Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1378657
www.machinerylubrication.com | May - June 2021 | 45 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. 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. e 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. is 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. e motors that fall into the A category aren't really used in manufac- turing anymore, but you may find them in some of your household items. e 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. e Class B and Class F are where you are going to find most of your motors today. ese 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. is 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 tempera- ture sensors on every electric motor in a facility, but I would recommend identifying which pieces of equipment might have a higher consequence of failure. ink 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 inspec- tion tasks, it is possible to minimize catastrophic problems without spending too much time inspecting relatively low-priority or low-main- tenance-cost motors. ML About the Author Jeremie Edwards is an Associate Technical Consul- tant at Noria Corporation. He is one of an elite few certified by the International Council for Machinery Lubrication (ICML) as a Machinery Lubrication Engineer (MLE) and did so in order to become the best advisor for clients when it comes to their continuing education needs. Before joining Noria, Jeremie served six years in the U.S. Army as a parachute rigger and was deployed in Afghanistan, Uzbekistan, Turkey and Germany. INSULATION CLASS A 5°C 5°C 10°C 10°C 15°C 60°C 75°C 80°C 105°C 125°C 40°C 40°C 40°C 40°C 40°C 130°C 155°C 180°C Design Temp. Thermal Margin Allowable Temp Rise Ambient Temperature E B F H 120°C 105°C START YOUR FREE SUBSCRIPTION www.machinerylubrication.com Figure 1 NEMA rating chart for electric motors. ML ML