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dotuntokun
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Join Date: Apr 2009
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Current Output From a 3-Phase UPS

05/07/2010 1:56 AM

Please I need to confirm that the current output from a 3-phase UPS remains the same, when being powered by the grid and when in UPS mode.

A client of mine has argued that the current output from a 120kVA UPS (400V phase -phase) will be about 57A per phase when on battery mode.
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Tornado
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#1

Re: CURRENT OUTPUT FROM A 3-PHASE UPS

05/07/2010 2:15 AM

120,000VA ÷ (400V x √3) ≈ 173 A. How is the 57A being derived?

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dotuntokun
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#2
In reply to #1

Re: CURRENT OUTPUT FROM A 3-PHASE UPS

05/07/2010 3:22 AM

He divided the current by 3. that is 173A/3 = 57A.

He reckoned that when the UPS is supplying the load via the batteries the current per phase will be 57A.

I am sure this cannot be correct.

but I just need confirmation that it is not.
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Tornado
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#3
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Re: CURRENT OUTPUT FROM A 3-PHASE UPS

05/07/2010 3:37 AM

The division by √3 already makes the proper correction for each single phase. Dividing by 3 again is completely incorrect. But it's sometimes tough to deal with pointy-haired bosses/clients. All join in now for the Dilbert theme song!

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hithuanand
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#4
In reply to #3

Re: CURRENT OUTPUT FROM A 3-PHASE UPS

05/07/2010 7:49 AM

Yeah me too agree to tornado.
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dotuntokun
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#5

Re: Current Output From a 3-Phase UPS

05/07/2010 9:12 AM

Thank you very much.
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msamad
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#6
In reply to #5

Re: Current Output From a 3-Phase UPS

05/07/2010 10:38 AM

The UPS output current totally depends on the load, not on the UPS rating. The rated current of the UPS determines the maximum current that UPS can supply. It is absolutely true that the current of UPS output remains same if the UPS output voltage and the load are same, regardless of the power supplied from utility or from the battery bank. In some circumstances, the UPS load may change if the utility supply fails (example: for the plant safety shutdown system, the UPS load increases when the AC power is lost due to the activation of safety shutdown system) and the UPS current may also change when the UPS is powered by the battery bank. So, discuss with your client and find the correct reason why he/she feels the UPS load is 57A when it is on the battery.

The maximum current/phase of a 3 phase, 120 KVA UPS is 173 A, not 57A – it is for sure.

- MS

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hithuanand
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#7

Re: Current Output From a 3-Phase UPS

05/07/2010 11:11 AM

Amp hour tells the capacity of battery. So 57 amps for more time but if load draws current maximum it can supply is 171 for proportional less time.
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