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Jack, here where I work we have a lot of compound shunt DC motors. We utilize field weakening to achieve higher RPMs by varying the current to the fields. You can only weaken the fields so much and the machine will become unstable. We know where full fields should be set and we also know where things will become unstable if they lower the current too much. One of the safe guards for monitoring the field strength is the "field loss" circuit. It watches to see that fields do not go lower than a pre-set level. If the fields drops below a certain level the "FL" relay drops out and opens the main contractor. Of course all hell breaks loose when this happens because all that energy has to go somewhere. There is comparing logic in the program which monitors Current Feed back, Speed reference, Ramp tracking, Linear time for both acceleration and deceleration. Everything begins with current feed back. We check this before we attempt and drive adjustments. Since you have the LEM maybe you can add logic.
What we have is quite complicated for several reasons but if your looking to play around with field weakening adding some pre-sets and comparing inputs might give you the control your looking for. I will look around for some of our old tech manuals they gave us for training which might be helpful to you and Paul.
I’ll see what else I can dig up
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Mike
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