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  #1  
Old 03-04-2005, 02:17 AM
Erin G. Erin G. is offline
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I work at a plant that uses three phase, three wire power. None of the three phase transformers are grounded. The transformer in question is a 2400 to 480 volt AC, delta - delta.

We ordered some 277 volt light fixtures that will not work. A co-worker is sure that since you can measure 277 VAC from any one leg of the three phase to ground, then you should be able to hook up a 277 volt lighting ballast, one side of the ballast to a phase, the other to ground, and the light will work. When we actually do this our 277 volts to ground drops to 159 volts. The more lights we put in the circuit, the more the voltage drops. I'm pretty sure that a transformer has to have it's own reference to ground (center tap to ground on wye, H1 on delta) before you can get a usable 277 volt power supply.
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If I'm wrong, why wont our lights work? If I'm right, how do I explain this to my co-worker?
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Old 03-04-2005, 07:40 AM
mozikluv mozikluv is offline
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hi erin

sorry i can't give you a descent answer to your query, but you should repost this at the General Electronics Chat. am sure you will have what you want to know. or you can PM "Bipin". he is an electrical engineer.
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  #3  
Old 03-04-2005, 09:23 AM
pebe pebe is offline
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Quote:
Originally posted by Erin G.@Mar 4 2005, 03:17 AM
I work at a plant that uses three phase, three wire power. None of the three phase transformers are grounded. The transformer in question is a 2400 to 480 volt AC, delta - delta.

We ordered some 277 volt light fixtures that will not work. A co-worker is sure that since you can measure 277 VAC from any one leg of the three phase to ground, then you should be able to hook up a 277 volt lighting ballast, one side of the ballast to a phase, the other to ground, and the light will work. When we actually do this our 277 volts to ground drops to 159 volts. The more lights we put in the circuit, the more the voltage drops. I'm pretty sure that a transformer has to have it's own reference to ground (center tap to ground on wye, H1 on delta) before you can get a usable 277 volt power supply.
[COLOR=blue]
If I'm wrong, why wont our lights work? If I'm right, how do I explain this to my co-worker?
You are quite right. Your lights require a single phase supply, but you cannot get that from a Delta wired transformer, because there is no neutral connection. You need a Star connected transformer.

A possible way to get single phase might be to connect a Star 3phase tranforner to the 3 phases to create a neutral, but I'm not sure. Another way would be to use a 480v/277v autotransformer.

Or you could wire two (equal wattage) lamps in series across the 480v phases and run them at slightly reduced brightness.
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Old 03-05-2005, 07:29 PM
Erin G. Erin G. is offline
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Thanks for the response. I found some helpful schematics at www.kilowattclassroom.com, but apparently my co-worker did the same research, because he wasn't in the mood to debate the subject any further.

Fortunately, the existing ballast has a multi-tap primary, with a 208 volt lead available. We were able to get 208 VAC from the delta secondary feeding the circuit to the lighting ballast without any significant increase in amps.
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  #5  
Old 03-26-2005, 12:35 PM
JayTech JayTech is offline
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You are right in that to use the apparent 277VAC which you sometimes see on one leg (any one leg) of the transformer,
you must have a neutral reference which is valid.
A DELTA output has NO valid neutral. THAT IS why it is a delta and not a 'wye'.
It is a triangle shape symbol for its output windings, and only provides a stable predictable usable power output when taking any two legs to obtain a single phase voltage (which will be 480 volts); (or all three legs for three phase).
The fact that you observed 277 volts is a coincidence of luck (good or bad you pick). Ground is a meaningless (or better to say ambiguous to the unwary) place to return your meter lead to, and read a valid voltage, on a delta winding, since the delta winding doesn't reference to ground (it is unconnected to it).
I can ground my meter lead to my ankle and read all kinds of weird voltages from circuits if I choose, but none of the readings observed will have any validity to an AC power circuit. I will get readings though.
BE wary of readings obtained with sensitive electronic meters, as they will give indications based on leakage currents and weak / non-existant / non-wired return paths.
The proof that it is not a valid power output voltage is: that when you load it, it goes down so much, with not even an appreciable load.
A valid load on a valid lighting transformer would barely cause a drop in voltage (maybe 5 percent) with normal load factor.
If you want to use that transformer, you must buy single phase 440 volt ballasts.
If you want to use the 277 volt ballasts, buy a transformer that has a delta primary (440 volt); Connect it to the load side of your existing 440 volt delta transformer, and your new transformer must provide a wye-configured output, rated at 277 volt (each leg to neutral of the wye) output secondary.
If it is still unclear what is going on, email or post again.
JayTech.
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  #6  
Old 03-14-2008, 09:49 AM
Buddrow Buddrow is offline
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Default light diagram

Quote:
Originally Posted by Erin G. View Post
Thanks for the response. I found some helpful schematics at www.kilowattclassroom.com, but apparently my co-worker did the same research, because he wasn't in the mood to debate the subject any further.

Fortunately, the existing ballast has a multi-tap primary, with a 208 volt lead available. We were able to get 208 VAC from the delta secondary feeding the circuit to the lighting ballast without any significant increase in amps.
Hi
Can you tell me where to get the schematic for this setup. I have a customer who has a similar problem and he insist the multitap ballast will not work.

thanks
Buddy

EDIT: Keep the discussion on the forums, or request a contact through PM. Posting your URL up in this way constitutes advertising. Thank you - Dave

Last edited by Dave; 03-14-2008 at 09:57 AM. Reason: Removed link
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  #7  
Old 03-14-2008, 09:58 AM
Dave Dave is offline
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This thread is nearly 3 years old. It is unlikely you will get any assistance on this matter.

Dave
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