Rewound and Rewired BLDC Makes a Half-Decent Generator

What’s the best way to turn a high-powered brushless DC motor optimized for hobby use into a decent low-RPM generator? Do you take a purely mechanical approach and slap a gearbox on the shaft? Or do you tackle the problem electrically?

The latter approach is what [GreatScott!] settled on with his BLDC rewinding and rewiring project. Having previously explored which motors have the best potential as generators, he knew the essential problem: in rough terms, hobby BLDCs are optimized for turning volts into RPMs, and not the other way around. He started with a teardown of a small motor, to understand the mechanical challenges involved, then moved onto a larger motor. The bigger motor was stubborn, but with some elbow grease, a lot of scratches, and some destroyed bearings, the motor was relieved of both its rotor and stator. The windings were stripped off and replaced with heavier magnet wire with more turns per pole than the original. The effect of this was to drive the Kv down and allow better performance at low RPMs. Things looked even better when the windings were rewired from delta to wye configuration.

The take-home lesson is probably to use a generator where you need a generator and let motors be motors. But we appreciate [GreatScott!]’s lesson on the innards of BLDCs nonetheless, and his other work in the “DIY or buy?” vein. Whether you want to make your own inverter, turn a hard drive motor into an encoder, or roll your own lithium battery pack, he’s done a lot of the dirty work already.

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Scrap a Hard Drive, Build a Rotary Encoder

There’s something to be said for the feel of controls. Whether it’s the satisfying snap of a high-quality switch or the buttery touch of the pots on an expensive amplifier, the tactile experience of the controls you interact with says a lot about a device.

[GreatScott!] knows this, and rather than put up with the bump and grind of a cheap rotary encoder, he decided to find an alternative. He ended up exploring hard drive motors as encoders, and while the results aren’t exactly high resolution, he may be onto something. Starting with a teardown of some old HDDs — save those magnets! — [Scott!] found that the motors fell into either the four-lead or three-lead categories. Knowing that HDD motors are brushless DC motors, he reasoned that the four-lead motors had their three windings in Wye configuration with the neutral point brought out to an external connection. A little oscilloscope work showed the expected three-phase output when the motor hub was turned, with the leading and lagging phases changing as the direction of rotation was switched. Hooked to an Arduino, the motor made a workable encoder, later improved by sending each phase through a comparator and using digital inputs rather than using the Nano’s ADCs.

It looks like [GreatScott!]’s current setup only responds to a full rotation of the makeshift encoder, but we’d bet resolution could be improved. Perhaps this previous post on turning BLDC motors into encoders will help.

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