth on MY one is indeed too high for ordinary use.

    So I do not USE it. That wasn't hard.

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    JST, thank you for your reply.

    You wrote:

    Quote Originally Posted by JST View Post

    2) With a "high leg", "farm", or "lighting tap" transformer (all the same thing), one winding of the delta is center-tapped for grounding. That gives 240 V 3 phase, with the option of also getting 120-0-120V single phase from the tapped winding. The grounded point is a regular neutral for the 120/240 portion, but NOT a standard neutral for the 3 phase.
    Will X4, the center tap, become the neutral? And should this then be bonded with the ground in one, and only one, location? What did you mean by "NOT a standard neutral for the 3 phase"?

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    Quote Originally Posted by gbent View Post
    ..........................

    Will X4, the center tap, become the neutral? And should this then be bonded with the ground in one, and only one, location? What did you mean by "NOT a standard neutral for the 3 phase"?
    The center tap on one winding becomes the ground point for the secondary, yes. It gets grounded to whatever NEC ground you have, building steel, the green wire of the 480 supply, a etc. Yes, pick ONE to connect it to, and bond THAT to the distribution breaker box for the transformer (if any).

    It is NOT the neutral for the 3 phase because it does not have the same voltage to all the wires. It will be the neutral for the two wires at the ends of the tapped winding, with 120V to each, but will be about 200V from the "high leg" or "stinger" leg, which is conventionally wired with orange wire or ID to show what it is.

    The option of a lighting tap exists for 240V 3 phase, but 208V naturally has a normal neutral, with 120V from that neutral to all phase wires. If you have the option of using 208, that could be a good move, because it is simple with less to remember.

    But 208 is not always acceptable. Most 240V motors don't really "like it", although they may be able to accept it. Higher current for the same power, and less power than marked if you stay within the marked current. It's 13% less voltage than nominal 240V.

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    Quote Originally Posted by gbent View Post
    JST, thank you for your reply.

    You wrote:



    Will X4, the center tap, become the neutral? And should this then be bonded with the ground in one, and only one, location? What did you mean by "NOT a standard neutral for the 3 phase"?
    You have the wrong transformer for that goal is what.

    If you want a re-derived Neutral at the secondary?

    You need a Delta-Wye transformer // 'drive isolation' transformer specifically wound for that tasking.

    Those DO HAVE a proper "Neutral-Neutral", at the "Y" point intersection.
    AND NOT just a convenience tap on one winding. "208" or not is a separate issue. As has been linked. Already.

    That simple. Really. It is.

    Yah cannot "rationalize" a way around it with any amount of keyboarding and wishful-stubborn.

    Transformers are more stubborn that you are. They are all wound-up about being wotever they were wound-up to be.

    They just don't give a damn about being 'convinced' by online opinions.


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    Thanks again JST, very much. I'll get it finished this morning.

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    Quote Originally Posted by gbent View Post
    Thanks again JST, very much. I'll get it finished this morning.
    Dumb question - You indicated you only need 240 single, if that's just 240V single phase why are you looking at three phase transformers at all? All you need is one of the extremely common 240/480 x 120/240 single phase transformers in the appropriate size. As noted it's a "separately derived system" so you just ground the center tap on the 120/240 side and be done with it. The 240/480 side you just configure for 480 and leave that center tap unconnected.

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    I think these Northwest Lineman College transformer videos are very instructive.

    See Transformers - YouTube

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    Quote Originally Posted by wp6529 View Post
    Dumb question - You indicated you only need 240 single, if that's just 240V single phase why are you looking at three phase transformers at all? All you need is one of the extremely common 240/480 x 120/240 single phase transformers in the appropriate size. As noted it's a "separately derived system" so you just ground the center tap on the 120/240 side and be done with it. The 240/480 side you just configure for 480 and leave that center tap unconnected.
    Spot-on.

    My case? I'm NOT changing Voltage. Not significantly, anyway. But would use Delta-Wye even so.

    I actually NEEDED the "drive isolation" feature to tame a passle of rude-bugger Thyristor DC Drives, load-side ...and isolate a Phase-Perfect as well.

    The Delta-=> Wye was so I could have the same distribution and shared supplementary ground rods as the local Neutral of the Wye output off the Diesel Gen set, "XOR" RPC "XOR" Phase-Perfect.

    So the loads can all be "five-wire" and "don't have to care" which source is active.

    It's more of a lab that a shop. And I just don't LIKE corner-grounded Delta.

    YMMV, otherwise.

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    Quote Originally Posted by wp6529 View Post
    Dumb question - You indicated you only need 240 single, if that's just 240V single phase why are you looking at three phase transformers at all? All you need is one of the extremely common 240/480 x 120/240 single phase transformers in the appropriate size. As noted it's a "separately derived system" so you just ground the center tap on the 120/240 side and be done with it. The 240/480 side you just configure for 480 and leave that center tap unconnected.
    Easy answer, I already owned a sufficient 3 phase transformer.

    It's wired and working. Thanks.


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