Building No. 6 houses a system in which various ingredients flow from their respective hoppers into a mixer, in batch sequence (as opposed to continuously). After the ingredients are mixed for X time, the resulting slurry transfers to an extruder. The extruded items are then batched into an oven, after which they are batched into a cooling chamber (the cooling is done by fans, and the heated air is exhausted out the chamber and to the outside). After cooling, the parts move to the next stage.
So far, this description could be for a cookie-baking machine, and the next stage might be frosting or packaging. However, one of the hoppers holds plastic pellets, and instead of making food items, this system makes plastic parts that are various shapes and colors. It operates on a “recipe” (PLC instructions) basis, so for example green widget no. 21 might use recipe 1053C.
There’s a lot that has to happen in a particular sequence, all timed properly, so things arrive at the next stage in the condition and at the time they are supposed to. The operators have been complaining that this or that motor has been “sluggish” (or they use a similar term). When queried by maintenance about what they mean, they say things don’t get started on time at the various stages.
How might you investigate this?
Answer to quiz.
It could be a given recipe has time delays built in or in some other way isn’t correct. Pass this aspect to the system engineer, and ask for some recipes to be verified as correct so you can observe the operation with the possibility of a recipe error ruled out.
To rule out motor problems:
- Check the power at the motor. What is the voltage, voltage imbalance, and power factor?
- Check each motor for bearing heat and vibration.
If everything so far checks out okay and the problem persists, then you’ll want to ensure the motors are not being asked to do more than they are intended to do. In these kinds of systems, poor cleaning procedures or poor adherence to the cleaning procedures can result in an overly thick slurry. Ask the system engineer to investigate that potential problem area.
Physically inspect the moving parts of the system, which in this case would be the conveyors and any switching gates or other mechanical mechanisms. A build-up of plastic dust, process drip, or other gunk may be working against the product flow the motors are intended to create. This system should have some automatic means of detecting this resistance to flow, such as motor current monitoring (more torque required equals more current drawn), or even proximity switches that work in concert with timers to calculate the flow. If there is no such means, discuss this with the system engineer.
If no problems have been found thus far, check any motor gear boxes for lubrication issues and check any motor VFDs for proper set-up. Also use the system drawings to verify that each motor is the correct one for the application; it is possible that a failed motor was replaced with the wrong motor (not just the rating, but also the design as in Design B versus Design D).
In every PLC-controlled system, you want to check the final control elements first. In this system, that would be the motors and the associated equipment. And that part’s now done. You’ve also had the systems engineer verify the PLC, which is normally done last but in this case we did it first because there are multiple recipes.
Now what remains is to check the inputs. For example, does the cooling chamber provide temperature information to the PLC? If so, calibrate that instrument loop and be sure to visually check the instrumentation wiring to ensure there’s no induced voltage from power wiring affecting the temperature signal.
If everything checks out — and the operators still say there’s a problem — see if they can run one of those verified recipes. Have the timing at each stage written out ahead of time. Use a stopwatch to check that the system is doing as the recipe says it should. There is almost no chance at this point that a problem actually exists. But if it does, the cause does not lie with the equipment condition or performance. The system engineer will need to check the recipes against the materials to determine what changes must be made either there or in the equipment design.