I was once consulted to resolve a problem with the small in-house-designed BLDC (brushless DC) motor in one of our company's products. A significant number of the newly assembled motors were behaving erratically or locking up altogether, but no failed electrical components were found.
The motors had optical sensing for commutation, with a plastic shutter wheel between the planes of the three LEDs and the three phototransistors. The motors were enclosed, but the cover was removed for inspection.
Ambient light was my first thought, but I was assured that these particular motors had never worked correctly, even with the cover on. Just to be sure, I tried shading it with my hand. If my finger was very close to one of the sensors, the commutation state would change and the rotor would rotate to another position. It returned to its former position when I withdrew my finger. Shading the whole works with a clipboard or similar object had no effect.
My memory flashed back to times when the polystyrene cover of my VOM (volt-ohm meter) had acquired a static charge. The pointer would rest well upscale from zero, and would respond to my hand if I brought it near the cover. To discharge the plastic, I only had to breathe on it. I exhaled slowly into the motor. It started up and continued running. My colleagues thought I was playing a joke on them -- I'd just breathed life into a dead motor. I explained my hypothesis, which the experiment had confirmed.
The next day, I brought in a piezoelectric air ionizer which I'd bought some years earlier to discharge phonograph records when cleaning them. It had two sharp points slightly recessed in a plastic housing, a brush on the bottom, and a handle which one squeezed and released to generate the high voltage. In a dark room, the corona could be seen at each tip. One squeeze of the handle sufficed to resuscitate a motor. I recommended several, to avoid potential RMAs (returned materials authorizations).
My record discharging gadget became official production equipment until we acquired ionized air blowers for production. I suggested some design improvements to reduce the susceptibility to static charge and also prevent its retention. Once discharged and assembled into the product, however, the motors worked fine with no indication of electrostatic issues. The design therefore remained static.
This entry was submitted by Dick Neubert and edited by Rob Spiegel.
Dick Neubert has a long and diverse history in electronics and (mostly real-time) programming. His design work ranges from high-performance disk head servo systems to computerized automation systems for sawmills. He has an MS in engineering sciences.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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