We have a DC motor for our plastic

[ndelaughter]

We have a DC motor for our plastic extruding equipment. We have had some issues lately that we will see a trend on the motor current of very large oscillations, like it's hunting.

We have checked wires, we have meagered the motor and not found anything that sticks out. Ground readings are all ok.

The oscillations will start off relatively small then start to just get wild and rapid.
See the pictures of the trends.

https://files.engineering.com/getfi...-3773d901f634&file=PXL_20220321_161628211.jpg

https://files.engineering.com/getfi...-efc67d4bb8e8&file=PXL_20220321_161638350.jpg

https://files.engineering.com/getfi...-7ce81f5800ba&file=PXL_20220321_161610573.jpg

Eventually the swings get so large the motor just kills itself. No errors on the motor controller, just turns itself off.
We can restart it (usually) and it goes right back into the same problem.

Has anybody experienced this? Know what it is? Have suggestions?

 

[BrianE22]

You might need to offer a bit more info for the motor people. What is the red on the graph? The blue? I think it is speed and motor current. Maybe a smaller window of the data would be helpful. How big is the motor? Is it closed loop PID control (the oscillation sounds familiar)?

 

[FacEngrPE]

Your system is unstable to point out the obvious.
The mechanical system and the motor control loop are resonating against each other.
Questions
Have any adjustments to the control system been made recently?
Have any components been changed recently?
Has the viscosity of the stuff you are extruding changed?
Is there something about the nozzle that is different?
Can you change the speed of your feedback loop?

Often changing any one thing will solve a resonance problem.