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polerz
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Join Date: Jul 2010
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Calculating Transformer Impedance

10/29/2010 11:23 PM

If you have a transformer name plate which dictates: Isecondary(rated) = 3000A Vsecondary(rated) = 720V Is the transformer impedance not: Z=V/I Z=720/(sqrt(3)) / 3000A Z=.13 ohms?

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polerz
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#1

Re: Calculating transformer impedance

10/30/2010 12:09 AM

Also, can someone explain to me how they get the following rule of thumb formula for calculating short circuit power: Short circuit MVA = 100*S / (X%) where S = Rated transformer MVA X% = percentage impedance of the transformer Thanks

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kvsridhar
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#2
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Re: Calculating transformer impedance

10/30/2010 12:53 AM

Excerpt from the webpage : http://www.federalpacific.com/university/transbasics/chapter5.html

The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. Impedance is expressed as a percent. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by the multiplying the reciprocal of the impedance timed the full load current . Thus, if a transformer has 5% impedance, the reciprocal of .05 is 20 and maximum short circuit current is 20 times the full load current.

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DamnGud EE
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#3
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Re: Calculating transformer impedance

10/30/2010 10:57 AM

Dear Kvsridhar,

Sir, Excellent website. Quite intriguing and Informative. Keep up the good work. Thank you.

Regards,

DG EE

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kvsridhar
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#4
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Re: Calculating transformer impedance

10/30/2010 11:14 AM

Glad you liked it

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#5
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Re: Calculating transformer impedance

10/30/2010 11:38 AM

Excellent information, but what type of insulation which allows the transformer to run at 220 degree centigrade ?
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kvsridhar
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#6
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Re: Calculating transformer impedance

10/30/2010 11:53 AM

i am no expert in this field, but Federal says it is Nomex....

You will probably get more information HERE

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