Quote:
Originally Posted by drbobwoolery
If the current sensors are not too high, actually measuring all three, and seeing that they add to zero may be the easiest way to detect a number of parts failure faults at once. Whenever the three currents do not sum to zero, a general fault condition would be generated.
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Interesting. I agree that the 'vector sum' of the currents should be 0. And I agree that if the currents do not add up, there are a number of things that could be wrong.
How fast do you need to read the 3 phase currents so that they'll add to 0? Or do you need to average them, since you are sending a bunch of pulses?
I know that you can get a current signal (magnitude) from a hall effect sensor, or three of them I guess - but how do you calculate .. or measure, I guess, the phase angles?
I've seen a lower tech method used - put all three 'output' conductors (or bus bars, I guess) through a current transformer and see if anything is measured. If so, bad things are happening and current is 'escaping' somewhere. I think the alarm reads as 'ground fault'.
I have taken apart some older industrial VFDs that use this lower tech method. None of my employer's more modern VFDs have blown up yet, so I haven't had the opportunity to take them apart and see how it's done now.