




#187089  06/13/09 02:34 AM
Re: Open Delta BuckBoost
[Re: BigJohn]

Member
Registered: 02/06/03
Posts: 938
Loc: Wi/ Paris France { France for ...

I have a couple of buckboost transformers that I think are grossly undersized, and I'm hoping someone can check my math:
Two 3kVA, 600V, deltaconnected autotransformers should have a line current of 5 amps, correct?
3000VA * 2 * 0.87 (because it's an open delta) / 1.73 / 600V primary = 5.0A
Thanks a bunch.
John BigJohn., I came up complety diffrent number than your numbers. Here is my verison.,, 3000VA X .87 ÷ 1.73 ÷ 600 = 2.51 amps The 1.73 is allready taken care for triphase { it dont matter if open delta or close delta or wye format } But just wondering why you add X 2 in there so if you try that again like my formaila and also I did used the electrician caluacator to see and the answer and it came pretty close it show 2.8amps on that one. Merci,Marc
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#188048  07/19/09 03:20 AM
Re: Open Delta BuckBoost
[Re: BigJohn]

Broom Pusher and
Member
Registered: 10/19/00
Posts: 2703
Loc: Anaheim, CA. USA

BigJohn;
Please pardon the latency in my delay.
The key factor for sizing a Voltage Buck/Boost Autotransformer arrangement, is the capacity of the _Secondary Winding(s)_  in Amperes.
Two examples:
1: Load requires 5.0 KVA @ 230 Volts, supply Voltage is 208 Volts:
a: 230V  208V = 22 Volts difference. b: Most common Secondary Voltage for small Transformer = 24 Volts. c: Voltage output will be 232 Volts for this arrangement. d: 5,000 VA / 232 Volts = 21.56 Amps. e: Minimum Ampacity for Secondary Winding = 21.56 Amps. f: 21.56 Amps x 24 Volts = 518 VA. g: Minimum Secondary Winding capacity = 518 VA (Transformer rating) h: Closest common size of Transformer = 0.750 KVA (750 VA), which is good for 31.25 Amps through the Secondary Winding(s).

2: Load requires 7.5 KVA @ 208 Volts, supply Voltage is 240 Volts:
a: 240V  208V = 32 Volts difference. b: Most common Secondary Voltage for small Transformer = 32 Volts. c: Voltage output will be 208 Volts for this arrangement. d: 7,500 VA / 208 Volts = 36.06 Amps. e: Minimum Ampacity for Secondary Winding = 36.06 Amps. f: 36.06 Amps x 32 Volts = 1,154 VA. g: Minimum Secondary Winding capacity = 1,154 VA (Transformer rating) h: Closest common size of Transformer = 1.5 KVA (1,500 VA), which is good for 46.88 Amps through the Secondary Winding(s).
 
Now for your scenario.
If the Load requirement is 5220 VA _PER PHASE_, the 3 Phase VoltAmps will be 15,660.
If the _TOTAL 3 PHASE LOAD_ is 5220 VA, the PerPhase Load will be 1740 VA.
Let's use the 5220 VA PerPhase Load requirement for this discussion...
Your Load requires an input Voltage of 480 Volts, 3 Phase 3 Wire. The existing supply Voltage is 600 Volts 3 Phase.
Need to setup the "Autotransformer" connections to "Buck" the Voltage down by 120 Volts, so the output from the Transformer(s) will be capable of supplying no less than 5220 VA @ 480 Volts PerPhase.
5220 VA / 480 Volts = 10.875 Amps. The Transformer's Secondary Winding(s) must be able to carry at least 10.875 Amps.
10.875 Amps x 120 Volts = 1,305 VA.
Minimum Transformer rating = 1,305 VA (1.3 KVA).
A 2.0 KVA 600V x 120V Transformer has a Secondary Ampacity of 16.67 Amps.
A 3.0 KVA 600V x 120V Transformer has a Secondary Ampacity of 25.0 Amps.
Connect Two of the Transformers in an OpenDelta arrangement, with the Windings of each Transformer setup in a SeriesSubtracting connection.
The 2.0 KVA Transformers would be sufficient, as the maximum Amperes that could be drawn through the Secondaries is 16.67 Amps, and the Load's maximum Amps is 10.875 A.
The 3.0 KVA Transformers would be good for 25.0 Amps per Phase, which may be a better choice (if within budget).
Good luck.
Scott
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Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

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