20 kHz transformer secondary conversion to DC output

gluk69

Joined Oct 5, 2018
7
Hi everyone,
I'm trying to transfer a small power (few Watts) from a static coil to a rotating one, by feeding the primary coil with a pulsed squared signal @20kHz.
At the sec. coil output I'm measuring about 10.5~11 V but at the capacitor pins I'm getting only 4.6~4.7 V. What could be wrong? I think the frequency is appropriate to be handled by the diode full bridge formed by 4 1N4148, isn't it?

LowQCab

Joined Nov 6, 2012
3,056
What is a "Rotating-Coil" and exactly what is its relationship to the "Fixed-Coil".
Is there any type of Armature to concentrate the Magnetic-Field ?, or just Air ?
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MrChips

Joined Oct 2, 2009
28,472
How are you measuring voltages?

I am going to hazard a guess that you measured 11V peak-to-peak at the coil using an oscilloscope. This would give about 5.5V peak value.

Subtract about 1V for the diodes would give 4.5VDC at the capacitor.

gluk69

Joined Oct 5, 2018
7
What is a "Rotating-Coil" and exactly what is its relationship to the "Fixed-Coil".
Is there any type of Armature to concentrate the Magnetic-Field ?, or just Air ?
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You have to think about two concentric coils in which the wider one is fixed and the other one is rotating around the winding axis inside the cylindrical space in the inner of the first one. They are separated by around 1mm air radial gap (no armature).

gluk69

Joined Oct 5, 2018
7
How are you measuring voltages?

I am going to hazard a guess that you measured 11V peak-to-peak at the coil using an oscilloscope. This would give about 5.5V peak value.

Subtract about 1V for the diodes would give 4.5VDC at the capacitor.
MrChips, you're totally right. So nothing wrong with the circuit but only in my mind . This means only that I will have to rise the primary voltage. Thankyou

LowQCab

Joined Nov 6, 2012
3,056
There will be poor energy transfer with no means of concentrating the Magnetic-Flux in a useful manner.
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gluk69

Joined Oct 5, 2018
7
Probably what you say is true but I don't want to make such a big modification to my project

Dodgydave

Joined Jun 22, 2012
10,826
I would try better diodes like mbr1040 Shockley types.

Ian0

Joined Aug 7, 2020
7,441
I would try better diodes like mbr1040 Shockley types.
I think you mean Schottky (A Shockley diode is like a diac)

argile_tile

Joined Feb 24, 2023
9
>> from a static coil to a rotating one, by feeding the primary coil

> There will be poor energy transfer with no means of concentrating the Magnetic-Flux in a useful manner

OP has not explained his experiment.

You would get bumpkiss from such a thing unless the coils were very well designed. how does the rotor cross B? what is B? The complaint of inefficiency means nothing without such information. I'm surprised you got anything. (I've tried hand wrapping transformers: they get next to nothing without design)

ronsimpson

Joined Oct 7, 2019
2,708
I am going to hazard a guess that you measured 11V peak-to-peak at the coil using an oscilloscope. This would give about 5.5V peak value.

gluk69

Joined Oct 5, 2018
7
>> from a static coil to a rotating one, by feeding the primary coil

> There will be poor energy transfer with no means of concentrating the Magnetic-Flux in a useful manner

OP has not explained his experiment.

You would get bumpkiss from such a thing unless the coils were very well designed. how does the rotor cross B? what is B? The complaint of inefficiency means nothing without such information. I'm surprised you got anything. (I've tried hand wrapping transformers: they get next to nothing without design)
I ignore how much could be B, but I know how much power this system is able to transfer from stator to rotor circuit and I suspect it os sufficiente for my purpose.
I added some pictures below, hopefully more explicative than many words.

MisterBill2

Joined Jan 23, 2018
14,661
the diodes, 1N4148, are signal diodes. Try the system with a 0.47 mFD capacitor and no load. The coil resistance may be to high, and the power feeding the supply coil may not be enough.

MisterBill2

Joined Jan 23, 2018
14,661
If you can add a ferrite core inside the coils that will improve the power transfer without needing to modify what exists already. Unless that is where the bearings are located. The voltage doubler suggestion will increase the voltage but not increase the power..

gluk69

Joined Oct 5, 2018
7
MisterBill2 inside the inner coil there's the DC motor and in between the inner coil and the outer (static) one there's the air gap which is about 1mm thick radially. I think I'm running too short in room to achieve a ferrite core anywhere.
The voltage doubler suggestion will increase the voltage but not increase the power
My hope is to be able to store more energy in the secondary circuit cap by means of an increased voltage after the diodes H bridge and not to transfer more power.

BobTPH

Joined Jun 5, 2013
6,508
Have you considered a slip ring? That would possibly be simpler.

gluk69

Joined Oct 5, 2018
7
Have you considered a slip ring? That would possibly be simpler.
Yes, but that would be simply too noisy

MisterBill2

Joined Jan 23, 2018
14,661
Electrical noise or mechanical noise? All of the slip rings I have seen were silent. And for low power they will not need to be very big.

LowQCab

Joined Nov 6, 2012
3,056
"" My hope is to be able to store more energy in the secondary circuit cap by means of
an increased voltage after the diodes H bridge and not to transfer more power. ""

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Power and "Energy" are basically the exact same thing,
possibly just in different forms.

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Changing from one form of Energy to another always involves losses.
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Transferring Power is the only thing that You are attempting to do.
( aside from wasting Power as Heat due to all the cumulative losses/inefficiencies along the way )
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