Analyzing Hobby Motors with an Oscilloscope

We always like finding new excuses reasons to use our test equipment, so we couldn’t help but be intrigued by this tip from [Joe Mosfet]. He uses the ever-popular Rigol DS1054Z to demonstrate the differences between a handful of brushless motors when rotated by his handheld drill at a constant RPM. Not only is he able to identify a blown motor, but it allows him to visualize their specifications which can otherwise seem a bit mystifying.

One wire from each motor is used as the ground, and channels one and two are connected to the remaining wires. Despite the DS1054Z having four channels, [Joe] is actually only using two of them here. The third channel being displayed is a virtual channel created by a math function on the scope.

After wiring them up, each motor got put into the chuck of his drill and spun up to 1430 RPM. The resulting waveforms were captured, and [Joe] walks us through each one explaining what we’re seeing on the scope.

The bad motor is easy to identify: the phases are out of alignment and in general the output looks erratic. Between the good motors, the higher the Kv rating of the motor, the lower voltage is seen on the scope. That’s because Kv in the context of brushless motors is a measurement of how fast the motor will spin for each volt. The inverse is also true, and [Joe] explains that if he could spin his 2450Kv motor at exactly 2450 RPM, we should see one volt output.

Beyond demonstrating the practical side of Kv ratings, [Joe] also theorizes that the shape of the wave might offer a glimpse into the quality of the motor’s construction. He notes his higher end motors generate a nice clean sine wave, while his cheaper ones show distortion at the peaks. An interesting note, though he does stress he can’t confirm there’s a real-world performance impact.

Last year we featured a similar method for identifying bad brushless motors using a drill press and an oscilloscope, but we liked that [Joe] went through the trouble of testing multiple motors and explaining the differences in their output.

[via /r/multicopter]

Tiny Improvised Grinder/Saw Packs a Punch

They say necessity is the mother of invention. Sometimes the necessity is simply avoidance of unpleasant tasks such as cutting down 3500 header pins by hand. [Nixieguy] and his coworkers were faced with 50 prototype boards bearing 70 overly long pins apiece. He saved them from cutting them all down by hand by making a tiny improvised circular saw/grinder.

[Nixieguy] started by laser-cutting a combination tool holder and grinding platform. His laser failed before he could fashion a guard to keep the pin bits out of the motor or cut all the pieces he had in mind. The grinder is made from a 10A brushless RC motor, a motor driver, and a servo tester. [Nixieguy] machined an adapter to connect the disc to the shaft.

The transformer is there to hold the thing down during use since it’s so lightweight. He’s wearing two pairs of gloves because the pin cuttings were hot enough to sear skin. [Nixieguy] is planning on a complete redesign including a motor guard and the ability to adjust the depth. Maybe he can turn it into a chainsaw, maybe not.