One way power flow within transformer with low pass filter

[Salvador12]

I will make a setup and i'm almost sure how it should work, but I want to check. Sorry for my previous threads being somewhat confusing.

So the idea is this. I have a high frequency generating coil part of a circuit and I need that high frequency to be converted down to mains frequency.
I will make the high frequency coil generate PWM wave into the primary of a transformer with a capacitor in series. So the primary loop as can be seen in the drawing is a series LC.
the secondary contains a low pass filter.
If my understanding is correct and please do correct it if necessary two things should happen

1) Power should be able to flow from the high frequency side through the transformer/filter into any load on the low frequency side
2) Power from the low frequency side (if load become source) cannot flow back into the high frequency side , at least not by any large margin because the frequency is low and the high frequency side has low capacitance capacitor + series LC resonance would present a very high reactance to the low frequency

Therefore due to differences in frequency power can only flow from left to right in the drawing and not vice versa or it can flow from high frequency side to low frequency side but not back, correct?

Another question is this, given the primary side and secondary side work on high frequency, the transformer therefore can also be a high frequency transformer , but as a high frequency transformer its turns ratio would be different than a typical mains transformer, yet the secondary side of the transformer still has mains 50 Hz across it, how would these factors work, because as far as I'm aware a low pass filter does permit low frequency power flow both ways right? It's only the high frequency that is cut off, so the low frequency power would be able to reach the transformer secondary ?
But if I'm right given the transformer primary works with high frequency and would present an almost open circuit for low frequency it would make the transformer secondary a very high inductance load for the 50Hz current so that would block the low frequency power from passing through the transformer. One thing i'm not sure is how would the low frequency power affect the secondary coil given it would have a lower turns ratio than a mains transformer and therefore would heat up with 50Hz current passing through it?

 

[KlausST]

but the transformer primary inductance changes with load,

in a transformer there are several "inductances"

let´s talk about a normal 50 Hz mains transformer.

There is the primary magnetizing inductance. It is in the range of mH and thus causes only a few mA of magnetizing current.
It will not change with (much) load. It is in parrallel to the load (current).
Then there is the coupling between primary and secondary. (for sure it technically works with magnetizing via the core)
But it "adds" to the magnetizing current. It often is referred as "coupling" between primary and secondary.
And in series to this "coupling" there is the stray inductance. Usually rather small maybe in the low microHenrys.

Klaus

 

[Salvador12]

I don't see the purpose of 50 Hz series LC circuit in your hypothetical inverter. Practical 50 Hz inverters have a series inductor to reduce pwm carrier and an additional LC low pass but no series capacitor.

Yes well , I'm sorry if I did not describe this earlier but the series capacitor is a must in my design because the generating coil is on a rotary axis and the capacitor serves as brushless current coupling. Now if you know this, do you still don't see a purpose of using the series resonance of an LC given the capacitor is fixed and limited in it's value?

--- Updated May 6, 2024 ---

in a transformer there are several "inductances"

let´s talk about a normal 50 Hz mains transformer.

There is the primary magnetizing inductance. It is in the range of mH and thus causes only a few mA of magnetizing current.
It will not change with (much) load. It is in parrallel to the load (current).
Then there is the coupling between primary and secondary. (for sure it technically works with magnetizing via the core)
But it "adds" to the magnetizing current. It often is referred as "coupling" between primary and secondary.
And in series to this "coupling" there is the stray inductance. Usually rather small maybe in the low microHenrys.

Klaus

Sure I get that, but practically speaking as you said the load impedance just adds to the magnetizing current since now it is so much harder to "magnetize" the core given the back EMF of the secondary winding.
So for practical purposes in a series resonant circuit I would think that it would be more practical to factor the inductance such that resonance happens at transformer max rated power

 

[FvM]

Need to specify inverter voltage/current and calculate actual capacitor values. For higher power, you'll end up with multiple 10 or even 100 uF for a 50 Hz resonant circuit. Don't think it's in your available capacitance range.

 

[KlausST]

Hi,

So for practical purposes in a series resonant circuit I would think that it would be more practical to factor the inductance such that resonance happens at transformer max rated power

for a resonant you need L and C.
Now you say "L" varies with load.

I get what you mean.
You see just the current. But you need to look into the phase shift, too.

For an "L" you (ideally) have 90° phase shift. V leading I.
You need this phase shift for resonance. Because the phase shift means "stored energy".
All resistive load causes "useful power", often resulting in heat.

So when you have a transformer .. and there is no load connected at the secondary side, you get almost pure 90° phase shift (only a little off 90° because of real power in magnetizing loss and copper loss).

As soon as you connect a load at the secondary .. the current will also be seen as primary current (coupled, and transforemd with (inverse) transformer ratio).
And not only as simple current, but also with it´s phase shift.

Add a resistor at the secondary then you get
* the 90° phase shifted magnetizing current
* plus the 0° shifted "transformed" secondary current.
at the primary side.
This 0° shifted (secondary) current wil not contribute to a resonance. Instead it will dampen the resonance.
The total phase shift depends on the currents and will be somewhere inbetween 0° and 90°.

