# Transformers - Why output current decreases for step-up xfmr?

1. Oct 30, 2013

### tomizzo

Transformers -- Why output current decreases for step-up xfmr?

So I've been wondering if anyone in PF could explain to me the reasons currents can either increase or decrease in a transformer depending on the wiring.

I understand electromagnetic fields/forces, magnetic flux, and I even could tell you the input and output currents given a transformer with the number of turns of each winding.

However, I never learned the reason why in, for example, a step up transformer is able to increase voltage but the current decrease. I understand that these values have to inversely change from one another to keep constant power, but I want to know why a changing magnetic flux through more coils creates less current and greater voltage and why changing the flux through fewer coils creating lower voltage and higher current.

Any help?

2. Oct 30, 2013

### meBigGuy

power in == power out

10V 1 amp in = 10 watts
20V 0.5A out = 10 watts

3. Oct 31, 2013

### wirenut

Could not have said it better, meBigGuy! Most people I try to explain that to can't wrap their head around it.

Last edited by a moderator: Oct 31, 2013
4. Oct 31, 2013

### Staff: Mentor

Conservation of energy may explain "why" at a high level, but it doesn't really explain "how". The "how" is that current creates a magnetic field, but a changing magnetic field opposes the current, creating the voltage drop. And since the primary and secondary are sharing the same changing magnetic field, they are mirror images of each other.

http://en.wikipedia.org/wiki/Transformer#Induction_law

5. Oct 31, 2013

### meBigGuy

I said power in == power out and maybe you wanted to know more about how that actually occurs.

First, why does the flux created by 2 turns cause twice the voltage on 4 turns? Basically the voltage needed to put current through 1 primary turn will creates the flux that will generate the same voltage on 1 secondary turn. But there is only so much energy in the field so when you take it out with 2 turns (at twice the voltage) you can only get half the current before depleting the available energy supplied by the primary.

6. Nov 1, 2013

### cabraham

Ampere's law. Flux relates to amp turns, voltage relates to flux and frequency. The amp-turns are the mmf (magneto-motive force). The induced voltage on the secondary is determined by the constant voltage source driving the primary and the turns ratio. So a 20 volt source attached to a primary w/ 20 turns results in a core flux at the source frequency. This core flux is changing and a secondary voltage exists. Under no load, if the secondary is 10 turns, then the secondary voltage is 10 volts.

But we attach a 10 ohm load across the secondary. Current is now established in the secondary. But this current has an mmf that *opposes* that from the primary source input. The tendency is for the secondary load current mmf to reduce the core flux and consequently reduce terminal voltage. The emf generated by this load currnet mmf is "counter-counter-emf. The load current is counter-mmf.

But the primary is supplied by a CVS (constant voltage source). A CVS will force constant voltage at the xfmr primary terminals by supplying any necessary current. When the primary terminal voltage drops due to load current cancelling core flux, the CVS outputs larger current to the primary. The primary current is simply (CVS voltage value minus xfmr terminal voltage)/primary winding impedance. The CVS value is fixed, so that when the primary voltage drops, the primary current increases since a larger voltage difference gets dropped across the primary winding impedance.

The increased primary current cancels the cancellation of core flux due to load current. Equilibrium is reached when amp-turns balance. To correct the flux drop due to 10 amp-turns in the secondary, we need 10 A-t in primary. With 20 primary turns this computes to 0.50 amp.

Claude

7. Nov 1, 2013

### sophiecentaur

Get your 'causes and effects' in the right order and it may make better sense.
1. You apply your supply volts to the primary. 2. This causes a secondary voltage (transformer ratio). 3. That secondary voltage will cause a certain current to flow through the load (I = V/R).
4. That secondary current will result in a primary current (transformer ratio again).

So the turns ratio doesn't 'cause' a lower or higher current in the load.