Understanding and Setting Electric Motor Inspection Tasks

Jeremie Edwards, Noria Corporation

“Failure is central to engineering. Every single calculation that an engineer makes is a failure calculation. Successful engineering is all about understanding how things break or fail.” - Henry Petroski

All manufacturing facilities are different, and each has its own needs. Most of these facilities do have some common components such as electric motors, gearboxes, pumps and bearings.

Inspections and condition monitoring for each:

When we think about equipment maintenance, what drives the equipment should be a primary concern. It doesn’t matter if you are talking about a gearbox, a pump or a chain drive; if it is run by an electric motor (as is often the case) and that motor fails, no amount of gearbox care is going to fix the problem.

Not all electric motors are equal. Some will be carefully maintained and lubricated, while others will not. Even motors that are not carefully maintained usually make it onto an inspection schedule. In any case, there are a number of things that we can look for on any electric motor during inspections.

Certain aspects should be obvious, like operating temperature, sound, obvious shaking, etc. These are all things that even a layman should be able to spot if they have seen what “normal” operating conditions look like.

But it is often the case that all our other duties get in the way of even the clearest signs of problems. As younger folks enter workplaces where apprenticeship programs are not as prevalent, a lot of the “old knowledge” that we could assume everyone has is being lost. Of course, not all old knowledge still holds true. We need to address which knowledge is fact and what has become fiction as our understanding of lubrication and maintenance has developed.

Let’s start with temperatures. Now, I have to admit that I have been as guilty of this as anyone out there, but just because a motor is too hot to touch doesn’t mean that the motor is “running hot.” Don’t get me wrong, I would love it if all electric motors could run at 125° F (52°C) all the time while being in an ambient temperature of 77°F (25°C), but we all know that this just isn’t the case. More often than not, I see electric motors running upward of 175° F (79°C) and even up to 225°F (107°C). Looking at these issues with lubrication in mind, my focus would be on the grease, and more specifically, the base oil inside of that grease.

Lower operating temperatures for an oil means a much longer life (this can be seen in a drastic way when you look at the oil change frequency difference in hydropower turbines versus steam turbines). For this reason, I had long thought that an electric motor too hot to touch meant that it was automatically degrading the lubricant prematurely, but this isn’t necessarily the case.

Figure 1 NEMA rating chart for electric motors.

Too hot to touch?

According to ASTM C1055, 140°F (60°C) is about the temperature where it is unsafe to touch something for more than 5 seconds (any longer and there could be permanent burn damage). Most electric motor greases are going to be rated for an operating temperature of closer to 350°F (177°C), which is much hotter than we should ever touch. Now, this doesn’t mean that we should run our motors this hot, but we should understand a few things while we are looking at a motor’s temperature, like the difference in temperatures that the bearings might be exposed to versus the temperature of the outside housing. Keep in mind that there are a number of factors that determine the outside temp of the motor housing, but in relation to the bearings, the housing will typically be about 50°F-77°F (10°C-25°C) cooler than the actual bearing temperature. The range is so large because different designs will dissipate this heat at different rates.

So what are we really looking at, or what should we be looking for? Most electric motors out there have a NEMA rating listed on them. This is where we start. Your electric motors should have a NEMA rating designation of either an A, E, B, F or H. Figure 1 helps explain these ratings a bit better.

The motors that fall into the A category aren’t really used in manufacturing anymore, but you may find them in some of your household items. The E category is going to be some of your light-duty applications, but once again, you’re probably not going to find these doing a whole lot of work in a large manufacturing facility. The Class B and Class F are where you are going to find most of your motors today. These are designed for higher temperatures, as you can tell from the chart, and continuous duty.

Now that we know what temperatures our motors are rated for, we have an idea of what our grease is tested to, and we know that the bearings and windings are going to be a bit hotter than the housing of the motor, but what do we do with that information?

We can’t just focus on one aspect of electric motors or any other piece of equipment. We have to look at the criticality of the equipment to help determine what inspections we should put in place to keep an eye on it. If you’ve been to one of Noria’s classes or have been a Machinery Lubrication reader, you have probably heard of the Optimum Reference State. This helps us match the reliability needs of a specific piece of equipment with our operating needs, operating conditions and budget.

I would never recommend placing real-time vibration and temperature sensors on every electric motor in a facility, but I would recommend identifying which pieces of equipment might have a higher consequence of failure. Think of it this way: If this specific motor fails, is it going to cause a cascade effect in other equipment? Is it going to shut down production for the entire facility, or maybe a production line? Maybe this motor is in a location where you would have to hire a crane to move it or even have the motor airlifted into position. Replacing a motor in these scenarios is going to cost 50 times more than motor itself.

By taking the above aspects into consideration when setting inspection tasks, it is possible to minimize catastrophic problems without spending too much time inspecting relatively low-priority or low-maintenance-cost motors.

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