Moisture, not time, is the enemy
A motor degrades from condensation cycling as ambient temperature swings, not from sitting still. Store spares indoors with humidity under 60 percent. Where that's not practical, energize built-in space heaters to keep winding temperature slightly above ambient and stop condensation before it forms.
Test going in, check periodically
Take a megohm reading before storage and log it, then repeat quarterly for anything long-term. A downward trend flags moisture intrusion long before it's a real problem.
Bearings need attention too
A motor sitting in one position for months risks false brinelling, small race indentations from stationary vibration. Rotate the shaft by hand monthly to redistribute grease and move rolling elements off the same contact points.
A few other habits
● Coat exposed shafts and flanges with rust preventive
● Leave drain plugs in the storage position specified, not sealed shut
● Store horizontally on the original shipping supports unless the manufacturer specifies otherwise
● Label each unit with its last test date and reading
BOTTOM LINE A megger reading before it goes on the shelf and another before it comes off is the cheapest insurance a spare motor gets.
