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Long-Term Motor Storage – 4 Key Factors

RSAW motor storage area

It is common knowledge to those involved in the electric motor storage business that maintenance philosophies and procedures vary.  But getting it right isn’t that complicated.  Here are four key variables that EASA and our own Carick “Joe” Howard found when looking at seven different motor manufacturers and the similarities and differences from each source.   They are:

  • Environment
  • Moisture Protection
  • Bearing Maintenance
  • Insulation

Let’s take a quick look at each one.


This was a consensus top choice.  Your electric motors should be stored in a clean, dry and vibration-free area. Often this requires air that is ventilated and is a) free from dust, and b) offers protection against the infiltration of a motor by insects and vermin.  Temperature and humidity controls are also important, with recommended temperatures ranging from 40-140° F (5-60° C) and relative humidity ranges of less than 50% to less than 75%. Finally, the recommended maximum vibration level is not to exceed 0.15 IPS (3.8 mm/s) or 0.8 mils (0.02 mm).

Moisture Protection

In addition to humidity, our sources also agreed on the importance of protecting the windings from moisture.  The most common recommendation is to utilize onboard space heaters to keep the winding temperature about 10° F (5° C) above ambient. If a motor is not equipped with space heaters, single-phase AC power is an alternative means to heat the windings (e.g., trickle heating).

Bearing Maintenance

On the flip side, there were wide variations in the recommended methods for the maintenance of bearings while in storage. The recommendations for sleeve bearings are very different than those for grease lubricated rolling element bearings. The rolling element bearing recommendations range from no maintenance at all, to rotating shafts 30 rpm for 15 seconds every month.  When designing a maintenance process for end users, ask yourself these two questions:

  • Does the end user have a motor storage procedure?
  • Does the motor manufacturer have specific motor storage provisions to maintain warranty coverage on new equipment?

For sleeve bearings, the recommendations are wide ranging as well, stretching from monthly rotation with oil, to no rotation with Tectyl® 511 sprayed in the housing every six months for two years. Beyond the two-year storage mark, removal and coating of the bearings with rust inhibitor is advised.

Insulation Resistance

All of our sources recommended performing periodic insulation resistance measurements. One of the key recommendations was to correct the megaohm reading to 40° C (104 F°). The frequency of megaohm testing ranges widely from once before storage and once before installation to testing the IR and PI monthly while in storage. Across the sources, the recommended insulation resistance values varied from a stated 10 megaohms minimum to calculated values utilizing the formula (machine rated voltage + 1000 / 1000).

In Conclusion

Examination of currently available information revealed a wide disparity in motor storage recommendations. No standard yet exists for defining what a motor owner should do to protect assets placed in long-term storage. It’s probably best to take a conservative approach and begin with a review of the storage standard of the manufacturer of the machine. Once this information has been evaluated, a storage review discussion between the motor owner and the storage provider can be held to develop a specific and agreed upon storage plan.

For more information on this topic, click here to see our full article on the EASA website.

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RSAW's team of thought-leaders on all things motor repair, sales, storage, and field services contributed this post.