Conservation of energy in transformers

[jamie1121]

Homework Statement

"Why does increasing the load on the secondary coil circuit increase the current in the primary coil circuit"

Relevant Equations

Qualitative explanation

This question is given as an exercise in my book. I can't figure out whether this is a poorly worded question or if I misunderstand. The answer I can come up with is that power is dissipated over the load so more power is needed to be supplied by the ac source. This seems too hand-wavy to me though, why would it not increase in the first place? How could it know that there is an increase in power needed if there was no change in current or magnetic flux from the secondary side to signify the change? Surely the component would just have a smaller potential drop?

 

[cnh1995]

How could it know that there is an increase in power needed if there was no change in current or magnetic flux from the secondary side to signify the change?

The secondary current changes based on the load impedance. This proportionally changes the primary current such that Pin=Pout.
I am not sure if I understood your question correctly.

 

[hutchphd]

This assumes the driver of the primary provides a set voltage. The current in the secondary of the transformer produces back emf reduces the field and makes the primary work harder (more and things current). If the secondary is open there is no current in the secondary
If the load is not purely resistive it gets a bit complicated but the Math is straghtforward and things are reflected back through the magnetic coupling to the primary.

 

[jamie1121]

This assumes the driver of the primary provides a set voltage. The current in the secondary of the transformer produces back emf reduces the field and makes the primary work harder (more and things current). If the secondary is open there is no current in the secondary
If the load is not purely resistive it gets a bit complicated but the Math is straghtforward and things are reflected back through the magnetic coupling to the primary.

Please check my understanding, what you're saying is that an emf is produced in the opposite direction in the secondary coil so the net emf in the secondary coil would decrease but from faraday's law this would mean that the rate of change of flux linkage in the secondary decreased(*), which cannot happen since it is the same as in the primary coil.
If that is correct, I understand this partly but I still have two queries
-Could you elaborate a little more on the back emf, what causes it, the fact that resistance is added makes sense that the current would decrease, but from kvl surely the potential around the second circuit back to the coil must be the same?
-If (*) is correct, surely this means that the input voltage to the primary coil has to increase since induced emf on the other side is only dependent on rate of change of flux linkage(not on current)

 

[cnh1995]

Primary current has two components.
One component is responsible for establishing magnetic flux in the core (as per Faraday's law) and is therefore called 'magnetizing current'. It flows as long as the primary winding is excited by a voltage source, and does not depend on the secondary load condition (ideally). Due to this magnetizing current, a sinusoidally varying magnetic flux is established in the core. This varying flux induces an emf E in the secondary winding based on the turns ratio of the transformer.

The other component of primary current depends on the secondary load. If you connect a resistance R across the secondary, the secondary current will be E/R. This is the load current, and it gets proportionally reflected in the primary circuit through the magnetic coupling, such that
primary current*primary number of turns= secondary current*secondary number of turnns. In other words, the primary voltage source "sees" the secondary impedance and adjusts its primary current proportionally to achieve the power balance.
Look up ampere-turn/mmf balance in transformers.
If no load across the secondary, no secondary current, no reflected primary current, only magnetizing current in the primary.

The total primary current is the phasor sum of the magnetizing current and the reflected component of the secondary current.
In practice, the magnetizing current is only 3-5% of the load current. Hence, it is mostly ignored in basic discussions of the transformer.

 

[DaveE]

The transformer core contains (nearly) all of the magnetic flux from both the primary and secondary windings, so they are strongly coupled. Faraday's law says that if the flux changes, a voltage is induced to oppose that change. Of course the flux in the core is equivalent to the sum (difference, in practice) of the winding currents, and the induced voltage will appear in both windings.

When the secondary current increases this creates an induced voltage in the primary winding that will tend to increase the primary current. Whether that primary current actually can increase depends on the circuit it is connected to. The circuit connected to the transformer is often neglected in discussions of how they work which can create confusion. There is an underlying assumption in the way this is posed that the magnitude of the AC flux in the transformer doesn't change, it doesn't have to be that way. For example what if the primary winding wasn't connected to anything, or was connected to a high impedance current source, then it's current wouldn't change at all.

When you call something a transformer, you are implying that the flux magnitude doesn't change. Or, more accurately that changes in the winding currents don't store additional energy in the magnetic field of the core. If that energy was stored then it would behave as an inductor. In practice the differences aren't absolute. There are common circuits used in practice that rely on devices that operate both (simultaneously) as inductors and transformers.