Motion System Design

Help motors survive wet environments

When electric motors are installed in wet or damp areas, their lives are inevitably shorter than if they lived in a dry climate. However, using a little caution can extend motor life. Open Drip Proof (ODP) motors have openings in their frames that allow passage of external cooling air over and around their internal windings. It is this open access to the inside of the motor that makes ODP motors not suitable for wet environments. Yet there are many situations where OEMs specify an ODP motor — probably to save upfront costs — where a totally enclosed motor would have been a better choice. If an ODP motor is already in place, here are a few suggestions to make the most of it.

First, the motor should be shielded from direct impact of rain and other elements, being careful not to restrict airflow to and around the motor. Placing a shelter over the motor is fine, as long as it is well ventilated so that hot air is not trapped inside. Next, it's important to realize that ODP motors are built to be mounted in a certain orientation: Many of these motors have “venetian blind” type louvers in the end housings to make water that is falling from above deflect away from the motor's insides. This works fine except when motors get mounted to a wall or with feet up. In the latter case, unless the position of the end housings is changed relative to the motor's base, louvers will have a funnel effect, thereby sending rain, snow, or other debris into the windings. 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.

Totally Enclosed Fan Cooled (TEFC) motors are more adaptable to outdoor and high moisture areas. Their enclosed design does not have openings that allow free exchange of air between the inside and outside of the motor, but even so, they are not airtight or watertight. With a bit of caution - including the following considerations - they can work quite well for years.

As with ODP motors, efforts should be made to shield a TEFC motor from direct impact of rain and other elements. Water falling or splashing directly on the frame can work its way into the motor via the frame to bracket fits (openings in the brackets where the shaft extends out) or at the conduit box. The key to extending the life of TEFC motors used in these settings is to give this moisture a way out. Failure to do so allows moisture to damage the motor's internal components. Most TEFC motors have “weep holes” to protect against internal moisture build up.

Weep holes are located at the low spot of the end-housings to drain any collected moisture out of the motor enclosure. If a motor is to be mounted in a different position, care must be taken to ensure proper weep hole orientation. This can be done by repositioning the brackets so that the weep holes are at the motor's lowest point.

On motors that are mounted at odd angles where weep holes cannot be properly repositioned to the lowest point, the problem can be remedied by carefully drilling a small hole at the lowest point. Caution: Be sure that power to the motor is disconnected and the drill bit does not touch or damage the windings or motor bearings.

Another source of water is when a motor is installed in a high-humidity environment. Water can collect in a TEFC motor from condensation that occurs as a result of repeated heating and cooling cycles. When the motor gets hot, the air within it 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 amounts of water can accumulate. If left unchecked, this will lead to insulation failure. Once again, be sure to position weep holes properly so that water can drain.

Where motors run continuously, heat generated by normal operation can keep windings dry. But when a motor is used infrequently and subject to temperature swings, two methods can reduce the susceptibility to failure caused by moisture. The first method is to use heaters installed within the motor. The objective is to keep the temperature inside the motor 5° to 10° warmer than the surrounding air to prevent condensation inside. The second method is a system called “trickle heating;” a source of low-voltage single-phase power is applied to the three-phase motor windings when the motor is at rest. The result is a low energy, single-phasing condition that produces heat in the windings, rotor, and indirectly in the motor's shaft and bearings — an effective system for preventing condensation in motors at rest.

Special thanks to Baldor Electric Co. for this month's tips. For more information, visit or call (479) 646-4711.

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