Optocoupler selection? - Page 1
Forum section: electronics / beginners

author: Things
author_rank: Regular Contributor
timestamp: December 13, 2012, 12:50:38 pm
content: Hey all,In a project I'm working on, I want to be able to measure the rotational speed of a sensorless 3phase brushless motor.My idea is to use a optocoupler connected to one of the motor poles.However, I'm having some difficulty in determining what I need to look for in the optocoupler. The main issue here is, the motor being a huge inductor, will generate some pretty nasty kickback. Optocouplers seem to be rated based on current input and voltage output, how does this work? Can i use any optocoupler as long as I limit the input current to the LED, even if the motor is kicking back ~100V? (Jut a guesstimate based on ~24V drive voltage).Dan

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 13, 2012, 02:05:42 pm
content: If you have any PC fans lying around how bout you grab the hall effect sensor out of one and use it? They should detect the polarity of the magnetic field flip every 180 degrees.

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 13, 2012, 09:55:32 pm
content: Can't use one of the H-bridge drive signals as tacho?

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 14, 2012, 04:12:48 am
content: If you're trying to measure the set speed, you could just get it from the driving side, but if you're trying to measure it including slippage under load, you'd need something like a hall effect sensor.

author: Kremmen
author_rank: Super Contributor
timestamp: December 14, 2012, 09:14:32 am
content: Hm, this was a BLDC, not an induction motor. A BLDC is not supposed to have any slip, because the rotor is permanently magnetized (i.e. it has permanent magnets in there). An induction motor, as the name implies, creates the rotor flux by magnetic induction in the rotor cage. For induction to work there must be a varying mag field across the conductors and that is where slip enters the picture. No speed difference between rotor and the stator field - no induction - no torque. Whereas a BLDC rotor is already happily magnetized and no slip is needed.So, excluding stalls due to overload, there should be no slip in a BLDC. Slipping would actually introduce gross torque oscillations that would probably induce a stall right there, under any appreciable load.So the driver speed reference can be used as a rotation speed observer. Subject to implementation details there will be some variation, but not too much. You don't specify what kind of BLDC motor this is - a small toy motor used in RC models or an industrial servo or something in between. Anyway the control system between speed setpoint and actual shaft speed can vary subject to speed profiles vs torque limits and control schemes.Usually however, the controller feedback is taken from either phase voltages or currents or both. In a toy motor with open loop V/f control this feedback may not be there, but is easily achieved either by a simple current transformer or voltage probe ("simple" is subject to stator voltage level of course). Just detect the fundamental frequency of either signal and you are basically done.