# Homework Help: Quick question on transformers

1. Apr 20, 2010

### Magma828

I need to know the purpose of using different numbers of coil turns for the primary and secondary coils on a transformer, provided the turns ratio remains constant.

Put in to context, what is the difference between a transformer with 10 turns on the primary, 20 turns on the secondary and a transformer with 100 turns on the primary and 200 turns on the secondary? The turns ratio is the same for both, but surely there's a different result?

2. Apr 20, 2010

### collinsmark

Hello Magma828,

What things would change in a single coil if you increased the number of windings (keeping everything else equal, such as the gauge of wire, material of core and wire, radius of windings, etc.)? Then think about multiple sets of windings after that.

3. Apr 21, 2010

### Magma828

I guess the resistance of the coil would increase, and there would also be greater magnetic flux linkage. A higher magnetic flux linkage = a higher induced emf, but according to the (simple) theory I know, only the ratio matters when you do it for two coils.

4. Apr 21, 2010

### collinsmark

That's a great start.

And you're right, that the mutual inductance falls out of certain equations when you model the transformer using simplistic approximations (such as the voltage ratio merely being a function of the turns ratio -- not the actual number of turns themselves).

So, so far you've indicated that the series resistance of the coil(s) would increase with a higher number of turns.

But there is one other aspect (besides just the series resistance) of a single coil that is a function of the number of turns. Can you think of that? It turns out that this aspect has an impact on a 2-coil transformer performance, even when you model it simplistically, particularly when the secondary coil has no load (is an open circuit).

5. Apr 21, 2010

### Magma828

Um.. is it the time taken for the current to change direction or something like that? Or loss of magnetic flux?

I'm a bit lost now :P

6. Apr 21, 2010

### collinsmark

In terms of physics/engineering, what do you call wire wound up in a coil, which may or may not contain a metal core? (Hint: its main characteristic involves reactance, not resistance, -- and it's not a capacitor.) What are some of the physical properties which dictate its electrical properties?

[Edit: Think of a wall transformer attached to a wall outlet (mains), but is not powering anything or even plugged into anything (only the wall itself). Some current still flows through the primary coil, and there is some power loss due to the resistive heat dissipation in primary coil. Yes, the primary coil's series resistance is involved with this pointless power loss to some extent, but there is more important factor involved, even if we ignore hysteresis in the core. This other aspect is particularly important in minimizing the current in the primary coil when the secondary coil is open-circuit. Another hint: Since there is no current flowing through the secondary coil, it's a function of the primary coil's physical configuration.]

Last edited: Apr 21, 2010