You work as a maintenance electrician in a mid-sized manufacturing plant. You report to the plant electrical engineer, who reports to the plant engineer. The latter called both of you into his office to discuss a problem he wants you to solve.
There’s a motor that opened its overloads a few times over a couple of months. A month ago, the problem stopped, and there haven’t been any service calls for it. Since there was no problem being reported, nobody from maintenance pursued the issue further.
The plant engineer then showed you his phone. “I took this while out in that area of the plant an hour ago,” he said. It was a picture of the motor controls enclosure standing wide open with a floor fan blowing into it.
“I told them that was not permitted,” he continued. “They gave me some lip about it, and we ended up in Bob’s office.” Bob is the plant manager. The plant engineer then stressed to you how important it was to figure out what was going on. The plant electrical engineer then informed you the motor is correct for the application and the heater strips are correctly sized.
What might have caused this problem, and how can you ensure it won’t return now that the floor fan isn’t artificially upsizing the overloads?
Answer to Quiz. The fact that it’s this one motor and not motors all over the plant would indicate the precipitating cause is not voltage imbalance, low voltage, or poor power quality. The fact that the strips don’t blow right away (or close to it) is also informative. That would likely rule out a problem with the motor itself, such as shorted windings or worn bearings.
But there could be a combination of marginally okay things that add up and cause the motor to pull enough excess current that an overload opens. So as part of your troubleshooting, you need to check all of those things and see if that’s the case. For example, a voltage imbalance of 2% would not cause the heaters to open. But combine that with a few shorted windings, clogged vents, and excess axial runout, and now you have a situation where heat builds up over time and the motor draws enough current to open its overloads.
You should also find out if the operators are starting, stopping, and restarting this motor. And if so, are they short-cycling it so that it doesn’t have sufficient time to cool between restarts?
However, the most likely reason is that the motor’s output torque isn’t sufficient to drive the load presented to the motor. That is the classic definition of “motor overload.” We know the motor was properly selected for the application, but we must add the caveat “as designed” to the application. If the load has been inadvertently increased beyond what it was intended to be, then you have found the reason for this problem. Some ways the motor will see a load that is higher than design:
- Gearbox with low oil or contaminated oil.
- Misalignment. If the output shaft is side-loaded, extra torque is required to drive the motor and the bearings run hot.
- Soft foot, pedestal warping, or related problems that convert motor torque to wasted motion (vibration).
- Dirt in the load’s moving parts. For example, the motor is driving a conveyor and the rollers are “high friction” due to being dirty.
- Jamming of material. For example, the motor drives an auger and the material fed into the system contains large chunks the system wasn’t intended to handle.
- Operator actions. For example, walking on the conveyor or dropping a box onto it instead of setting the box on it.
- Reconfigurations that added friction to the system. Think of why you don’t pull wire through more than three 90s. The same principle applies to loads a motor must drive. If, for example, a section was added to a conveyor to take the load up three feet, over six feet, and back down three feet that’s a lot of friction added to the system. The solution will be to “straighten the track” or upgrade to a bigger motor.
While it’s great that the plant electrical engineer confirmed that the motor meets the design specs, someone needs to confirm that the load also meets the design specs. That includes not just the actual load (e.g., gearbox) but also the apparent load (e.g., soft foot). So check these things.
Finally, keep in mind that if simply cooling the overloads with a floor fan was enough to prevent them from opening, the amount of overload can’t be much. It may just be a matter of fixing a few little things that added up.
