Some tests on mono and bifilar coils

[Farmhand]

Conrad your spinner is an awesome setup for doing these tests, I wanna join in. If I use my rotary spark gap I can attach another shaft to the end of it and use it to spin a small rotor for testing.

It has a small universal motor rated for 240 VAC, but I just use DC to run it and can get over 6000 RPM with it by applying about 50 volts DC. It must do over 20000 RPM with 240 volts and no load  . However if I use a small rotor made from plastic I will be restricted in RPM by safety reasons. I can use the rotor that is already in the old pulse motor or I can use another very similar, if I use enough magnets on the right size rotor I should be able to get fairly high excitation frequency, and try all the same polarity excitation to resonance in the coil as well, (like all north facing magnets).
I should have thought of using that spark gap motor and shaft before, it should be fairly easy to setup similar to yours and even almost mirror some of your experiments. And do some that you might not get time to do. I doubt I will be able to present the results as nice looking as yours, but the numbers should be good, and it will be a lot of fun to spin some magnets again.  I've already got a real neat bifilar coil but I don't have a monofilar equivalent yet, I'm sure I can wind one to match well enough. Time to disassemble the pulse motor prototype.

If I use the same rotor then I already have a "MOI" for it I think, or am part way there, at least. I won't be able to spin that rotor at any more than 3000 or 4000 RPM, It was tested up to 5000 RPM but that was some months ago, it'll need inspecting. If I make a smaller rotor I can spin it faster and put the magnets closer together. I think 60 or 70 mm will be the smallest I can go and I have 8 diametrically magnetized tube magnets. No flying magnets unless the rotor itself breaks.

Impressive test and results presentations.

Cheers

[synchro1]

@Conradelektro,


Even though I'm away from my shop down here in Costa Rica, I performed an inventory of my equipment and accessories, and determined I have ample material and instruments to re-conduct a magnetic field strength comparison test between the monofilar and series bifilar solenoid coils. I am also equipped with my video camera, so I'll be able to upload a video to youtube.


I have two skeins of magnet wire, a ferrite rod, and a multi meter with good battery, also: I have two new six volt dry cell lantern batteries. I need to buy a compass, some dielectric tape and magnetic material in town. I plan to wrap the coils of equal length wire and gauge on each end of the ferrite rod and see if the Ohmic resistance is half what it is in the monofilar in the SBC, then see if I can produce a field of equal magnetic strength with one six volt battery in the bifilar over the two batteries in series, for twelve volts in the single wire coil. I plan to run this test and upload the video shortly. I'm convinced you did something wrong to get those false results at two hundred milliamps. I think Milehigh helped rig that test for failure by causing a super saturation of the coils!


If these coils differ by twice the resistance, half the voltage should produce the same power in the SBC of lesser ohms!  

[conradelektro]

@Farmhand: the little 12 V motor only costed about 8.-- EUR (see a photo at http://www.overunity.com/14235/some-tests-on-mono-and-bifilar-coils/msg386184/#msg386184).

One has to look carefully at the motor specs, only the motors for race cars go up to 14.000 rpm. The motor should not be too powerful or you will not see the drop in power demand. It is better to have a weaker motor than a stronger. I attach the specs of my 12 V motor (measurements are in millimetre). It was difficult to find bolts which fitted into the tapped holes at the motor face. Eventually I found some bolts which I had salvaged from old computer equipment, probably from old hard disk drives which I took apart. But one could fasten the motor with a bracket as well.

There is an error in the specs: the maximum power draw is 2.45 A (not 24.5 A).

You might have ball bearings and an axle. The coupling between axle and motor was more expensive, it costed about 12.-- EUR. The coupling is essential because it must be good to avoid vibrations. I can spin the whole thing up to 14.000 rpm (10.000 rpm easily without much vibrations).

I am sure you will know where to get similar stuff in your country. It pays to have a practical and simple set up. Your motor sounds a bit too big.

By the way, the 12 V DC motor was MileHigh's idea.

I am planning to add a rotor with six magnets (instead of the ring magnet) to get three times the frequency (which would be 700 Hz max, 500 Hz easily without much vibrations). I guess I will have to balance that 6 magnet rotor. But there will be the danger that the magnets fly off.

Greetings, Conrad

[gyulasun]

Could it be a change in the rotor speed that causes the variance ?

..

