Transformer circuit quistion

Discussion in 'Homework Help' started by TheSpArK505, Oct 18, 2014.

Sep 25, 2013
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Hi everyone .

To answer a question u need to understand it 100%...and that's what I'm going for .
I have a question about transformers...and it says:

The transformer secondary is connected to an electrical load , and it delivers its rated current at rated voltage and 0.85 pf lagging , find primary voltage ?

Can any one explain please : what does rated means , and is the power of the load is only due to secondary voltage and as nothing to do with the primary voltage ...and also the underlined part please ??

2. MrChips Moderator

Oct 2, 2009
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Can you please post the complete question and not just part of the question?

3. Kermit2 AAC Fanatic!

Feb 5, 2010
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transformers 'transform' the voltage, current from one side to the other.

The power input = the power output theoretically.
Real world we know there are losses.
The 'power factor' is one of the corrections we apply to determine real world conditions
In the real world power in is ALWAYS more than power out.

Rated levels are given in a piece of equipments specs.

12 V 25 VA transformer(watts)
operates from 110 VAC

So you plug the numerical values found there into your calculations.

Sep 25, 2013
92
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the question says that a 50 KVA , 2400/240 V, 60 Hz before mentioning any load .....Where does the 50 KVA come from ?

5. studiot AAC Fanatic!

Nov 9, 2007
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And did you miss the lecture that explained VA ?

Also known as VARs or Volt-amps reactive.

6. crutschow Expert

Mar 14, 2008
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VA is volt-amps which is just the product of the volts and amps that the transformer is rated for. It is the phasor (phase vector) sum of the real and reactive power.

7. The Electrician AAC Fanatic!

Oct 9, 2007
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Volt-amps (VA) are not the same as volt-amps reactive (VAR).

8. studiot AAC Fanatic!

Nov 9, 2007
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You are quite right I misspoke in haste.

The proper story is explained here very well.

http://en.wikipedia.org/wiki/AC_power

Apr 5, 2008
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Sep 25, 2013
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Thanks guts......some of u miss understood me ...my question that there is no load.....where does the 50 KVA come from?
also..does the KVA = Vp,rated*Ip,rated=Vs,rated*Is,rated

11. crutschow Expert

Mar 14, 2008
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3,385
The 50 KVA is the transformer maximum rating. That actual KVA depends upon the load. If there is no load the KVA is essentially zero.