Now add a pure inductor at the secondary, causing a 90° shifted secondary side current. Then you get
* the 90° phase shifted magnetizing current
* plus the 90° shifted "transformed" secondary current caused by the secondary inductor.
at the primary side.

Now this secondary inductance really contributes to the resonance. It will shift the resonance frequency.
It will also not increase your electrical bill, because it all is inductive current (ignoring unwanted loss). This "inductive" power is not usable.
This energy is just pushed back and forth.

Now add a pure capacitor at the secondary, causing a -90° shifted secondary side current. Then you get
* the 90° phase shifted magnetizing current
* plus the -90° shifted "transformed" secondary current caused by the secondary capacitor
at the primary side.
It surely will also have influence on the resonance and will shift the frequency.

****
And because the phase shift will also be transformed from secondary to primary ...
* we don´t call it inductive current, because indcutive current is expected to be 90° phase shifted.
* we call it "coupled" current instead

Klaus

 

[Salvador12]

Need to specify inverter voltage/current and calculate actual capacitor values. For higher power, you'll end up with multiple 10 or even 100 uF for a 50 Hz resonant circuit. Don't think it's in your available capacitance range.

Well If my capacitor value can't change then the only way I can achieve maximum efficiency power transfer is at resonance and the only way to increase total power by increasing voltage because IIRC for a fixed capacitance just one way to increase energy stored and that is by increasing the potential difference aka voltage.

 

[FvM]

Yes I know, but this makes the concept almost unrealizable. Limitations are inductor voltage, resonator Q, inductor size (~ I^2 L). I'd consider high frequent inductive transmission in the first place.

 

[Salvador12]

Hi,


for a resonant you need L and C.
Now you say "L" varies with load.

I get what you mean.
You see just the current. But you need to look into the phase shift, too.

For an "L" you (ideally) have 90° phase shift. V leading I.
You need this phase shift for resonance. Because the phase shift means "stored energy".
All resistive load causes "useful power", often resulting in heat.

So when you have a transformer .. and there is no load connected at the secondary side, you get almost pure 90° phase shift (only a little off 90° because of real power in magnetizing loss and copper loss).

As soon as you connect a load at the secondary .. the current will also be seen as primary current (coupled, and transforemd with (inverse) transformer ratio).
And not only as simple current, but also with it´s phase shift.

Add a resistor at the secondary then you get
* the 90° phase shifted magnetizing current
* plus the 0° shifted "transformed" secondary current.
at the primary side.
This 0° shifted (secondary) current wil not contribute to a resonance. Instead it will dampen the resonance.
The total phase shift depends on the currents and will be somewhere inbetween 0° and 90°.

Now add a pure inductor at the secondary, causing a 90° shifted secondary side current. Then you get
* the 90° phase shifted magnetizing current
* plus the 90° shifted "transformed" secondary current caused by the secondary inductor.
at the primary side.

Now this secondary inductance really contributes to the resonance. It will shift the resonance frequency.
It will also not increase your electrical bill, because it all is inductive current (ignoring unwanted loss). This "inductive" power is not usable.
This energy is just pushed back and forth.

Now add a pure capacitor at the secondary, causing a -90° shifted secondary side current. Then you get
* the 90° phase shifted magnetizing current
* plus the -90° shifted "transformed" secondary current caused by the secondary capacitor
at the primary side.
It surely will also have influence on the resonance and will shift the frequency.

****
And because the phase shift will also be transformed from secondary to primary ...
* we don´t call it inductive current, because indcutive current is expected to be 90° phase shifted.
* we call it "coupled" current instead

Klaus

so what your saying is you need not just the frequency but also to keep the phase shift right to achieve maximum efficiency in terms of power transfer at resonance?

--- Updated May 7, 2024 ---

Yes I know, but this makes the concept almost unrealizable. Limitations are inductor voltage, resonator Q, inductor size (~ I^2 L). I'd consider high frequent inductive transmission in the first place.

Yes this worries me too, you are most likely right.

 

[KlausST]

so what your saying is you need not just the frequency but also to keep the phase shift right to achieve maximum efficiency in terms of power transfer at resonance?

1) What is your "maximum efficiency"? How do you measure it? Frequency is not a measure of efficiency at all.

An inductive current does not result in real power. So it does not affect efficiency.
The same is with capacitive current.

2) You talk about resonance. This is when X_L = X_C. Which "L" are you taking about? (I already asked this)
Fact is that, what you call "inductive current" indeed is not necessarily 90° phase shifted current, thus it is not involved in resonance.

But if the secondary current is phase shifted (caused by load) how are you able to maintain resonance.

And a parameter for resonance is "Q". It is determined by the real power, but real power depends on load. So Q depends on load. Do you ignore this?

***
To be honest ... to me this thread is like a discussion about a new idea to invent a perpetuum mobile. They all have in common to miss to prove the idea with the laws of physics. Resulting in an endless and fruitless discussion.
There are free electronics simulation tools. Use it to prove or at least show the working principle. Then I´m willing to spend more time.

Indeed .. I lost what your goal of this whole discussion is.

Klaus