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I wound my own motor, and tested it to be about 300 RPM per volt, and if my noob math is right, it should be able to handle about 1600 watts with a ~40 volt supply without burning my nice but thinly insulated wire.

My idea now is to drive this motor with an engine of some kind to get it to turn a little more than 12000 RPM, run the 3 phases through a powerful 1600v 150a rated diode bridge, and hook a capacitor on the other end.

The problem is, I borrowed a friend's oscilloscope and found that when I turn it at about 1,000RPM with a drill, I am getting HUGE voltage spikes. There's obviously little to no load, but i'm getting 4-500v spikes, maybe even up to 1kv. They're very very short spikes.

I get the picture that these will disappear with a load on the motor, but I want to ensure the several thousand hertz DC pulses I am getting at the other end are being somewhat dampened by a capacitor. Is there some math I can do to determine what voltage rating and how many farads/microfarads I need to balance the pulses out? I'm worried about getting a cap rated for 500v and it being too much, but I'm also worried about it blowing the eff up when I actually start putting power through it. I've intentionally blown up tiny electrolytic caps and they are basically bullets. Don't really want to find out what a hand grenade sized one will do.

If there's no way to bridge the super high frequency pulses in a small form factor, I suppose I can just wind the motor to a lower v/rpm, but either way, I want to do it safely.

My end goal is to get 1,000 watts out of it, although 1600 would be outstanding. I doubt the system is going to be more than 10% efficient, but we'll see.

As a side note, I plan on shielding the whole thing with brass mesh to keep EMI down, as I imagine it will produce quite a lot.

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    \$\begingroup\$ Is it a permanent magnet motor, or does it have brushes? Without brushes, short sharp pulses seem unlikely. Try loading it with a high value resistor, just to draw 1 W, and test again? \$\endgroup\$ – tomnexus Mar 3 '15 at 7:16
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    \$\begingroup\$ Do the spikes repeat every rotation of the generator? Perhaps measure each winding's voltage alone, turning slowly, and then also its short circuit current, again turning slowly. They should all be the same, sinusoidal and no spikes shorter than 1 per (revolution * poles * 2), right? \$\endgroup\$ – tomnexus Mar 3 '15 at 7:26
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    \$\begingroup\$ If it's a brushless motor, surely it's producing AC, not DC, since there is no commutator? Are you planning to rectify the output to get DC? \$\endgroup\$ – Reversed Engineer Mar 3 '15 at 9:25
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    \$\begingroup\$ However you are rectifying the output, please see radio-electronics.com/info/circuits/diode-rectifier/… \$\endgroup\$ – Reversed Engineer Mar 3 '15 at 9:28
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    \$\begingroup\$ Coil cooling will determine continuous wattage possible. \$\endgroup\$ – Optionparty Mar 3 '15 at 17:07
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If what you say is correct, I would recommend that you use a load of about 10 ohms (150 watts). This should keep the spikes below 500v which will allow you to use a 10 micro-farad 600V AC capacitor. Check the results with the O'scope and see if that takes care of the problem.

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  • \$\begingroup\$ Metal film type work well for high voltage AC snubbing. "Y" class if possible (more expensive, but more reliable.) \$\endgroup\$ – rdtsc Aug 10 '15 at 13:06

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