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susheel1225
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Join Date: Jun 2011
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Transformer Capacity

12/12/2011 11:02 AM

We have totally 50 motors and total hp of all motors is 352 and tranformer is 250 kva. How to calculate the capacity of this trasformer
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PWSlack
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#1

Re: Transformer capacity

12/12/2011 11:05 AM

The capacity of the transformer is stated in the original post: 250kVa. No calculation is necessary.

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TonyS
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#2

Re: Transformer Capacity

12/12/2011 2:25 PM

You're overloading the transformer!

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WJMFIRE
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#3

Re: Transformer Capacity

12/12/2011 3:11 PM

746 watts x 352 hp = 261.1 kw. See any issues here?

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wareagle
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#4

Re: Transformer Capacity

12/12/2011 4:53 PM

261 wk/0.80 pf = 326 kva. Any issues here? However the op did not say that all the motors ran at the same time.
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msamad
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#5

Re: Transformer Capacity

12/12/2011 7:20 PM

You didn't provide enough information. As already indicated, you didn't mention the diversity of the motor loads, largest motor size, motor starting method, future expansion etc. In the case of the possibility of running all the motors simultaneously, your transformer is undersize for sure. In addition to the running loads, you also must consider the starting of your largest motor and its starting method. Make sure your transformer's reserve capacity is at least 4 to 5 times more than the largest motor's kva.

For example, out of the 50 motors, consider one is 10 hp (7.5 kw, 460V) and all of the motors may simultaneously run. Assuming motor's p.f is 0.8, the kva size of the 10 hp motor is 7.5/0.8 = 9.3. In this case, total simultaneous running load is 352 x 0.746 / 0.8 = 328 kva. So, the minimum size of the transformer capacity is: 328 + 5 x 9.3 = 375 kva.

The transformer also should have the reserve capacity.

- MS

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anand purushothaman
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#6

Re: Transformer Capacity

12/12/2011 11:28 PM

Dear Susheel,

Please take the diversity factor into consideration apart from the parameters mentioned by other. It is dependent on the type of industry.

A rule of thumb is 2. that is . You can load the transformer 2 times of its connected load.

Regards,

Anand Purushothaman.

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pnaban
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#7

Re: Transformer Capacity

12/12/2011 11:47 PM

Guesswork is not recommended. Fix a datalogger/power recorder and from the graph of kVA you can judge the transformer size required.

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Magesh
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#8

Re: Transformer Capacity

12/13/2011 1:37 AM

You are connected 352 hp.. But we dont know what is the maximum demand..if you are not using any starter, it takes around 6 to 8 times of load current.. So you cant start the multiples of motors at a time.. If you maintain unity powerfactor you can load upto 250 kw..
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