




#128261  01/02/03 03:56 PM
Re: Open Deltas

Moderator
Registered: 04/10/02
Posts: 2561
Loc: WestSouthern InnerNortheast ...

Examples: In the first case, If there were a pair 10kVA transformers, their capacity is 17.3kVA, and not 20kVA
30,000 VA total nameplate for three pots 0.5774 = 1/sqrt3 17,321 VA rating using only two 10kVA pots In the second case, if there was one transformer removed from a three 10kVA pots, there capacity is reduced to 17.3kVA
20,000 total nameplate VA for two pots 0.8660 = sqrt3/2 17,321 VA actual capacity for two pots In the third case, for two transformers serving a 20kVA load, the total nameplate capacity needed is 23.1kVA
20,000 VA load  served by two pots 1.1547 = 2/sqrt3 23,094 VA total nameplate transformer capacity for the two pots; id est, two 10kVA pots cannot adequately serve a 20kVA load.
Cooper Power bulletin R201902 may be a useful reference.
[This message has been edited by Bjarney (edited 01022003).]

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#128262  01/05/03 04:57 AM
Re: Open Deltas

Broom Pusher and
Member
Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA

Bjarney has covered it very well!
Just for the sake of redundancy, I'll add some "Simple Stuff":
The Open Delta bank is able to deliver 86.6% of the KVA into 3 Phase loads.
Example: If one pot is 6 KVA and the other is 4 KVA, the output KVA is 8.66 KVA  not 10 KVA. See Bjarney's descriptions and formulas above.
If an existing Closed Delta network (three Pots connected together in a parallel / delta scheme), and one pot fails, the remaining two pots can continue to deliver 3 phase 3 wire AC  but the maximum output KVA is only 58% of the original KVA rating.
Example: 3  10 KVA pots connected in a delta scheme has an output power of 30 KVA max. One pot fails, due to a secondary coil becoming barbequed. Network can continue to deliver 3 phase 3 wire power, with a maximum of 17.4 KVA apparent power output  as opposed to the original 30 KVA of apparent power available.
The normal rating of a 3 Phase 4 Wire Closed Delta will have a center pot which is at least 200% larger than a single "Outer" pot.
Example: a 45 KVA 3 Phase 4 Wire Closed Delta would have a center pot rated at 25 KVA, and the two "Outer" pots rated at 10 KVA each.
Open Deltas are similar for 4 wire applications, but the "Single Phase" pot might be as large as 400% of the "Kicker" pot. All depends on the calculated loads to be driven.
For 3 Phase 3 Wire settings, the pots are typically sized equally  on both Open and Closed Delta networks.
Scott s.e.t.
P.S. The term "Pot" is referring to a Single Transformer  or more precisely a "StandAlone" single phase Transformer.
Bjarney's examples and formulas are much better than my stuff! I'll need to add this to the tech reference section!
S.E.T.
_________________________
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

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