Emf question

1. Mar 6, 2006

matt85

What would be the change of the induced emf in the secondary coil of a two coil system ( without ferromagnetic core ) if both the amplitude and the frequency of the current in the primary coil would double?:

emf would stay the same
emf would be halved
emf would be 4 times larger
emf would double
emf would be 4 times smaller

I figure that it is either 4 times larger or doubled...I know that it won't stay the same. How should I go about getting the answer?

Thanks a bunch,
Matt

2. Mar 6, 2006

Staff: Mentor

The input amplitude doubling will give you a first doubling of the output. But the doubling of the input frequency will give differenct results depending on the output impedance of the drive stage, and the leakage inductance and the mutual inductance and the load impedance. If the transfer function is in the stable part of the curve, then doubling frequency will have no effect. But if the doubleing of frequency occurs in the low part of the xfmr transfer function curve, then you could get up to a doubling of the output voltage from the 2x frequency change. Do they give you any more details about the drive circuit impedance and the load and the coil characteristics?

3. Mar 6, 2006

matt85

Nope, I do know that doubling the frequency of the current in the primary coil will double the emf, as will doubling the amplitude of the a.c. voltage and current in the primary coil (also will double emf).

4. Mar 7, 2006

mukundpa

Let
IP = Im*sinwt
dIP/dt = w*Im* coswt

e = - M(dIP/dT)

will this help?

MH

5. Mar 7, 2006

Staff: Mentor

But those are inductor equations, not transformer equations. Quiz question -- what is different about a transformer compared to just an inductor? What does a finte coupling coefficient and finite load impedance do to these equations? Why?

6. Mar 7, 2006

mukundpa

Just want to corelate the rate of change of current in the primary depends on the frequency of the input, will change the rate of change of flux through the secondary in the same ratio.

7. Mar 7, 2006

Staff: Mentor

Yes, and then what happens? What is the effect of there being a load current induced in the secondary? What does the secondary EMF do?

8. Mar 7, 2006

MP

9. Mar 7, 2006

Staff: Mentor

Yes, but my point is that the total secondary voltage depends on the load at the secondary. That's what makes a transformer a transformer. The back emf from the load current is what balances things out and gives a transformer its unique characteristics. I suppose it comes down to whether the question is asking about what the "induced" emf is or the "final total emf" is in the secondary. I also suppose that the OP could state that he is assuming an infinite load impedance, which would give him zero load current and zero back emf.... (not much of a transformer problem then though....)

10. Mar 7, 2006

mukundpa

Because of this I thought it in that way.

Sorry

MP

11. Mar 7, 2006

Staff: Mentor

No need to be sorry, you might be 100% correct. The lack of a core will alter the magnetic coupling coefficient, but if wound correctly, you can still get significant coupling, which will induce a back emf via the load current, which will change the overall emf values that the transformer stabilizes at. I think the original question is not asked very well. It's important for students who are learning about transformers to understand why the transformer equations are the way that they are -- and they come about because the load current induces a back emf that stabilizes the current and voltage transfer functions in the passband of the transformer.

Well OP, have we confused you sufficiently now? :-)

12. Mar 7, 2006

matt85

It was 4X by the way, and I got it right. Thanks for the help.