When an electric motor is expected to be stored for an appreciable time before it is placed into service, certain steps should be taken to ensure that it will be suitable for operation when it is needed. The practical limitation we need to recognize is that much of. what we do when putting a motor into long-term storage has to be undone when the same motor is moved into operation.
Our recommendations for storing motors.
The suggested time periods are not absolute; a number of practical considerations might call for a different schedule. While factors such as heat, humidity and vibration affect motors both large and small, it may not be practical to protect smaller motors as diligently as a larger machine.
If you expect to place the motor into service within a few weeks, the only necessary precaution is to protect the motor from weather. Store it indoors, if possible. If an indoor storage location is not possible, cover the motor allowing for a breathing space at the bottom of the cover. If the motor is tightly wrapped in plastic and placed in the sun, the result is a solar still temperature extremes plus humidity result in condensation inside the motor. For motors that have space heaters, the heaters should be energized to maintain the winding temperature 5-10′ C (10-20′ ) above the ambient temperature.
For larger (above-NEMA sized) machines, and those expected to be in storage for an extended period of time (several months or years), additional precautions are recommended to protect the windings, bearings, and machined surfaces.
Suitable Storage Areas:
Motors should be stored indoors in a clean, dry area with minimal ambient vibration. Motors should be stored in accordance with their intended use.
Store horizontal motors in a horizontal position and vertical motors in a stable vertical position. Precautions should be taken to prevent rodents, snakes, birds, or other small animals from blocking ventilation or drain openings; loosely wrap the motor, covering openings.
Prepare Motor Surfaces
There Should be some sort of rust preventative coating on the exposed machined surfaces of the motor. A viscous corrosion inhibitor should remain in place for the duration of the storage period. In humid and rainy/snowy environments, it is a good idea to routinely paint as much of the interior surface as practical. If sleeve bearing surfaces are coated, it will be necessary to dismantle and clean the coating before the machine can be placed into service.
Bearing Damage Possible
Even when a motor is not rotating, it may still be subject to vibration. Proximity to rail lines, busy roads, and/or productions floors will all contribute to the ambient vibration. Even low-magnitude vibration, over time, can cause bearing damage such as false brinelling. This occurs when the machine bearings are stationary but subjected to repeated vibration. Such damage may not be visible to the naked eye until the machine has been placed into operation, when it quickly worsens. I know of one mill where all stored motors are placed on old conveyor belting, to dampen the ambient vibration from nearby machinery.
Motors places into long term storage should have the shaft rotated at regular intervals both to prevent false brinelling and to redistribute lubricant on the metal surfaces to prevent corrosion. The recommended time interval in monthly depending on the size of the machine. Larger 2 pole machings require more frequent attention than smaller (NEMA-frame) machines. In the case of heavy machines with heavy rotors and long frames, regular rotation of the shaft is critical to prevent the shaft distortion cause by rotor sag. This is a consideration for machines rates approx 1500 hp (1000 KW) and larger. Such machines may need to be rotated weekly. As an extreme example, very large generators a kept rotating slowly at all times to prevent sag. While uncommon, for critical, very large machines, the rotor can be removed and suspended vertically to prevent sagging.
TIP: Stop the keyway in a different o’clock position each month. By designating 5 hour increments with all shafts following the same rule, you can easily tell if a motor was missed. For example, if the keyway position for September is 12:00, October will be 5:00, November is 10:00, December 3:00, and so on.
This puts the rolling elements in a different orientation each time and avoids rocking the rotor back and forth between just two positions.
If you know that the motor has been subject to vibration while in storage, inspect the bearing visually for damage before installing the motor. If bearings are damaged, they should be replaced. From a practical standpoint, it makes sense to obtain baseline vibration readings when the motor is placed into service and to check a motor again within a week or two afterwards.
The bearing cavities of grease lubricated motors should be filled completely if long-term storage is expected. If moisture is found in the grease, it is likely that the bearings have sustained rust damage and should be replaced. When the motor is put into service, purge the excess grease by running the motor without a load for at least 10-20 minutes with the drain plug removed. If the motor was stored for several years, the grease has probably hardened and the drain pipe is plugged with dried grease. If so, the best course of action is to dismantle the motor, clean out the old grease, and repack with fresh grease.
Bearings that are oil lubricated are shipped without oil and should be filled to maximum capacity as soon as possible after they have been situated into storage. If at all possible, add enough oil to cover the bearings completely, without overflowing the stand tube or labyrinth seal. The oil should contain a rust and corrosion inhibitor. Water is heavier than oil, so at 3 month intervals draw a oil sample form the drain to check for moisture.
A motor should never be moved with oil in the reservoir. If oil is sloshed over the stand tube, capillary action may continue to siphon oil from the chamber. Before putting the motor into service, the oil should be drained and replaced as well -(drain it, movie it, then re-fill it).
