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Welcome to Just Answer!.99% of the time the motor either runs at the proper speed stamped on the motor or it stops entirely... I have never seen one simply run too slow and not burn out within half an hour or so. Motor speed however is stamped on the motor, usually 850 RPM on fans over 24 inches in diameter, and 1050 RPM for the smaller fans.
Most of the time the cause of the problems you are having are due to several factors acting in combination:1. Hot weather over 100F... especially weather over 105F. That begins to exceed the temperatures the units were designed for.2. Unit is over 3 years old, and the condenser coil looks clean, but there is actually dirty deep between the aluminum fins... that makes the unit run hot and go off on its internal overloads.3. During heat waves, electrical use on the areas entire power grid is higher than the grid was designed for, so the voltage drops.... electric motors run a lot hotter during low voltage conditions. (called a brown out) and can even cause a motor to burn out.The combination of all three or even one of these will cause the problem you are having.
______________I will keep this question open with no time limit as long as you rate my answer so far *positively..Thanks!We can go from there with coil cleaning instructions if you need them and a range of tests.Phil2.