Simple to make Hilden-Brand style motor

[gyulasun]

Hi Scianto,

Thank you for the detailed answers, I appreciate them.

There is one thing I do not understand: you say that for half period (when the MOSFET is off) the motor runs by the generated half-sine voltage (shown in the scope shot). 
I do not think this works like that because the cathode point of diode Dq is at 2 x 36V with respect to the negative point of battery Bzm and is at 1 x 36V with respect to the drain point of the MOSFET.  This means that in the 1st case the diode anode needs higher than 72V peak AC to conduct and in the 2nd case the diode anode needs higher than 36V peak AC to conduct, ok? 
So in the schematic as you show the diode Dq cannot conduct and from this it follows that the generated half waves during the switch OFF periods are unloaded, this is how I see it.  Am I wrong?

Thanks,  Gyula

PS: please what is the type of the MOSFET you were using for this test and showed the scope shot for?

[scianto]

>> I do not think this works like that because the cathode point of diode Dq is at 2 x 36V ...
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You may be right that it is unloaded voltage trace, this seems to me more probable. I am not sure too. I need some time to explore it more, but I rather prefer to continue testing, building now the motor into a bicycle for practical run tests.
But I will come back for deeper understanding of this phenomenon. Thanks for you input.


>> please what is the type of the MOSFET you were using for this test and showed the scope shot for?
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That was (not any more, burned in flames) IRFP250N.

[gyulasun]

Hi scianto,

Thanks for the answers. 
I have had a look at the data sheet on the IRFP250 ( http://www.datasheetcatalog.org/datasheet/fairchild/IRFP250.pdf ) and in Figure 2,  Page 3, the graph shows how the maximum drain current values are derated in the function of case temperature. Assuming your coil DC resistance say 1 Ohm, your 36V battery voltage establishes about 36A peak current during the ON time of the FET, this abuses the FET when it gets up to 70-80 degree Celcius case temperature (even though you use heatsink and ventillation).
Another abuse for the FET is when you switch it off, the inductive spike created by the collapsing field in the coil can be in the some hundred Volts range (because of the fast 25-30 nanosec switching speed the MAX4420 provides, induced voltage Vi=L*dI/dt) and this FET has a 200V max drain-source breakdown voltage, small for this task (no wonder the FET eventually got ruined as you wrote).

I included below the scope shot you showed in your video because I wish to refer to the narrow voltage spike's amplitude of being only about 36V, seen at the biggining of the induced sinewave.  This means that some Zener or VDR device may have clamped the drain-source voltage (and you did not draw it in the schematic, this is not complain  ) or if you did not use any such protection, then the FET already was abused and it showed a 36V breakdown voltage limit instead of the data sheet-specified 200V.

I would like to show you two possible schematics that can utilize the induced voltage (and the spike too) that appears across the coil: they simply add the rectified voltages to the DC supply voltage, helping the motor (or whatever application).

first is here: http://www.overunity.com/index.php?topic=8731.msg233576#msg233576  that uses two reed switches what you can replace with an N and a P channel power MOSFET, driving BOTH ON or OFF at the same time for your application too.

the second is here: http://www.energeticforum.com/inductive-resistor/4314-cop-17-heater-rosemary-ainslie.html#post56075   this seems simpler for your application by showing 1 switching device only (you do not need the shunt resistors) and includes the series diode D1 that separates automatically the output of the DC power supply from the recovered induced voltages (in Ossie's circuit the two switches do this, you drive them ON or OFF at the same time so that B1 never "sees" B2).

I wish you good luck for the bycicle tests, I agree the practical tests are what eventually count.

rgds,  Gyula

PS I assume the vertical scale was set as 10V/DIV in your scope shot.

[scianto]

After burning the IRFP250 I installed an IGBT  FGA25N120ANDT 1200V 50A 312 W. Now tuning the system to it.

>> Assuming your coil DC resistance say 1 Ohm
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In fact it is 50 mili ohms and 260 µH now.


>> FET has a 200V max drain-source breakdown voltage, small for this task
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Yes, too little. I had to use 200 V varistor on D-S of the FET. It was cutting the spikes very well but this way energy is waisted.


>> narrow voltage spike's amplitude of being only about 36V, seen at the biggining of the induced sinewave.
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I've noticed that the amplitude of this spike is always limited by the charged battery regardless of the voltage powering the circuit.


>> This means that some Zener or VDR device may have clamped the drain-source voltage (and you did not draw it in the schematic, this is not complain  )
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No, I didn't have any diode or varistor there, the charged battery was clamping it.


>> I would like to show you two possible schematics that can utilize the induced voltage (and the spike too) that appears across the coil:
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Thanks. Several months before I was experimenting along this line. In fact, this is my conclusion, this circuits gave me nothing better. The high voltage nanoseconds spikes were still there and more parasitic oscillations were produced.
In some cases these circuits may work well, if Time On and Time Off of the two FETs are well matched and the diodes are very fast (below 20 nS) with almost zero capacitance, and all the wires connecting the FETs, diodes, coil are very, very short, thick, heavy soldered.
I am going to test these kind of circuits again.

[gyulasun]

Hi Sciento,

Yes, the voltage spike amplitude is 36V when you measure it directly across the coil with the scope, due to the clamping action of the charging battery of course,

EDIT:  and that is true if your scope ground clip is placed onto the common +/- point of the two batteries!  Because the spike appears across the coil.

HOWEVER your scope shot showed the scope zero level was at the source point of the MOSFET, is that right? 
If yes then there is no way (in a normal circuit you show in the schematic) the spike should be limited to 36V.  Normally if everything is ok, the top peak of the spike should be at 72V,  (2 x 36V plus the 0.7V diode forward voltage) in your shown schematic. 

Your 50 milliOhm coil resistance demands switches in the several hundred Amper range if you use 36V supply voltage, the FGA25N120ANDT has 'only' 50A collector current rating (90A when pulsed, pulse width depends on junction temperature).

Good luck with the recapture circuit, maybe the second link I referred to is simpler, using a single switch. Start with low supply voltage like 5-10V to be within safety peak current limits for your switching device.

rgds,  Gyula