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Good, readily available records are essential for any motor storage program. One method is to attach a form like that in Figure 1 to each motor to document the storage dates, maintenance procedures completed, and the results of all tests performed during the storage period.

For motors in long-term storage, a good practice is to replace the form annually (or at other designated intervals). Store electronic copies of the previous forms for future reference, or simply keep them in an envelope attached to the motor.

Storage conditions

Short-term storage. Motors that will be in storage for just a few weeks primarily require protection from the weather (see “Indoor storage” and “Outdoor storage” below) and ambient vibration (more on this later).

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Figure 1: Motor storage tag.

Long-term storage. Motors slated for several weeks to several years in storage and all above-NEMA size machines require additional preparations to protect their machined surfaces, bearings and windings.

Indoor storage. If possible, store motors indoors in a clean, dry area. Place horizontal machines in a horizontal position and vertical motors in a stable vertical position.

Unless the storage area is climate controlled, prevent condensation from forming inside the motor by energizing the space heaters (if supplied) to keep the windings 5-10°C (10-20°F) above the ambient temperature. (For other ways to prevent condensation, see “Special care for windings” below.)

Outdoor storage. Don’t! Seriously, if a motor is too large to store indoors, it is likely to be a very expensive machine. It’s worth the cost to construct an enclosed storage facility. When outdoor storage is absolutely necessary, protect the motor with a waterproof cover (e.g., a tarp), allowing a breathing space at the bottom. Wrapping it tightly in plastic and placing it outdoors will cause condensation to form inside the motor due to the temperature extremes and humidity.

Outdoor storage also requires preventive measures to keep out rodents, snakes, birds or other small animals that can damage the winding insulation. If insects are prevalent, keep them from blocking ventilation and drain openings by loosely wrapping the motor and covering all openings.

Shafts and machined surfaces

Apply a viscous rust/corrosion inhibitor (e.g., LPS2, Techtyl 502C or RustVeto) to exposed machined surfaces and sleeve bearings, allowing it to remain intact throughout the storage period. In humid and rainy/snowy environments, have the service center paint as much of the motor’s interior surface as practical, and coat the windings with a topical fungicide in tropical environments. (Note: Disassemble the machine and inspect the sleeve bearings before placing it into service.)

Bearing protection

Grease-lubricated motors. For long-term storage, completely fill the bearing cavities with compatible grease to prevent rust and corrosion staining that can occur if moisture collects between the balls and races.

Oil-lubricated motors. Do not ship or move these motors with oil in the reservoir. After placing the motor in storage, fill the reservoir with enough oil to cover the bearings but without overflowing the stand tube or labyrinth seal. Fill sleeve bearing machines to just below the labyrinth seal and vertical motors to the “max fill” line.

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Figure 2: False brinelling.

The oil should contain a rust and corrosion inhibitor and be moisture free. Check it every three months by drawing a sample from the drain. Since water weighs more than oil, any moisture will be evident.

Ambient vibration. This can damage motors, even when they are not rotating. Proximity to rail lines, busy roads, and/or production floors can all contribute to the ambient vibration. Even low-magnitude vibration, over time, can damage bearings while they are stationary–e.g., false brinelling (see Figure 2). Solutions vary. For example, one mill that had ambient vibration from nearby machinery stored its motors on scrapped conveyor belting.

False brinelling damages the bearing race at uniform intervals matching the spacing of the rolling elements. Although the damage initially may appear slight or even invisible to the naked eye, it often progresses rapidly once the motor is in service.

Shaft rotation. Turning the motor’s shaft at least monthly during long-term storage redistributes lubricant on machined surfaces to inhibit corrosion. Motors with ball or roller bearings also benefit from monthly rotation, since the rolling elements stop in different positions each time. Larger, 2-pole machines require more frequent attention than smaller (NEMA-frame) machines.

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Figure 3: Insulation resistance (IR) tests can be performed with a megohmmeter or motor analyzer.

