By Edward Cowern, P.E. – Baldor/ABB
When electric motors are installed in wet or damp areas, the life of the motor is almost always shortened from what would be expected in a dry situation. However, there are several cautions and suggestions that can extend the life of motors in these less than ideal situations.
Open Drip-Proof Motors
Generally speaking, open drip-proof motors are not suitable for wet environments. However, there are many situations where an equipment manufacturer chooses the open drip-proof motor (probably because of its lower first cost) for use where a totally enclosed motor would have been a better and longer life choice. If an open drip-proof motor is in place, a few suggestions can help extend motor life.
First, the motor should be shielded from the direct impact of rain, fog, snow, etc. In shielding a motor from the elements, caution should be used not to 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 that hot air is not trapped inside.
Next, it is important to realize that open drip-proof motors are built to be mounted with a certain orientation. 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 except when motors get mounted to a wall or with feet up (ceiling mounting). In the ceiling mounted case, 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 to shorten the life of the motor. In these cases, end housings should be rotated to put the louvers in the proper position to fend off rain rather than funneling it inside.
The use of open drip-proof motors outdoors or in wet areas is not ideal. In the event of a failure, the motor should be replaced with a motor 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, they will work well. The following suggestions will help extend the life of totally enclosed motors.
Totally enclosed fan cooled motors have “weep holes” at the bottom of the end housings. Weep holes or fittings are put there to allow condensation or other accumulations of moisture to drain. At times, motors are mounted in unusual positions such as with the shaft horizontal but with the base mounted on a vertical wall. In this case the 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. This, of course, is unacceptable. When motors are going to be used in different positions, 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 in applications such as the brush drives used in car washes and similar situations where water is apt to be falling on the motors continuously. 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 that are mounted at odd angles where the weep holes cannot be properly re-positioned to the lowest point, the problem can be remedied by carefully drilling a small hole at the lowest point. Caution must be taken to be sure power to the motor is disconnected and the drill bit does not touch or damage the windings or motor bearings.
Motors such as the Baldor “Washdown Duty™”, “Dirty Duty®” and “Severe Duty” are designed to seal the motor and prevent the entrance of moisture. However, try as we might, it is nearly impossible to keep all water out. Thus, it is vitally important that the weep holes be positioned so that water entering the motor either by direct impingement or by exchange of air saturated with dampness, can drain away freely rather than accumulating.
Baldor accomplished Washdown Duty rating by adding features like an improved paint system, Baldor’s Inverter Spike Resistant insulation system, Class F insulation with Class B (or lower) temperature rise, Mobil Polyrex® EM grease, customer-friendly drain plugs, and shaft seals. Baldor Severe Duty premium construction motors include features designed to protect against contamination, moisture, vibration and demanding duty cycles. Super-E electrical designs are available on Severe Duty motors to create a packaged solution that reduces energy consumption and handles the punishment of the application.
One other source of water in a motor is condensation that can occur as a result of repeated heating and cooling cycles. For example, when the motor gets hot, the air within the motor expands and pushes out. Later, when the motor cools, fresh moisture laden air will be drawn in as the air contracts. As this cycle repeats again and again, substantial quantities of water can accumulate. If left unchecked, it will lead to insulation failure.
Again, this highlights the importance of having the weep holes properly positioned so that water can drain before it accumulates in sufficient quantities to damage the motor.
Where motors run continuously, the heat generated in the motor by normal operation can keep windings dry. But when a motor is used infrequently and is subject to large swings in temperature, there are two methods which can be used to reduce the susceptibility to failure caused by accumulated moisture.
The first and most popular method is the use of heaters installed within the motor. In this case, cartridge heaters or silicon rubber strip heaters, are placed within the motor and are turned on during the non-operating periods. The object of this method is to maintain the temperature inside the motor approximately five to ten degrees warmer than the surrounding air. when this is done, condensation inside the motor is prevented and the motor will stay dry. The heater method is similar to the way light bulbs are used in closets where the climate is humid to prevent 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.
The second method of accomplishing the same result is a system called “trickle heating”. In this case, 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 heat in the windings, rotor, and indirectly the shaft and the bearings of the motor. This system is a good one for preventing condensation in motors that are at rest. Trickle heating is particularly good where there are groups of identical motors such as those used on aerators in pollution control lagoons.
One of the most difficult motors to protect in wet and damp environments is hazardous location or explosion proof. The difficulty in protecting these motors arises from several factors. First, due to explosion proof design requirements, gaskets cannot be used. Similarly, the joints between the end housings and the frame and the conduit box and frame cannot be gasketed or sealed. There must be metal-to-metal contact along these joints. This metal-to-metal contact is close fitting but nonetheless, it cannot seal completely. Also, in explosion proof designs, it is not possible to use normal weep holes. Thus, when explosion proof motors get used in wet environments, moisture that gets inside the motor can accumulate and stay there for extended periods of time. There are breather drain devices that are used in some motors such as the Baldor 1.15 service factor Class 1, Group D explosion proof motors. These specially designed breather drains allow moisture to drain from the motor while still retaining the explosion proof integrity. Again, as in the case of other motors with weep holes, care must be taken to make sure that the breather drains are at the lowest point on the motor.
Some of the options that are available to control moisture in explosion proof motors are the same as those used in totally enclosed motors. Space heaters can be installed in the motors to keep the internal temperature of the motor above the outside temperature during idle periods. This is an effective way to control the build-up of condensation.
One further key to protecting explosion proof motors, especially in outdoor situations, is to shelter them from direct rainfall. Again, as in the case of other motors, the sheltering must be done so that it protects the motor but does not restrict the air flow to and around the motor from the outside.
The installation of motors in outdoor, wet, or damp environments presents some unique problems but, by the proper choice of motor and some caution in installation, most situations can be successfully handled to yield good, long term operating results. The proper choice of motor enclosure and features followed closely by the proper location of the weep holes and in some cases, use of an auxiliary heating device or system to warm the motor during non-operating time, will result in an effective life-extending‑solution.
Motors such as the Baldor Washdown Duty™ and Severe Duty motors are specifically designed to handle difficult situations but even when using these specialized products, the basic cautions regarding proper orientation of the weep holes must be followed.