Quote:
Originally Posted by MPaulHolmes
I think I may have found a way around the noise issue TI was talking about with one of the major steps for doing sensorless FOC:
Ea = Va - R * I - L * dI/dt
The trouble is, you have to compute dI/dt, but the current waveform is noisy in general. I found the graphs of some of I(t) by printing out the data while the motor was running, and gosh darn it, even down to zero RPM, they are sine waves, but jaggedy due to the noise! So, just figure out the period and magnitude, which are more noise immune (when does it cross x-axis, when does it reach a peak) and then don't do a numerical derivative (i - iOld) / 0.0001), but instead do cos(theta) = d(sin(theta)/dtheta so then your derivative is not noisy!
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I have a story about a zero-crossing detector ... we had a line-to-line fault (short circuit) that caused all sorts of bad things with the breakers that were involved. That is expected.
The fault also caused our large AC Controllers (700 HP) in a different part of the mill to detect a change in utility line frequency (which was not correct). No big deal, right? That just causes a number on some display to be wrong. Not quite. The wrong calculated line frequency caused the controllers to change their firing sequence (converting 3 phase AC incoming to DC).
But the line frequency did NOT change, so the firing change turned on the transistors, not at 0V between phases, but at substantial voltage between phases .. and blew up the switching transistors on 4 controllers. This caused 18 hours of downtime while the controllers were fixed.
The company that sells the controllers is STILL (over 6 months later) trying to come up with a fix the does not break the controllers during every other condition.
I guess my point is that the noise can make it look like you have a period that is not correct. Perhaps it could be combined with a maximum rate-of-change, as a sanity check for the calculated period?
Or am I being too paranoid?