Why does Ohm's law not apply to transformers?

[AsrielDreemurr]

How can a transformer increase the voltage yet lower the current? Are the equations P=VI and V=RI contradictory?

Thanks in advance and sorry if I made any english mistakes.

 

[CheesyPeeps]

Hi, http://forums.mikeholt.com/showthread.php?t=160324discussion might help you with this.

 

[cnh1995]

Are the equations P=VI and V=RI contradictory?

Not at all. You'll need to study transformer magnetics first. Primary current "due to" the primary voltage source is called magnetizing current and it establishes the flux in the core. When we say V_pI_p=V_sI_s, I_p is the reflected secondary current, an extra current drawn from supply to totally cancel out the demagnetizing action of secondary mmf and maintain constant flux in the core(the one produced by magnetizing current). Magnetizing current is very very small compared to I_p, hence is neglected in explanations about the transformer. For power transfer, Ip is responsible. Get any good electrical machinery book and you'll see all these things explained in detail.

 

[sophiecentaur]

Are the equations P=VI and V=RI contradictory?

Not when you realize that the effect of a transformer is to modify (transform) the Resistance of the load to a different value, as seen by the supply. In order to understand this you have to get the causes and effects in the right order. The produce VI will be unchanged but the R of the load will determine the current through it for a given secondary Volts. The Load Current times the Secondary Volts is the Power dissipated and so is the Supply Current times the Primary Volts.
We all know that the Voltage is altered by the ratio of the numbers of turns in primary and secondary but also the Load resistance is transformed by the square of the turns ratio. So a mains transformer will supply high current at a low voltage (to a low resistance load) but demand a low current from the mains voltage supply (presenting itself as a high resistance).

 

[russ_watters]

How can a transformer increase the voltage yet lower the current? Are the equations P=VI and V=RI contradictory?

Increase the voltage vs what? Was the circuit run without a transformer before the transformer was installed? What was the current then?

The equations work fine, the problem is that you are not being strict/specific in your analysis.

 

[AsrielDreemurr]

I think I got it, thanks a lot!

 

[CWatters]

How can a transformer increase the voltage yet lower the current?

It doesn't lower the current. It's best to think of the current in the secondary as determined by the load not the transformer. No load => no secondary current I_s. If the load is a resistor of value R then I_s = V_s/R.

If the transformer increases the voltage (in the secondary compared to primary) then the current in the primary I_p will be greater than the secondary I_s. So it looks like the transformer is reducing the current in the secondary compared to the primary but in reality it's the other way around. The secondary current is dictating the primary current. This is due to the law of Conservation of Energy. If the transformer is lossless (aka "Ideal") then the power going into the primary must equal that drawn by the load on the secondary.

Conservation of energy...
P_p=P_s
but P=IV so..

I_pV_p=I_sV_s
rearrange
I_p = I_s* (V_s/V_p)

So if V_s > V_p then I_p > I_s