Chances are you have heard that mantra quite a few times if you have been watching the hit HBO cable television show, Game of Thrones. But, what does that mean for electric motors in outdoor motor applications, especially after last year’s polar vortex and this upcoming season?
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Posted on: November 18, 2014
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Generally speaking, most AC general purpose type electric motors are capable of withstanding temperatures as low as -20°C or -4°F without needing to take any environmental-based modifications into consideration. (Elektrim motors can typically tolerate temperatures as low as -30°C or -22°F). However, beyond this, there are at least two critical environmental adjustments that should be made.
One of the first considerations a maintenance professional should make concerns lubrication, and whether or not the standard lubrication used for their electric motor bearings are suitable for the environmental conditions it will be subjected to. Since bearing failures account for more than half of all electric motor failures, ensuring proper lubrication under stressful environmental conditions can help ensure optimal, uninterrupted performance.
During harsh winters when temperatures can drop below most motors’ ideal operating limits, bearing lubrication can become increasingly viscous or “tacky,” creating bearing friction. In most electric motors under 500 HP, grease lubricated bearings are utilized, and a low temperature grease should be applied when operating below -20°C to -30°C (-4°F to-22°F) to prevent unintended viscosity. If you know that the electric motor may be subjected to extreme low temperatures, it might be worth your while to request low temperature bearing grease when the motor is ordered or it may have to be applied later by your local motor shop. Usually only large electric motors (greater than 500 HP) use oil-lubricated bearings, which due to their more sophisticated seals, will require an oil-sump system to maintain in low-temperature applications. When in doubt, always confirm with the manufacturer that the electric motor is designed and tested for the low ambient temperature to which it will be subjected.
The second biggest consideration and challenge facing electric motors in extremely cold conditions is condensation, as moisture can cause deterioration of winding insulation and corrosion of metal parts. As you might expect, Totally Enclosed Fan Cooled (TEFC) motors are better suited for cold, outdoor and high moisture applications since they do not rely on the free exchange of air between the inside and outside of the motor for cooling. This design makes them significantly better adapted than their Open Drip-Proof (ODP) counterparts to repel rain, dust and other elements. However, despite the name, it does not necessarily mean that they are completely air or water-tight. Water and condensation can still work its way into the motor from a variety of different entry points, including through the brackets, junction box and during repeated heating and cooling cycles.
In the winter, when an electric motor is running, it is very hot, which causes the air within it to expand and push out of the motor. If run continuously, the heat generated by the motor can usually keep the windings fairly dry. However, if the motor is used intermittently and turned off, there is a sudden contraction of air as the motor cools, which can pull in moisture and accumulate until the insulation becomes a partial conductor of electricity. If normal voltage is then applied, the insulating material may fail and the motor may need to be rewound before it can be operated again.
Therefore, the first step to preventing condensation in cold outdoor applications is to give moisture from rain or created during cooling a way out. When an electric motor is operated at low temperatures, care should be taken to open the drain holes at the lowest point of the motor, which will allow any collected moisture within the motor an opportunity to drain out. Sometimes, however, motors are installed in a variety of positions that make the manufacturer’s pre-drilled drain holes no longer the lowest point of the motor. As a result, it may be necessary to drill additional drain hole(s) to allow adequate drainage.
The second way condensation can be prevented is to install space heaters within the motor. These are used to maintain a winding temperature 5°C to 10°C warmer than the surrounding air. Since the windings are now warmer (i.e. above the dew point) than the air outside the motor, no air contraction takes place to pull-in moisture and condense on the frame and winding. These flexible resistance heating strips or coils are installed inside the stator housing near the bottom of the motor or in the air chamber above the motor’s feet. When the motor is turned off, turn on space heaters for one hour or until the motor is cooled.
In the end, while most electric motors operate without too much headache in cold weather applications, it may be prudent to re-examine your application for any environmental concerns and/or modifications. While proper bearing lubrication and the prevention of condensation are two critical measures that can easily be taken to ensure your motor’s optimal operation, these are not the only concerns you might have. Care should also be taken in the selection of materials and construction of electric motors to prevent temperature-related embrittlement in severe environmental conditions. When in doubt, always ask your electric motor manufacturer—a five minute conversation can save you more than time and money.