Discussion in 'Homework Help' started by Shafkat Shishir, Jul 8, 2009.

1. ### Shafkat Shishir Thread Starter New Member

Feb 15, 2009
6
0
Hello every one,
I need the a little bit of solution of below this 2 questions.

1.) Why does the secondary voltage increase when we connect lagging load or unity p.f. Loads at the output.
**What happens when we connect leading loads at Terminal and the secondary side & why??????
........Preferred book by prof.(Pustin)
2.)why do we need lower winding ratings for an auto transformer to transmit the same power then a conventional two winding transformer?Why do we not use auto transformer to step voltages up or down in power system????

please if anyone knows then let me kow...from where I can get the proper solution.and if anyone of u have any HINT then please let me know....
Thanks
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• ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,791
771
One problem arise, unity p.f loads? is it possible.

Rifaa

• ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
I'm not sure this is a reasonable statement in general. Perhaps the question needs some further clarification as to the conditions you are assuming.

Mar 28, 2009
12
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hi , every one ! please for the question posted !

I think Shishir meant " why the secondary voltage increase die to capacitive load at secondary side "??

thanks every one !!

• ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
Clue: A pure resistive load is 'unity power factor'.

For the transformers, pick some convenient values (Volts / Current / Ratio) then use them on both a two winding transformer and an autotransformer.

• ### JDT Well-Known Member

Feb 12, 2009
658
85
1/ Not quite sure what you mean here but remember no transformer is perfect. They all have leakage inductance caused by a less than perfect magnetic coupling between primary and secondary. This shows up as a series inductance through the transformer. Connecting a capacitive load can make the output voltage rise. There is also the resistive losses in the copper and magnetic losses in the iron core. These will cause a reduction in output voltage with load current.

2/ In an auto transformer, the shared part of the winding only carries the difference between primary and secondary current. Therefore the wire can be thinner. This is only useful when the ratio between input and output voltages is fairly small. In a utillity power transformer the ratio could be 11kV in and 415V out so an auto transformer does not give any benefit.

Also a power transformer will often have delta connected primary and star connected secondary. The secondary star point is usually locally earthed. Isolation is important in case of lightning strikes on the overhead lines. The delta - star connection is also useful in reducing harmonics.

• ### Shafkat Shishir Thread Starter New Member

Feb 15, 2009
6
0
Thanks JDT but for the and 2 I am satisfy but u know for the ans of no. 1 here when we connect the capacitive LOAD though there is a neglegible loss but it is negligible so i think there is voltage arise all the time by ratioo..what you say!??!?!?

• ### Shafkat Shishir Thread Starter New Member

Feb 15, 2009
6
0
ya rjenkins you are right and here the pure resistive is is unity P.F.