Motors with spring-loaded spherical bearings are more difficult to turn, but they still require manual rotation to coat the bearings with oil. With sliding plate (i.e., Kingsbury) bearings, lift the shaft before rotating it–from below with a short jack and a bearing ball centered on the shaft, or from above with an overhead crane and eyebolts. To avoid bearing damage, be careful not to lift the shaft more than a few mils.

Machines with heavy rotors and long frames in ratings of about 2000 hp (1500 kW) and larger sometimes require more frequent (weekly) rotation to prevent shaft bowing caused by the weight of the rotor. As an extreme example, power plants often keep large turbine generators rotating slowly all the time to prevent sag. While it is uncommon, removing and vertically suspending the rotors of very large critical machines also can prevent sagging.

Special care for motor windings

Methods for preventing condensation. Motor windings must stay clean and dry to keep the insulation from degrading. Unless the storage area is climate controlled, condensation can form in the motor if the temperature of the winding dips below the dew point. As mentioned earlier, the usual way of avoiding this is to keep the winding 5-10°C (10-20°F) above ambient temperature.
If the motor has space heaters, energize them while it is in storage; if not, add them. Another option is to use the windings as a resistance heater by supplying low-voltage DC current (approximately 8-12% of rated amperage). An energy-saving alternative is to lower the dewpoint of the storage room with a dehumidifier.

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Insulation resistance (IR) tests. Measure and record the IR of the winding(s) before storing a motor even a few weeks, and again just before putting it in service (see Figure 3). Correct all IR readings to a standard temperature and address any decrease in IR before installing the motor. If a motor will be in storage for a long time, take IR readings annually.

Polarization index (PI) and dielectric absorption ratio (DAR) tests. For form coil windings, conduct a PI test in addition to the IR test. The PI test variables skew results for windings with lots of exposed conductor surface area, so use the DA ratio test for random windings and DC armatures (see Tables 1 and 2).

If the windings need to be cleaned and dried, measure the IR again. If it is greater than 5000 megohms, disregard the PI (see IEEE 43); otherwise, recalculate the PI.

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Carbon brushes

DC machines, wound-rotor motors and some synchronous machines have carbon brushes. For long-term storage, lift the brushes away from the commutator/slip rings to prevent a chemical reaction (sometimes called “photographing”) that can discolor the underlying commutator or slip ring. When practical, store the springs in the relaxed state to prevent a gradual loss of spring pressure.

Putting the motor into service

To ensure proper operation when removing a motor from storage and putting it into service, perform the following:

• Use compressed air to clean the outside of the motor, and visually inspect it.
• Assess the condition of the insulation system by measuring the IR with a megohmmeter.
• Oil-lubricated motors:
• Drain the oil before moving the motor to the installation site.
• If there is water in the oil, check for and replace any rusty bearings.
• If sleeve bearings received a protective coating, disassemble the machine and clean the bearings with an appropriate solvent before putting the motor into service.
• Fill the oil reservoir to the correct running level after installing the motor.
• Grease-lubricated motors:
• Moisture in the grease usually indicates rust-damaged bearings that need replacement.
• After several years in storage, the grease probably will be hard and the drainpipe will be plugged; usually it is best to disassemble the motor, remove the old grease and repack with fresh, compatible lubricant.
• Run the motor 10-20 minutes without the drain plug to purge excess grease.
• Vibration and alignment:
• If the storage area has ambient vibration, inspect and replace damaged bearings before installing the motor.
• After installing and aligning the motor, document the uncoupled baseline vibration levels; check the levels again after a week or two of service.
• For motors with rolling element bearings, check for bearing fault frequencies in the vibration spectra.
• On large machines that are susceptible to shaft sag, monitor the vibration levels during startup to avoid catastrophic damage.

High-cost machines obviously justify more precautions than inexpensive, readily available motors. What is not always apparent is that some “smaller” motors are equally important to production and can have enormous consequences if they fail.

References
IEEE Std. 43-2013: Recommended Practice for Testing Insulation Resistance of Electric Machinery. Institute of Electrical and Electronics Engineers, Inc. New York, NY, 2013.

Text: Chuck Yung, senior technical support specialist at EASA, Inc., St. Louis, MO USA     Images: EASA, stockphoto