MRO Magazine

Focus on Harsh Environments: Managing with motor moisture

Often electric motors in industrial environments are subjected to water and moisture, resulting in expensive-to-repair damage. And while the need to protect electric motors from rain, snow, sleet and ...

September 1, 2000 | By Bill Siuru

Often electric motors in industrial environments are subjected to water and moisture, resulting in expensive-to-repair damage. And while the need to protect electric motors from rain, snow, sleet and hosing-down may be obvious, it often is forgotten is that moisture can accumulate even in sealed motors or motors installed in supposedly dry environments. Here are some tips to keep motors dry so they do not expire before their time.

First if possible, select the right motor — one that is designed to tolerate a wet environment. Choices here include totally enclosed, fan-cooled (TEFC) motors and sealed motors that go under titles like “Wash Down”, “Severe Duty” and “Mill and Chemical.”

Open-type motors and equipment containing them can be effectively shielded from rain, snow and sleet. However, make sure not to shield them so that airflow needed for cooling is significantly decreased, thus causing the motor to run at higher temperatures and shorten its life.

Any shelters built over motors should include ventilation holes or louvres that admit air but not moisture.


Make sure to mount motors in the recommended orientation. For example, open-type motors fitted with venetian blind-type louvres in their end housings should be placed so water falling from above is deflected away from the interior of the motor. Problems can result if the motor is installed vertically or inverted so the louvres actually funnel rain or snow into the motors. If possible, rotate the end housing so the louvres are properly positioned to fend off the water flow.

One significant source of moisture is condensation. It results when motors repeatedly heat up and cool down. As motors heat up, air inside expands and is pushed out. Then when the motors cools, fresh, moist air is drawn in as the remaining air inside contracts. The moisture condenses out on to internal components, causing damage to the windings and corroding other components. As the heating and cooling cycle is repeated, substantial amounts of water can accumulate. This is not usually a problem with motors that run continuously, where the heat generated keeps windings and other internal parts dry. It is a definite problem in motors which operate infrequently or cycle so they will heat and cool with wide variations in temperature. As might be expected, the problem is greater in areas with high humidity than in arid locations.

Because of condensation problems, even TEFC and sealed motors can experience moisture damage. Therefore, they usually have weep holes so condensation can automatically drain out. Make sure the motor is installed so the weep holes are located at the lowest point. Otherwise, significant amounts of water can eventually collect within the motor housing. If this orientation is not possible, consider drilling new weep holes at the lowest point. If you do the latter, make sure the motor is disconnected from the power source and lock the moving parts before drilling. Also be careful not to touch or damage the windings or bearings with the drill bit.

Consider installing a heater for particularly tough problems, for instance, motors that run infrequently, cartridge or strip heaters can be placed inside the motor. Internal heaters should be wired so they are interlocked with the motor, meaning they are turned on when the motor is not running and turned off when it is running. Heaters should provide internal temperatures that are five to 10 degrees warmer than the surrounding air.

For trickle heating, low-voltage, single-phase power is applied to three-phase motor windings while the motor is stopped. Application of power results in a low-energy, single-phase condition that provides heat in the windings, rotor and, indirectly, the bearing of the motors.MRO

Bill Siuru is a regular contributor to Machinery & Equipment MRO.


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