A fan-motor failure on an air-conditioning or refrigeration system usually is a catastrophic event because it often goes unnoticed for a while and puts great stress on the system’s compressor, thus shortening its life.

An evaporator (indoor) fan-motor failure almost always results in the indoor coil freezing over and the suction line freezing all the way back to the compressor. And since the refrigerant isn’t being evaporated in the coil, liquid refrigerant is being drained back into the compressor. Then the natural law comes into play: “You can’t compress a liquid.” So on some compressor designs, this destroys the internal valves. But on all compressor designs, the liquid refrigerant dilutes the lubricant, which often results in a locked-rotor condition.

However, a condenser (outdoor) fan-motor failure can be even worse. Since the refrigerant isn’t being condensed, it causes the pressure on the discharge side to get very high and the compressor (since it isn’t being cooled) gets extremely hot, which breaks down the internal lubricant and contaminates the system.

Fortunately, indoor fan motors don’t fail too often because they are out of the weather and the design of the fan blades keeps the electrical current through the motor from rising when there is a blockage, such as a very dirty air filter. The main causes of failure are loss of lubricant, a failed run capacitor, a failed bearing in a belt drive, a little water dripping into the windings or a voltage problem. So technicians should carefully check to find the cause of a failure before changing the motor.

However, outdoor fan motors have more stressful conditions to deal with because they operate in all kinds of weather and in much hotter conditions, and the design of their blades causes them to draw more power when there is any blockage, as with a dirty condenser coil.

Because of the heat, probably the most common type of failure is caused by the lack of lubrication of the bearings. And though lubrication ports used to be the norm in their design, more manufacturers are deleting them to cut costs. Yet, where there are lubrication ports on any HVACR motors, oiling them must be a regular part of any preventive-maintenance program.

Of course, failed (rusted, leaking and bulged) run capacitors and voltage problems also can cause failures. But probably the second most common failure is when water gets into the motor windings, which can happen during a violent storm or when the technician is washing the outdoor coil.
You will notice most outdoor fan motors have closed covers on the side that faces upward, but open covers to allow cooling on the side that faces downward. The technician never should mount the motor where the open portion can receive rain (but I’ve seen it done).

Also, most HVACR equipment manufacturers will mount a shield on the motor shaft to keep water from reaching the bearings. Though these may be very hard to remove from the old motor, they absolutely must be put back on or replaced, or the new motor also will quickly fail.

As with indoor fan motors, the technician always should try to find out why the fan motor failed before replacing it because voltage or moisture problems can cause a repeat failure and I’ve seen some very clean-looking condenser coils totally clogged between the fins where the dirt couldn’t be seen.