unbalanced 3 phase load


  #1  
Old 01-19-03, 04:36 AM
bryaninolathe
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Question unbalanced 3 phase load

will an unbalanced phase/neutral load in a 3 phase 120/240 system cause flourescent ballasts to go bad?
 
  #2  
Old 01-19-03, 06:11 AM
brickeyee
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Do you really have a 3 phase system? They would normally be considered a 120/208. If this is a residential application, 120/240 is single phase 3 wire. Hot-neutral-hot. How old are the ballasts in question, how many are hooked up, and how are they hooked up? A multiwire circuit with older ballasts could have a problem.
 
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Old 01-19-03, 07:04 AM
bryaninolathe
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yes, the system is definately 3 phase 120/240. this is an older commercial building (20+ yrs). over the years circuits have been added with no apparant thought given to the sharing of the neutral with 3 different phases. the ballasts are old magnetic type and the tenent wants to replace them with new electronic ballasts. I suspect an unbalanced system and wonder if that should be straightened out before installing new lighting. any thoughts as to how an unbalanced load can affect the ballasts? I know electronic ballasts are even more sensitive to disruptive current and am concerned with having to warranty them.
 
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Old 01-19-03, 07:25 AM
J
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"3-phase 120/240" is an oxymoron.
 
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Old 01-19-03, 08:59 AM
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It's doubtful but maybe the building is hit with a straight 240 volt "Delta" service and any 120 volt load comes from a center-tapped transformer offering 5% maximum of the transformer's load capabilities. I can't imagine this but I suspect certain parts of the country might offer such a system if the 120 volt load is extremely small (less than 5%) with a very large (95+%) 240 volt 3-phase load... (If true, this sounds more like an industrial facility instead of "commercial".)

I'm curious to know what type of commercial business this is (its intended purpose) and also if there is a transformer being used, which is owned by the business and downstream of the utility meter. Also, what is the amperage of the service in this building.

I agree with John that "3-phase 120/240" is an oxymoron!

Kooter
 
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Old 01-19-03, 09:30 AM
brickeyee
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I would investigate exactly what power you have. You may have a high leg system that provides 120/240 by grounding the middle of one side of the delta. This allows the supply of 120/240 and 3 phase 120/208 at the same time.
The 3 phase in this case is a delta system. The neutral is only used with the 120/240 side of the delta. You probably should have a very experienced electrician look over what you have and asses it. It is very hard to have all the information needed without being on site.
Problems starting showing up with neutral harmonics with the first generation of electronic ballasts. There were probably some third harmonic neutral currents with the old inductive ballasts but they generally did not cause much of a problem. The newer electronic ballasts are required to have power factor controls in them to minimize the objectionable neutral harmonics.
 
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Old 01-19-03, 09:48 AM
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Great picture of 120/240 3 phase at
http://electrical-contractor.net/ubb...ML/000215.html
 
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Old 01-19-03, 10:00 AM
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This is actually a "240 Volt Delta" system (as noted on the transformer winding schematic) with a center-tapped neutral in the middle of one winding. The current capabilities of the 120 volts (in this center-tapped 240 Volt Delta configuration) is very small, which may be all that is necessary for this particular customer's 120 volt needs...

Kooter
 
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Old 01-19-03, 10:01 AM
J
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Ron, thanks for the link. With that picture and the explanation that brickeyee provided, all is clear now. Apparently, bryaninolathe got the answer he needed in the other forum.
 
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Old 01-21-03, 05:36 AM
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The single phase 120/240 ampacity of a three phase delta bank is not necessarily small and it is not limited to 5 % of the transformer's rating. It is quite common to see a three transformer, 240 volt delta bank with the center transformer much larger than the other two. The center transformer (the one with the center of its winding grounded) is sized to carry the three phase load plus the single phase load. The single phase current is in phase with the three phase load on that transformer. And don't forget about the open-delta connected banks that are ideal for a load such as a residence or small garage where the load is almost all single phase 120 or 240 volt, but the customer has a three phase 240 volt air compressor or air conditioner. This is a common load in my area. Not sure what the policy is now, but at one time the qualifying three phase load was 5 kva.

Ichabod, P.E.
 
 

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