AL Phy ~Current change in transformer

2008-04-11 12:59 am
If it is an AC supply step down transformer , let AC supply be V (fixed amount)

primary turn : secondary turn = N1 : N2

primary coil has internal resistance r1, current i1, primary voltage E1

secondary coil has internal resistance r2 ,current i2 , step down voltage E2

E1 : E2 = N1 : N2

secondary coil is connected to a resistor R

such that V= i1 ( r1) + E1 --------(1)

E2= i2 ( r2 + R) ---------(2)

I wanna know if R is decreased , will i1 increase?

Someone told me that you can assume E2 is a constant , when R decrease, i2 will increase then power in secondary coil will increase ( power = i2 x E2)
My problem is if E2 is fixed as constant then E1 show be in constant since
they are in ratio
Then in (1) V , r1, E1 are always constant i1 can not be changed or
increase
can someone sovle it for me
更新1:

Why i2 will increase?

更新2:

Do u mean at that particular instant the E2 is constant and that's why i2 increases?

更新3:

If so, how does the magnetic flux affeacted to decreases by Lenz Law? Should it be the magnetic flux by sencondary core increase due to increase in i2 and opposite to the direction of the magnetic flux by primary core ? So magnectic flux decreases and E1 decreases

回答 (2)

2008-04-19 4:30 am
✔ 最佳答案
When the load resistance R decreases, the processes taking place are as follows:
--- i2 increases (using the same symbols as in the question)
--- magnetic flux in the transformer core decreases (because of Lenz Law)
--- E1 decreases (since E1 is the back emf in the primary coil)
--- i1 increases (because the supplied voltage V is constant, refer to your equation (1))
--- mangetic flux in the core increases
--- E1 increases to its original value

It is true that the ratio of E1:E2 remains almost constant and is related to the turn-ratio.

You should also be aware that in your equation (1), the potential drop across the primary coil, (i1).(r1) and the back emf E1 are not in phase. Hence, the two terms cannot be viewed as adding together arithematically.
Thus for a constant V, i1 can increases with E1 (the back emf) keeping more or less constant just by a change of the phase angle between (i1).(r1) and E1.
2008-04-11 2:17 am
You should have some conceptual misunderstanding in the power consumtion in secondary coil.

please take a look in http://en.wikipedia.org/wiki/Transformer

for power, current and resistance relation in secondary coil.


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