Cyndi Nyberg
Former EASA Technical Support Specialist

Many service centers are also motor distributors, keeping a large stock of motors to sell. In addition, many hold motors in storage for specific customers as their spares. Many end users also keep spare motors on site to keep production moving when a motor fails or needs an overhaul. In any of these cases, whether a motor is stored for a month or many years, there are certain steps that must be taken to ensure that as soon as a motor is taken out of storage, it will be in good condition.

Keep in mind that the suggested time periods are not absolute; different environmental conditions might necessitate a different schedule. Also keep in mind that smaller motors may not need the same special attention that a larger or more critical machine will need.

First, determine how long the motor will be in storage. If it is just a few days or a couple of weeks, no special precautions need to be taken unless the motor has space heaters. If so, they should be energized to maintain the winding temperature 5-10ºC above the ambient. If a motor is put in storage for an extended period of time, there are a number of steps to take to protect the windings, bearings, and machined surfaces.

Find a suitable storage area
Ideally, motors should be stored indoors in a clean, dry and vibration-free area. How- ever, we know that this is not always possible. Therefore, if amotor must be stored outdoors, make sure it is at least covered with a tarpaulin. It should extend to the ground but not be airtight. Air must circulate around the motor to prevent condensation from forming, so the tarp should be secure enough that it does not blow off or allow moisture in, but not so tight that no air gets underneath. Store horizon- tal motors in a horizontal position, and vertical motors in a stable vertical position.

Depending on the region, different creatures may try to make a home in a stored motor. Pre- cautions should be taken to prevent rodents,snakes, birds, or other small animals from nesting inside the motor.

They can destroy the winding insulation. In areas where insects are prevalent, precautions must be taken to prevent them from gaining access to the interior of the motor. In addition to ruining a winding, they can block ventilation and drain openings.

Prepare motor surfaces
If a motor must be stored for a lengthy period, there should be some sort of rust preventative coating on the machined surfaces of the motor.

There is a choice to be made between a durable coating (which requires removal before the motor is placed into service) and one that will not interfere with the motor’s operation if left on (but may deteriorate over time). If necessary, recoat the surfaces. Check the coatings periodically and recoat as needed. This will be especially important in humid and rainy/snowy environments.

Bearing damage possible
Whether the motor is indoors or out, it may be subject to vibration. Proximity to rail lines, busy roads, and/or production floors will all contribute to the ambient vibration. Excessive vibration can cause bearing damage such as false brinelling. This occurs when the machine bearings are not rotating and subjected to repeated vibration. The damage will be uniformly spaced at the same intervals as the rolling elements.

For motors that are in storage for more than 12 months, the shaft should be rotated periodically (typically every 1 to 3 months) to keep the balls and raceways coated and to prevent false brinelling. It is also helpful to lock the shaft to prevent any movement.

If you suspect or know that the motor has been subject to vibration while in storage, inspect the bearings visually for damage before installing the motor. If the bearings are damaged, they should be replaced.
 
Grease-lubricated bearing cavities in stored motors should be filled completely. This is especially important for machines that are in storage for very long periods of time. The grease should be inspected periodically to ensure that there is no contamination or moisture present. Purge a small amount of grease through the drain and analyze. If it is contaminated, the grease must be purged and replaced. Purge the excess grease by running the motor without a load for at least 10 minutes with the drain plug removed. (Then replace the drain plug before applying load.)

Bearings that are oil-lubricated are shipped without oil and should be filled to the maximum capacity as soon as they are received. The oil should contain a rust and corrosion inhibitor if the motor is put into storage for an extended period of time. As with grease, the oil should be inspected periodically for moisture and contamination. It should be replaced if contaminated, or otherwise every 12 months the motor is idle.

As long as there is oil in the reservoir, the motor should not be moved. If it is necessary to move the motor to a different location without putting it into service, the oil should first be drained and then replaced as soon as it is in place immediately after moving the motor. Before putting the motor into service, the oil should be drained and replaced as well.

Special care for windings
Motor windings must be kept clean, dry and at a stable temperature to prevent insulation breakdown while the motor is in storage. If the storage area is not a dry environment, to prevent moisture accumulation on the windings, heating must be used to prevent condensation inside the motor. The winding temperature should be 5-10º C above ambient. Space heaters should be energized if they are supplied (or added if they are not supplied) while the motor is in storage. Another option is to supply low voltage DC trickle-heat to an AC winding, or low voltage on DC fields. Insulation resistance readings of the winding(s) should be measured before the motor is put into storage. Even if it is to be idle for a very short period of time, at the very least, test it before it is stored and just before it is put into service. That way, any drop in the insulation resistance can be addressed before the motor is installed.

If the motor is in storage for an extended time, insulation resistance (IR) readings should be taken at regular intervals, and corrected to the same temperature each time. Then, if there are any sudden drops in the readings, the windings can be cleaned and/or dried.

A polarization index (PI) reading should also be done in conjunction with the insulation resistance test for form coil windings. Because the PI for random windings is often less than 2.0, even on a new winding, the dielectric absorption ratio (DA) can be used for random coils. It is calculated by taking the ratio of the IR for 60 seconds over the IR at 30 seconds. A minimum value of 1.25 is required, as opposed to a minimum value of 2.0 for the PI.

 If the windings need to be cleaned and dried, take IR and PI readings again before putting the motor back into storage. Note: If the IR is greater than 5000 megohms, then the PI test may not give meaningful results.

Putting motor into service
Once a motor is taken out of storage, there are a number of things to check before installing it. First, inspect the motor and clean off any dust or dirt. Change the grease or drain the oil be- fore shipping. Finally, check the insulation resistance and polarization index (if form coil). Following these guidelines, any motor that is in storage for any length of time is much more likely to perform as expected.