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Only a perfectly unloaded AC motor runs at the base speed of 1800 rpm. A loaded motor experiences "slip", which increases as the load increases. Slip represents the inability of the rotor to fully keep up with the moving AC voltage waves generated on the stator. This results in typically finding motors rated at "1750 rpm" or "1725 rpm". For almost all practical purposes, these two example motors could be used interchangeably.
It should be noted that the most common effect of slip on motor applications is that the motor speed changes subtly as the loading changes. If an exact speed is needed, a closed loop drive or controller will need to be used.