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# Why does a transformer transfer power from the primary to the secondary one?

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#### Subhasha

##### New Member
I'm struggling to keep my mind wrapped around the function of the transformer and, in the process, regretting the days I snooze back in my Electromagnetics class as an EE student when I was a boy I'm searching for an abstract explanation, but not just an equivalent one. I want it to be rooted in the real mechanics of what's going on. I've found a few excellent sources on the web, but all of them seem to skirt this issue.

I've come across a few interesting clues, so I'm pretty nearly now, I guess, but still yearning

Fact 1: While it varies sinusoidally, the "peak-to-peak" flow in the center of the transformer, so to speak, is basically constant (for a given voltage applied to the primary) regardless of the load.

My initial intuition was that the difference in the "power" of the flux was what moved the energy from the primary to the secondary, but this would seem to contradict the idea. I assumed the primary would produce a lot of fluxes dependent on the current running through it, and the secondary would suck it up and create the current on its own. No chance, it feels like that.

Then, of course, there is the fact that the Flow Formula only includes voltage, time (frequency) and turns

Fact 2: The current in the main is (usually) 90 degrees out of phase with no load voltage and is roughly centered in phase at full load.

It seems really interesting and yet strangely rewarding. This will mean that the Volt-Amps (VA) of the primary remain stable and that the power factor varies as the actual load on the secondary decreases.

Yet I really don't know how the energy is being transmitted. It seems slightly like the flux is only there as an energy conductor or something, and the other phenomenon is a bit of energy transfer.

Will anybody see what I'm missing and explain what's going on?

Look up Lenz's Law.
1) Moving a conductor (wire) through magnetic lines of flux creates current flow in the wire.
1A) Moving magnetic lines of flux passing through a stationary wire creates current flow.

2) Current flowing in a wire creates a magnetic field that can be passed to an iron core.

Take one and two together, you can build a transformer.

Fact 1: While it varies sinusoidally, the "peak-to-peak" flow in the center of the transformer, so to speak, is basically constant (for a given voltage applied to the primary) regardless of the load.
Incorrect.

While there is some primary current when the secondary is open circuit, the primary current increases with loading of the secondary. The load on the secondary is reflected back to the primary by the square of the turns ratio.

ak

While it varies sinusoidally, the "peak-to-peak" flow in the center of the transformer, so to speak, is basically constant (for a given voltage applied to the primary) regardless of the load.
I assume you are referring to the core magnetic flux.
Yes, the transformer core (magnetizing) flux is a function of the applied voltage, and is basically constant with a change in load current (for a well-made power transformer).
The load voltage and current direction is such as to subtract from that flux, but that just causes the primary current to increase to maintain the flux constant.
So interestingly, the flux value does not significantly change, even though it is transferring the energy from the primary to the secondary.
It is sort of like an inelastic coupling between primary and secondary.

The generation of the magnetizing flux is inductive, so that's why its current is 90 degrees shifted from the voltage.
The load current providing power has to be in phase with the input, thus making its primary current in phase with the secondary.

Does that help?

I have to be honest I have never given the operation of transformers that much thought, electromagnetic induction was as far as my thinking went..however I did do some research to find out why transformers operating at higher frequencies can be made so much smaller.

This thread reminds me of another question that has been bugging me, but I will start another thread for that.

I did do some research to find out why transformers operating at higher frequencies can be made so much smaller.
Did you find the answer?
Transformers are designed only as large as needed to transfer the desired power while keep the magnetizing current below the core saturation value.
Since the the magnetizing current is determined by the transformer inductance and frequency, and is inversely proportional to frequency, then a higher operating frequency will allow a smaller transformer core while still maintaining the magnetizing current below the core saturation value.

a higher operating frequency will allow a smaller transformer core while still maintaining the magnetizing current below the core saturation value.

For example, aircraft use 400Hz for AC power to allow much smaller (lighter) transformers and motors.

transformer ……. higher frequencies can be made so much smaller.
One of the things that effect a transformer core material is (volts X time).
60hz=16mS My power line goes plus for (120V x 8mS) then negative for 120V X 8mS. So the transformer must with stand 960VmS.
In a switching power supply the "ac" might be 60khz not 60hz. So the volts time on the transformer is 1/1000 as much.
There are other factors that effect the transformer but that one is easy to see.

Did you find the answer?
Transformers are designed only as large as needed to transfer the desired power while keep the magnetizing current below the core saturation value.
Since the the magnetizing current is determined by the transformer inductance and frequency, and is inversely proportional to frequency, then a higher operating frequency will allow a smaller transformer core while still maintaining the magnetizing current below the core saturation value.

I did find the answer thanks, and I sort of understood it, its just a lot to get my head around, so I just accept it lol

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