Something about power calculation for transformer

Hi,

I'd like to get some guide from gurus here how to work out the Ampere for a step down transformer.

Given information:

3 phase input 415 VAC
3 phase output 210VAC
50kVA

How do we calculate the ampere for the transformer output?

This step down transformer will be using in Malaysia.

Thanks in advance.

(50.000/3)/(210/1.73) = 142 A

seppoalanen said:

(50.000/3)/(210/1.73) = 142 A

Click to expand...

Hi,

The 1.73 we got from 415/240 or ??

Well.... 1.73 is the square root of 3....

Here is a link to a page that talks a little bit more about 3 phase transformers and some different connection styles, with a couple of sample equations at the bottom.

Wow!!!

What a great & details site thx a lot...now a lot i can read already.

seppoalanen said:

(50.000/3)/(210/1.73) = 142 A

Click to expand...

Hi,

Based on your calculation the answer is 142A, but based on the sample given from the site, it's something like this:

What size fuses is needed on the primary side to protect a 3 phase 480v to 208v 112.5 kVA transformer?

* Important when dealing with 3 phase applications always use 1.732 (square root of 3).

I truly wonder what kind of technical knowledge is really here. Now I know and I'm scared.

You asked for the OUTPUT of the STEPDOWN xfrm. The right answer is 50000 VA divided by 363.7 = 137.5 amps

The input is

50000 divided by 718.8 = 69.6 amps

Hopefully you are just doing this for knowledge and not designing some system. If you are, you need to find someone who can help you out and verify everything you are doing is correct.

As you can probably see, DO NOT BLINDLY TRUST SOMEONE WHO POSTS ANSWERS. Mr. Stalcup gave a good reference. "seppoalanen" will get you and someone else killed.

Bruce,

I agree with you first statement "Wow!!!"


ckchew666,

Let me hold your beer when you flip the switch, you won't need it for long.


There was a post not that long ago...someone was trying to do a pear to pear communication at the end of the thread and a lot of people trying to help this individual someone brought up a good point, SAFETY we are all here trying to help each other, but when it comes to something like this... if someone gets hurt what are you going to say...well that’s what they said on ‘plcs.net’ I thought they new what they were talking about...hell have the time i'm full of it. but I still don't want to here about someone getting hurt.

But you also HAVE TO calculate the short circuit at the sekundary (210V) side. This is very important for dimensioning your cable and for the overload switch or fuse...
When using fuses the lowest short circuit is really important (may not be to small) and when using the overload switch, the maximum short circuit is important.
And then again the cable have to be able to carry the energi from the short circuit until the fuse/overload switch breakes.

Best regards

Dariusch

All the answers and methods used produce the same result, seppolanen just made an error in typing the result.
(50.000/3)/(210/1.73)
50000/3 = 16666.67
210/1.732 = 121.25
16666.67/121.25 = 137.45
OR
50000/(210*1.732)
210*1.732 = 363.72
50000/363.72 = 137.47

They are the same formula and give the same results...NOTE 50000va or 50KVA is the power that can be consumed and the ratio of the turn windings in a transformer will be proportional..ie 415/210 = 1.976 therefore the output current will be 1.976 times the input;
137.47/1.976 = 69.57A therefore the incoming fuse/breaker would be 86.96A or the next higher rating which would probably be a 100A. Note the incoming supply wire should be rated to match the fuse/breaker involved, in this case for a 100A

The secondary is a different subject, the circuits are wired and fused according to the needs of each branch.

There are formulas, primarily for motors, that will assist in making conversions for single or three phase power, current, and voltage. http://www.patchn.com/motorformula.htm

For a simple explanation of 3 phase and why 1.73 is used; http://www.patchn.com/three.htm

rsdoran said:

415/210 = 1.976 therefore the output current will be 1.976 times the input

Click to expand...

That is if the transformer efficiency is 100%. Which does not exist in nature.If the rated efficiency is 90%, then the current in the primary circuit would be 10% higher. That 10% would just cover the losses (windings heating, core magnetizing etc.) within the transformer itself.

Hi,


Don't worry, I'll leave the design job of the transformer to the professional, I just gave them the Input & Output rating + VA, so they'll design for our customer.

Even though, I'd like to know how they work out the amp as well, just to learn more

If I understand the replies, the input amp will be:

INPUT
(50.000/3)/(210/1.73)
50000/3 = 16666.67
210/1.732 = 121.25
16666.67/121.25 = 137.45
OR
50000/(210*1.732)
210*1.732 = 363.72
50000/363.72 = 137.47

OUTPUT

while the output amp will be

137.47/(415/210)
137.47/1.976 = 69.56 Amp

INCOMING FUSE/BREAKER

The incoming fuse/breaker shall be:
69.56Amp * 1.25 = 86.95Amp (or next higher rating - 100Amp)

Pls correct me if I'm wrong. Thanks...

Not exactly

ckchew666 said:

Hi,


Don't worry, I'll leave the design job of the transformer to the professional, I just gave them the Input & Output rating + VA, so they'll design for our customer.

Even though, I'd like to know how they work out the amp as well, just to learn more

If I understand the replies, the input amp will be:
INPUT THIS PART IS SECONDARY OR OUTPUT
(50.000/3)/(210/1.73)
50000/3 = 16666.67
210/1.732 = 121.25
16666.67/121.25 = 137.45
OR
50000/(210*1.732)
210*1.732 = 363.72
50000/363.72 = 137.47

OUTPUT THIS PART WOULD BE PRIMARY OR INPUT.

while the output amp will be

137.47/(415/210)
137.47/1.976 = 69.56 Amp

INCOMING FUSE/BREAKER

The incoming fuse/breaker shall be:
69.56Amp * 1.25 = 86.95Amp (or next higher rating - 100Amp)

Pls correct me if I'm wrong. Thanks...

Click to expand...

With a step-down transformer the low voltage calculation gives you amperage on the secondary (output). The formulas above can be used with both primary (input to xfrmr) and secondary (output from xfrmr). Eample;
50000/(415*1.732)
415*1.732 = 718.78
50000/718.78 = 69.56

In your case you have a step-down transformer which means basically that the Secondary current will be higher than incoming current, I mentioned turns ratio because for rule of thumb it can offer an approximation.

Knowing KW or KVA means you know the power capabilitiies, this will not change. Simple example; a transformer 4KW or 4000W with primary at 400v would mean 10Amps, the secondary at 200v would equal approximately 20Amps....see the correlation?
400 times 10 = 4000
200 times 20 = 4000
The secondary voltage is 1/2 the primary, therefore the current must increase on secondary to maintain power requirments.

Yes there is more to it but much of it can be "rule of thumb" calculattions that while not exact will be close.

I am still not sure what the "WOW" was for, the only error shown was to show too high a value for the secondary calculation by 5 amps (approximate), not sure that could cause a major issue personally. As I mentioned the formula seppolanen used is the same as the others, just expressed in a different form.

Picture is more than couple of words.