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9 comments
Associate

Join Date: Dec 2008
Posts: 26

How to calculate the required AH for ups

01/31/2009 4:04 AM

could any one help me how to calculate the required AH for ups.

my load is two motors have same spec.

p=2.2kw

I Rated=4.98amp

I starting=28 amp

i want this ups to supply for 30 min.

so could you please explain for me how to calculate it the required KVA and AH for batterys .

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Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
Good Answers: 12
#1

Re: How to calculate the required AH for ups

02/02/2009 2:06 AM

Hi there

Simply put:

C = x * T where

C = capacity in amp hours

x = current drawn

T = Time in hours

So in your case:

C = (4.98 * 2) * 0.5

C = 4.98 AH

But you do not want your battery to be fully depleted so then add some spare capacity by:

Cnew = C/0.8

Cnew = 4.98/0.8

Cnew = 6.225 AH

Now you need to take rate of discharge into consideration. You will need to look at the following charts to get your value http://www.powerstream.com/BB.htm

I have used 0.5

Cfinal = Cnew / 0.5

Cfinal = 6.225 / 0.5

Cfinal = 12.45AH

In conclusion you will need a 12.45 AH battery to run your load for thirty minutes at 9.96A average draw.

To get the kVA rating you need to multiply the voltage by the amps. As you did not provide the voltage - you can calculate this yourself.

Regards,

Craig

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Anonymous Poster
#2
In reply to #1

Re: How to calculate the required AH for ups

02/02/2009 4:36 AM

why u put T=2,

C = (4.98 * 2) * 0.5

although i want it for 30 minutes only

so could we make it like that

C = (4.98 *0.5)

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Associate

Join Date: Dec 2008
Posts: 26
#3
In reply to #1

Re: How to calculate the required AH for ups

02/02/2009 4:38 AM

why u put T=2,

C = (4.98 * 2) * 0.5

although i want it for 30 minutes only

so could we make it like that

C = (4.98 *0.5)

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Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
Good Answers: 12
#4

Re: How to calculate the required AH for ups

02/02/2009 5:07 AM

Hi there,

You said that you have two motors of the same spec. So I took your 4.98A as the current of one motor, so - (4.98 * 2).

If the 4.98A is for both then you do not need to multiply by two. The divide by 0.5 is to get 30 min. Remember T is time in hours.

Regards,

Craig

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Associate

Join Date: Dec 2008
Posts: 26
#5
In reply to #4

Re: How to calculate the required AH for ups

02/02/2009 8:42 AM

by the way,the supplier get for me 60 batteries a every one is 12 volt and 9.5AH

to supply this motor it`s voltage is 400volt 3 phase ac

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Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
Good Answers: 12
#6

Re: How to calculate the required AH for ups

02/03/2009 1:08 AM

Hi there,

That sounds reasonable - but it depends on the battery type. Refer to the link I posted to get your constants.

Regards,

Craig

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Anonymous Poster
#7
In reply to #6

Re: How to calculate the required AH for ups

02/04/2009 11:33 AM

Craig,

But most UPS output is single phase, 240V 50Hz and not 3 phase 400V 50Hz! how will thus guy get his motors to run?

Rgds,

Percy

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Associate

Join Date: Dec 2008
Posts: 26
#8
In reply to #7

Re: How to calculate the required AH for ups

02/04/2009 3:13 PM
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Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 298
Good Answers: 12
#9

Re: How to calculate the required AH for ups

02/05/2009 1:18 AM

Hi there,

Industrial UPS 3 phase and 525/400V. Masterguard (Ex Siemens) makes a very nice range. It is a bit pricey - but well worth it. You can even have a Profibus link to it - nice for reading status' and diagnostics.

Cheers

Craig

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Previous in Forum: Switchyard main bus sizing in one and half breaker scheme   Next in Forum: U.S.A., 1200 foot underground cable for 200 amp single phase 110/220?
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