# Transformer circuit quistion

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

1. ### TheSpArK505 Thread Starter Member

Sep 25, 2013
101
1
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
14,291
4,197
Can you please post the complete question and not just part of the question?

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,120
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.

4. ### TheSpArK505 Thread Starter Member

Sep 25, 2013
101
1
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
5,005
519
And did you miss the lecture that explained VA ?

Also known as VARs or Volt-amps reactive.

6. ### crutschow Expert

Mar 14, 2008
16,217
4,336
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
2,368
353
Volt-amps (VA) are not the same as volt-amps reactive (VAR).

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
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
16,983
2,965
10. ### TheSpArK505 Thread Starter Member

Sep 25, 2013
101
1
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
16,217
4,336
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.

12. ### TheSpArK505 Thread Starter Member

Sep 25, 2013
101
1
OK ...But is it true that KVA = (Vp*Ip)both rated=(Vs*Is)both rated

13. ### crutschow Expert

Mar 14, 2008
16,217
4,336
Neglecting the small transformer losses, yes.