Motors in wet and damp places seldom last as long as their dry counterparts.
Extending the life of damp-duty motors requires you to do more than what you would to protect dry-duty motors. For example, shield the motor from the direct impact of rain, sleet, and snow, taking care not to restrict airflow to and around the motor. If you put a shelter over the motor, make sure it's open, vented, or louvered — so you don't trap hot air inside. Another major factor is housing design. When choosing a motor for a damp location, choose a totally enclosed fan-cooled (TEFC) motor. This design provides durability orders of magnitude above the open-case design. Just selecting a TEFC isn't enough — these motors have special requirements.
TEFC motors have “weep holes” at the bottom of the end housings that allow condensation or other accumulations of moisture to drain. You must ensure these holes remain open — they can easily clog in a dirty environment. Make visual inspection part of your preventive maintenance procedure.
At times, motors mount in unusual positions. For example, if the shaft is horizontal but the base mounts on a wall, the weep holes are out of position by 90°. The only time they could do their job is when the motor is half full of water.
When installing a TEFC, make sure to position (or reposition) the end brackets so the weep holes are at the lowest point of the housing. This is especially important in applications such as the brush drives in car washes where water is apt to fall on the motors. In such situations, you can expect some water to enter the motor. The key to extending motor life is to give water an easy way out.
Suppose your motor mounts at an odd angle, preventing you from aligning the weep hole with the lowest point. You can remedy the problem, but this is a step best left to a rewind shop. Mark the lowest point in the case (with motor mounted), so the rewinder can drill a hole there. If you do this yourself, disconnect power and ensure the drill bit doesn't go far enough beyond the case to damage windings or bearings. Also be sure not to leave drill shavings inside the motor. This last caution is reason enough to have your rewind shop do the work.
“Wash down,” “dirty duty,” and “chemical service” motor designs are possible solutions. All three seal the motor and prevent moisture from entering. However, it's nearly impossible to keep all water out. Thus, no matter what kind of motor you have, position the weep holes so water entering the motor (either by direct impingement or by exchange of air saturated with dampness) can drain away freely.Match your remedy to your motor. Different motor designs present different challenges. The difficulty in protecting hazardous location or explosionproof motors arises from several factors. First, explosionproof design requirements prohibit gaskets. You must have metal-to-metal contact among the joints between the end housings and frame and the conduit box and frame — although this metal-to-metal contact is close fitting, it cannot seal completely. Also, explosionproof designs cannot include normal weep holes. Thus, when you use an explosionproof motor in a wet environment, moisture can accumulate inside the motor. Some manufacturers solve this problem by using breather drains. These allow moisture to drain without compromising the motor's explosionproof integrity. As with other motors with weep holes, you must ensure the breather drains are at the lowest point on the motor.
Some designs, like the open drip-proof motor, aren't suited to wet environments. Nonetheless, some users choose such a motor in misguided attempts at standardization. These decisions don't take performance into account, and it is usually more economical to replace the motor right away than endure the loss of production from a motor failure. If you can't, here are a few suggestions to help extend the motor's life.
The design of open drip-proof motors (see Photo on page 47) requires you to mount them in a certain orientation: You can't make adjustments by drilling weep holes. For example, many open drip-proof motors have “Venetian blind” type louvers in the end housings to make water that is falling from above deflect away from the inside of the motor. This works fine until you mount the motor to a wall or the ceiling. In the case of the latter, unless you change the position of the end housings relative to the base of the motor, the louvers will funnel rain, snow, and other debris into the windings, shortening the life of the motor. If you plan to mount your motor on the ceiling, rotate the end housing to put the louvers in the proper position to fend off falling rain and debris. You can tell what that orientation should be by looking at the direction of the louvers.The enemy from within. Even if you keep water from entering your motor, moisture can still accumulate through condensation, which is caused by the motor's heating and cooling cycles. When the motor gets hot, air inside it expands and some air exits the motor. When the motor cools, fresh moisture-laden air enters as the existing air contracts. As this cycle repeats, substantial quantities of water can accumulate and cause insulation failure if left unchecked. Motors that run continuously generate enough heat through normal operation to keep windings dry. But when a motor runs infrequently and endures large swings in temperature, you must reduce the effects of moisture accumulation.
The most popular method of reducing moisture accumulation is installing heaters in the motor. Cartridge heaters or silicon rubber strip heaters turn on during the nonoperating periods and off when the motor runs. This method maintains the temperature inside the motor approximately 5° to 10° warmer than the surrounding air, preventing condensation from accumulating inside the motor.
An effective method for preventing moisture in motors at rest, called “trickle heating,” involves the application of a source of low-voltage single-phase power to the 3-phase motor windings when the motor is at rest. This results in a low-energy, single-phasing condition that produces heat in the windings, rotor, and the shaft and the bearings.
Damp-duty motors have all the installation and maintenance requirements of dry-duty motors — and then some. Most damp-duty motor failures occur for reasons unique to damp-duty motors. By understanding and applying damp-duty motor concepts, you ensure maximum motor reliability.
Editor's Note: This text is an adaptation of the Cowern Papers, courtesy of Baldor Electric Co., Wallingford, Conn., edited by Mark Lamendola, EC&M Technical Editor. Cowern is an application engineer for Baldor.