Transformer question

[indulis]

While I've never designed an iron core transformer, I have designed more ferrite core transformers than I care to remember. Ferrite core transformers are typically 98% efficient. If core losses are any higher, it's considered a bad design. Core loss is a function of frequency and volume (and this is stupid... in KW/M^3... I've NEVER seen a one cubic meter ferrite core). Then theres skin affect (not a big problem at 60Hz, but at 500KHz that's a different story), eddy current losses, I^2*R losses, coupling losses if it's not wound well. Voltage, current , inductance and power are all proportional to the turns ratio.

[KevinIV]

Gazza, that is the problem, the turns ratio should have stayed the same. A power transformer needs to transfer energy from the primary to the secondary. How are you going to get power in the primary if the reactance is so high that you don't get any current in the primary? Conversely, how are you going to get current in the primary if you don't have enough voltage?


I use peak power to avoid confusion. V*V/R can give you different results depending on what numbers you use.

[audioguru]

Kevin,
You want a transformer primary winding to have a high reactance, when the secondary doesn't have a load. Then the transformer doesn't draw much current when it shouldn't.
When you add a load to the secondary then you want the magnetic coupling to transfer power from the primary to the secondary.

[indulis]

What is the primary current in ANY transformer?

The primary current will be the secondary current reflected to the primary by the turns ratio plus the magnetizing current.

Don't confuse reactance with resistance!!

[audioguru]

Hi Indulis,
Sure a transformer without a load has a high reactance in its primary winding.
If you remove its core then it is just a very low resistance piece of wire.

A resistive load on the secondary is reflected and stepped up or down to the primary in parallel with the very high reactance. Since the reactance is much higher than the reflected resistance then the reactance is neglegible and the primary appears resistive.

[indulis]

Guru

My comments were intended for Kevin...

You mean that the inductive reactance dominates (XL) the impedance (Z).

Without a core it's still an inductor (air core) with mutual coupling to another inductor... sure the coupling coefficient is crappy, but it's still "more" than just a piece of wire.

[audioguru]

At only 60Hz, the inductive reactance of the primary of an air-core transformer is like a piece of wire, a very low resistance.

[KevinIV]

I attempted to construct a step down autotransformer one time with magnet wire. I figured that without a load, if I got the turns right, I would have a simple voltage divider. Being totally reactive for the most part, it would consume very little power.

The primary reactance was about 1000 ohms, the secondary reactance was about 1000 ohms. I took a 20 ohm resistor and attached it to the secondary. Is that a problem? A 20 ohm resistor in parallel with a 1000 ohm reactance. Will the autotransformer still be 2:1.

[audioguru]

The power rating for a mains transformer depends on the size of the core and wire, the number of turns and a few other things. If the input is 120V and the output is 60V then a 20 ohm load would be 180W. It must be a pretty big transformer.

[Gazza]

What is the primary current in ANY transformer?

The primary current will be the secondary current reflected to the primary by the turns ratio plus the magnetizing current.

Don't confuse reactance with resistance!!

Im glad some one gets it.