Vs/Vp = Ns/Np This apply in transformer.

[7bear]

Vs/Vp = Ns/Np
This apply in transformer.
But I have read the following statement :
' Uasally , voltage ratio is larger than the turns ratio. This is because not all field lines from primary coil cut the secondary coil.'
Why? Is that mean some field lines 'go' to surroundings?

 

[T@P]

I am no expert, but from what i understand the magnetic field, shown by the field lines, is *mostly* confined to the iron frame or whatever. mostly, as in not all, so that may explain it. am I right? this is more of an educated guess rather than a real solution

 

[Cliff_J]

Well I'm no expert either and would love some answers on my thread in the EE section but here goes.

The primary will indeed generate a magnetic field, but since there is no perfect copuling some of the lines are not intercepted by the secondary. In the transformer model I'm reading about the author modeled it as the "leakage inductance" and its in series with the secondary. He assigns the leakage inductance as being the culprit for not having an infinite primary impedance when the seconday is unloaded.

 

[rayjohn01]

The last statement is not true -- impedance in the AC sense is dependant upon L and frequency . The best way to understand the transformer is via it's equivalent circuit
Unfortunately a real transformer is a very complex animal and it is best to take it a bit at a time . First ignore any resistance , second ignore any capacitance just deal with inductive aspects .
It is true that not all the magnetic flux from the primary links the secondary ( with cored transformers it may not be much ) but it does exist and effects high frequency performance .
The simplest equivalent is an 'L' of inductances i.e. one series element and one parallel followed by a perfect transformer.
The series element represents the 'leakage inductance ' it can be measured
by a short circuit applied to the secondary , the parallel element is the main primary inductance as measured by an open circiuit . These values are good
if Ls << Lp . From this it can be seen that at high frequencies Ls may
become important .
I have to give you a warning -- it can take the best part of a lifetime to understand all the vagaries of this beast.
Yours Ray.


Ps for those interested in how to post diagrams this is a 60 dpi Jpeg originating in Corel Draw -- it is enclosed in a rectangle to make sure that a whole page ( letter is not posted ) . 60 dpi keeps it small yet visible so as not to waste Forum memory . Ray . ALso notice you do not have to down load this to view just click on the diagram .That is just a jpeg attribute not Cdraw .