Hi Farmhand,

The unloaded Q  (X_L/R) of the mono or bifilar helical coils is a low value mainly due to their 77 Ohm wire resistance and I mention this because the low Q defines a wide 'bandwidth'  (B=f/Q  where B is the bandwidth, f is the resonant frequency of the LC tank).  Wide bandwidth means that the resonant voltage amplitude changes slowly across the tank in the function of the frequency.  Putting this latter otherwise: the selectivity of low Q LC tanks is very poor.  Taking f=96 Hz and Q=2.7 we get B=96/2.7=35.5 Hz and placing the half of this B below and above the 96 Hz we get a frequency range between 96-17.7=78.3 Hz and 96+17.7=113.7 Hz  i.e. changing the speed of the prime mover between speeds of say 78 and 113 Hz the resonant voltage changes only a little across the tank and it is difficult to find the highest value at the expected 96 Hz resonant frequency, the response is rather flat, especially flat when a load resistor is put across the tank.  And the resonant voltage changes across the tank as the impedance of the tank changes in the function of the frequency.

So I do not really think the speed would matter much in the impedance of the tank, it surely influences but a little.


Hi Conrad,

You wrote:

the critical values are DC resistance of the coil and inductance of the coil. The inductance should be 201 mH to result in an impedance of 345 Ohm (instead of 602).

I put in bold the inductance of the coil and I think we have to consider a much dependable component of the LC tank, your 10 uF capacitor in our calculations.  At 96 Hz the 10 uF capacitor has about 165.8 Ohm reactance.  And to get a resonance at 96 Hz with this capacitor, the coil must have about 274.8 mH inductance, okay? (L=X_L/2*Pi*f   and  X_L=X_C)
Now this shows that somehow the coil i.e. its core behaves nonlinearly in the tank circuit when the resonant current flows through the coil.

Now the Z impedance of the LC tank can also be calculated by formula Q*X_C because X_L and X_C must be the same value at resonance, so first let's calculate the Q with the X_L=165.8 Ohm value, that is Q=165.8/77=2.1  and Z now comes as 2.1*165.8=348.1 Ohm,  now this is very close to your 345 Ohm load resistance!

There is also the problem of "impedance of the coil" and "impedance of the LC circuit", which should be different?

Yes, they are different,  the impedance of the coil is always X_L=2*Pi*f*L  while the impedance of a parallel LC circuit at resonance is Z=Q*X_C or Z=Q*X_L because X_L=X_C at resonance and there are some other formulas for it in textbooks too.

The experimental value of Z = 345 Ohm (halve of the unloaded tank voltage) seems to be the best. And it must be the Z of the tank circuit. The measured Z = 240 value of the coil (at 100 Hz) with the LCR meter is not far off if we consider that the LC circuit should have more losses than the coil alone. Also the value Z = 412 Ohm for the tank circuit from the Thevenin Theorem is not too bad.
The value Z = 602 Ohm for the coil calculated via the inductance and the DC resistance of the coil is pretty bad and I can not explain it?

Yes, I agree and obviously the Z=602 Ohm came from the 357 mH or so inductance value (what we believed as true from the separately measured coil)  but we did not check what inductance value has really been involved at the actual resonance frequencies (for which I took 96 Hz above for one calculation example) with the rightly assumed stabil value 10 uF capacitor.
If you feel like checking the Z impedance of your 10 uF capacitor with your LCR meter at 100 Hz and at 1 kHZ just out of curiosity, please do it. Maybe it can also show a Q value for that capacitor I wonder.

Thanks for measuring the capacitance between the two winding end or start wires of your helical bifilar coil, the 26 nF is a pretty high value, this must have something to do with the paper insulators between the winding layers. We can notice that this is a much different value from the cca 6 nF self capacitance of this same bifilar coil.

Greetings,  Gyula

[conradelektro]

If you feel like checking the Z impedance of your 10 uF capacitor with your LCR meter at 100 Hz and at 1 kHZ just out of curiosity, please do it. Maybe it can also show a Q value for that capacitor I wonder.

@Gyula: thank you for the explanations. It will take me some time to understand it. There will be no experiments till Monday, but I can study in the evening.

I could do the Z measurement of the 10 µF cap (the one I used in all the tests) with my LCR meter, C = 10.083 µF:


at 100 HZ: Z = 157 Ohm,   ESR = 0.33 Ohm,   Q = 480,   D = 0.002,   Phi = -90°

at 1 KHz:   Z = 15.8 Ohm,   ESR = 0.11 Ohm,  Q = 140,   D = 0.007,   Phi = -90°

at 10 KHz: Z = 1.57 Ohm,   ESR = 0.06 Ohm,  Q = 27,     D = 0.037,   Phi = -88°


Greetings, Conrad