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Operating Motors in Wet Environments

Feb. 1, 2008
When electric motors are installed in a wet or damp area, their lives almost always are shortened from what they would be in a dry environment. By heeding

When electric motors are installed in a wet or damp area, their lives almost always are shortened from what they would be in a dry environment. By heeding several cautions and following a few suggestions, however, one can extend the life of motors in less-than-ideal conditions.


Generally speaking, an open drip-proof motor is not suitable for a wet environment. In most cases, a totally enclosed motor is a better and longer-life option. Still, because of its lower first cost, an open drip-proof motor often is the choice of equipment manufacturers. If an open drip-proof motor is in place, follow these suggestions to help extend its life:

  • Shield the motor from the direct impact of rain, fog, snow, etc. Use caution, however, not to restrict airflow to and around the motor. Putting a shelter over a motor is a fine idea, as long as the shelter is well-ventilated or louvered so that hot air is not trapped inside.

  • Remember that an open drip-proof motor is built to be mounted a certain way. For example, many open drip-proof motors have “venetian blind”-type louvers in their end housings to deflect water falling from above. This works well, except when motors are mounted to a wall or ceiling. In the case of a ceiling-mounted motor, unless the position of the end housings is changed relative to the base of the motor, the louvers will have a funnel effect, directing rain, snow, and other debris into the windings, shortening the life of the motor. In such cases, end housings should be rotated to put louvers in the proper position to fend off rain, rather than funnel it inside.

The use of open drip-proof motors outdoors or in wet areas is not ideal. In the event of a failure, an open drip-proof motor should be replaced with a motor more suitable for an outdoor or wet environment.


With a bit of caution, totally enclosed fan-cooled motors will work well in outdoor and high-moisture areas.

Totally enclosed fan-cooled motors have “weep holes” at the bottom of their end housings. These weep holes allow condensation and other accumulations of moisture to drain. At times, motors are mounted in unusual positions, such as with the shaft horizontal and the base vertical. In such cases, weep holes are out of position by 90 degrees, and the only time they could do their job would be when the motor is half full of water. Obviously, this is unacceptable. When a motor is going to be mounted in an unusual position, care should be taken to reposition the end brackets so the weep holes are at the lowest point of the motor. This is especially important when water is apt to be falling on a motor continuously. In such situations, water always can be expected to enter a motor; the key to extending motor life, then, is to give water an easy way out. With motors mounted at odd angles, where weep holes cannot be repositioned to the lowest point, this can be accomplished by drilling a small hole at the lowest point. Caution must be taken to ensure power to the motor is disconnected and the drill bit does not touch or damage the windings or motor bearings.

“Washdown,” “dirty-duty,” and “chemical-service” motors are sealed. However, try as we might, it is nearly impossible to keep all water out. Thus, it is vitally important that weep holes be positioned so that water entering a motor drains freely, rather than accumulates.

Condensation can occur as a result of repeated heating and cooling cycles. For example, when a motor gets hot, air within the motor expands. Later, when the motor cools and the air contracts, fresh, moisture-laden air is drawn in. As this cycle repeats, substantial amounts of water can accumulate and, if left unchecked, lead to insulation failure. Again, this highlights the importance of properly positioning weep holes so that water can drain before accumulating in sufficient quantities to damage a motor.

When a motor runs continuously, the heat generated by normal operation can keep windings dry. When a motor runs infrequently and is subject to large swings in temperature, failure can result from accumulated moisture. To reduce susceptibility to failure:

  • Consider the use of internal heaters. In this case, cartridge heaters or silicon-rubber strip heaters are placed inside of a motor and turned on during nonoperating periods. The object is to keep the inside of the motor approximately 5°F to 10°F warmer than the surrounding air. This prevents condensation inside of the motor and keeps the motor dry. This is similar to the use of light bulbs in closets in humid climates to prevent the growth of mildew on clothing and leather goods. When internal heaters are used, they are interconnected with the motor starter to turn on when the motor is not running and off when the motor is running.

  • Consider “trickle heating,” by which a source of low-voltage, single-phase power is applied to three-phase motor windings when a motor is at rest. This results in a low-energy, single-phase condition that produces heat in the windings, rotor, and — indirectly — shaft and bearings. Trickle heating is particularly effective where there is a group of identical motors.


Among the most difficult motors to protect in wet and damp environments are hazardous-location, or explosion-proof. The difficulty in protecting these motors arises from several factors:

  • Because of explosion-proof-design requirements, gaskets cannot be used. Similarly, the joints between end housings and frames and conduit boxes and frames cannot be gasketed or sealed. There must be metal-to-metal contact along these joints. This metal-to-metal contact is close-fitting, but cannot seal completely.

  • In explosion-proof designs, normal weep holes cannot be used. Thus, when an explosion-proof motor is used in a wet environment, moisture can accumulate inside of the motor and stay there for an extended period of time. In some motors, specially designed breather drains are used. These devices allow moisture to drain from a motor without compromising explosion-proof integrity. As in the case of weep holes, care must be taken to ensure breather drains are at the lowest point on a motor.

Some of the options for controlling moisture in explosion-proof motors are the same as those for controlling moisture in totally enclosed motors. Space heaters can be installed in motors to keep the internal temperature above the outside temperature during idle periods. This is an effective way to control the buildup of condensation.

A key to protecting explosion-proof motors, especially outdoors, is shelter from direct rainfall. As in the case of other motors, sheltering must be done so as not to restrict airflow to and around a motor.


The installation of motors in wet or damp environments presents unique problems, but, with the proper choice of enclosure and features, followed closely by the proper location of weep holes and, in some cases, the use of an auxiliary heating device or system, long-term successful operation can be achieved.

A retired engineer and former district manager for Baldor Electric Co., Edward H. Cowern is a well-known technical writer, having been published many times in trade journals and authored numerous technical papers used by Baldor sales and marketing personnel. He has a bachelor's degree in electrical engineering from the University of Massachusetts.