Water torture
Wet or damp environments kill electric motors.  However, altered louvers, weep holes and heaters can fend off moisture and extend service life.

by Ed Cowern

When electric motors are installed in wet or damp areas, motor life is almost always shortened.  However, there are cautions and suggestions that can extend the life of motors in these less-than-ideal situations.  This article will outline several.

Open drip-proof motors
Generally speaking, open drip-proof motors aren't suitable for wet
environments.  However, there are many situations where an equipment manufacturer chooses this type of motor (probably because of its lower first cost) when a totally enclosed motor would've produced better results.

If an open drip-proof motor is in place, a few suggestions can extend its life.

First, shield the motor from rain, fog, snow, etc.  In doing so, do not restrict air flow to and around the motor.  Thus, putting a shelter over the motor is a fine idea, as long as the shelter is well-ventilated or louvered so hot air can escape.

Next, open drip-proof motors are built to be mounted with a certain orientation.   Many have "venetian blind" type louvers in the end housings to deflect water away from the inside of the motor.  This works fine except when mounting a motor to a wall or ceiling (with feet up).

In the ceiling-mounted case, unless you change the end housing position relative to the motor base, the louvers will act as a funnel, directing rain, snow and other debris into the windings.  To avoid shortened motor life, rotate the housings so the louvers can fend off rain rather than funnel it in.

Using open drip-proof motors outside or in wet areas is not ideal.  If the motor fails, replace it with one more suitable for an outdoor or wet environment.

Totally enclosed fan-cooled
Totally enclosed fan-cooled motors are more adaptable to outdoor and high-moisture areas and, with a bit of caution, work very well.  To extend life, follow a few suggestions.

Totally enclosed fan-cooled motors have "weep holes" at the bottom of the end housings.  Such holes or fittings allow condensation or accumulated moisture to drain.

At times, motors are mounted in unusual positions, such as with the shaft horizontal but the base mounted on a vertical wall.  In this case, the weep holes are out of position by 90 degrees.  The only time they could do their job would be when the motor is half full of water.  This is unacceptable.

When using motors in different positions, reposition the end brackets so the weep holes are at the lowest point of the motor.  This is especially important in applications where water falls continuously on the motor.  In this situation, some water can always be expected to enter the motor.  The key to extending motor life is to give it an easy way out.

On motors mounted at odd angles where the weep holes can't be properly repositioned to the lowest point, fix the problem by carefully drilling a small hole at the lowest point.   Disconnect power to the motor, and don't touch or damage the windings or motor bearings with the drill bit.

Some designs seal the motor in order to prevent water from entering.  However, it's nearly impossible to keep all water out.  Thus, it's vitally important to position the weep holes so water entering the motor (either by direct impingement or by exchange of air saturated with dampness) drains freely rather than accumulates.

Another source of water is condensation from repeated heating and cooling cycles.   When the motor gets hot, air within the motor expands and pushes out.  Later, when the motor cools, fresh moisture-laden air is drawn in as the air contracts.  As this cycle repeats, substantial quantities of water accumulate.  If unchecked, it leads to insulation failure.  Again, this highlights the importance of properly positioned weep holes.

In motors running continuously, heat generated by normal operation keep the windings dry.   Infrequently used motors are subject to large swings in temperature, but two methods reduce failures caused by accumulated moisture.

The first and most popular method is the use of heaters installed within the motor.   Here, cartridge heaters or silicon rubber strip heaters are placed within the motor and turned on during non-operating periods.  This keeps the internal temperature 5 to 10 degrees warmer than the surrounding air.  It prevents condensation and keeps the motor dry.  Internal heaters interconnect with the motor starter.  They turn on when the motor isn't running and off when the motor is running.

The second method is a system called "trickle heating."  Here, a source of low-voltage, single-phase power is applied to the three-phase motor windings when the motor is at rest.  This results in a low-energy, single-phasing condition that produces direct heat in the windings and rotor, and indirect heat in the shaft and bearings.

Explosion-proof motors
One of the most difficult motor lines to protect in wet and damp environments is explosion-proof, or hazardous location.  The difficulty arises from several factors.

First, due to explosion-proof design requirements, gaskets aren't used.  Similarly, the joints between the end housings and frame, and conduit box and frame, can't be gasketed or sealed.  Metal-to-metal contact is required along these joints.   This contact is close-fitting but, nonetheless, can't seal completely.

Also, in explosion-proof designs, it's not possible to use normal weep holes.  Thus, when using these motors in wet environments, moisture accumulates and stays there for extended periods of time.

Breather drain devices are used in some motors.  These drain moisture from the motor while retaining the explosion-proof integrity.  Again, as in the case of other motors with weep holes, breather drains must be at the lowest point on the motor.

Some options available to control moisture in explosion-proof motors are the same as those used in totally enclosed motors.  Install space heaters to keep the motor's internal temperature above the outside temperature during idle periods.

One further key is to shelter them from direct rainfall.  As in the case of other motors, shelter in a way that protects the motor without restricting outside air flow to and around the motor.

Summary
Motor installation in outdoor, wet or damp environments presents unique problems but, through the proper choice of motor and some caution in installation, most can be handled to yield good, long-term operating results.

The proper choice of motor enclosure and features, plus following the aforementioned suggestions, will extend motor life.

Ed Cowern is a retired engineer who spent 21 years as a district manager for Baldor Motors and Drives.

This article appeared in the February/March 2000 issue of MRO Today magazine.  Copyright, 2000.

Back to top

Back to Uptime archives