I recently bought some cheap brushless motors on ebay, to investigate the possibility of using them in a variation of my paint viscometer. Since they don't use brushes, they should have much lower running friction -- and therefore be more-sensitive to drag produced by a viscous liquid.
This post is regarding some learnings with respect to those motors. First, while they are sold as "variable reluctance" motors, they aren't. That was an easy call, all it took was to inspect the motor. A variable reluctance motor should have teeth for the motor's stator coils to attract, and the motors I bought do NOT have "teeth".
The other clue is that, since a variable-reluctance motor has no permanent magnets, it shouldn't exhibit any "cogging" when unpowered and rotated by hand. My motors DO exhibit cogging.
These motors are configured in the standard delta configuration, which only requires three wires. This is the same wiring that is used in the BLDC's used in hobbyist-style drones. An ESC would likely work to drive my motors, but sensorless ESC's don't do a good job of running motors slowly -- they sense the induced voltage on an un-driven motor winding to determine the position of the motor magnets, and that voltage is very low at low RPMs -- exactly where my viscometer motor needs to operate.
So the only way to use this type of motor will be to attach something to the motor that provides the necessary angular-displacement information to properly drive the motor.
The other difficulty is that it's not obvious to me (so far) on how to drive this type of motor with constant currents. The usual motor drive IC's are so-called "H" bridges -- half bridges -- that switch the motor terminals between Vcc and Ground. No current drive there! So no back EMF to measure?
At some point I think I just need to build a testbed to see what I can do, in as simple a manner as possible.
No comments:
Post a Comment