Always higher voltage. I've heard these can handle 72v so shoot for the sky if you can. There is a relationship between rotor field voltage and stator voltage on these. If you go 72v on the stator and 5v or more on the rotor, you can get more torque but keep the higher rpm range. Were you testing a 30amp esc on it? Because 30amps won't push that alternator very hard, especially only at 12v power supplies. My esc is the mystery cloud 200amp 2-7s lipo capable. They are $30-45 depending on where you buy them. I know the voltage still isn't very high, but it can get you at least 5hp out of it. Amp wise it is probably at the limit of the alternator. Rc speed controllers jump in price with anything higher in voltage, but price wise, idk if the one I got can be beat. I know there are 100-200amp 72v ebike controllers that can run sensorless but they are usually at least $200. I'd look at Kelly controllers as there isnt much else over 4hp for a decent price. Also I haven't tried it myself, but I heard if you short the field coil to itself, you can run the motor as an induction using and induction motor controller which Kelly controls also has which is only $150 for a 72v 200amp which should definitely be the limit for the alternators...but I wouldn't buy it myself because I'm not 100% sure if I can get it tuned in to work correctly, and if it wasn't as I hoped, it wouldn't be useful for any other motor I have. Now that I am talking about it though. I am seriously contemplating it because that is serious horsepower for cheap. Altogether total cost for someone to build this not including battery would be only $200 for a motor capable of 17hp max. That is insanely cheap. And shorting the field coil is less complication than powering the field separately. It's interesting stuff..must tell the wife I need to get one lol.