Motor failures happen for many reasons. Abuse, lack of oil, bad bearings, insulation failure (end-of normal life, oil, moisture), dirt or other foreign material (reduced ventilation), single-phasing, unbalanced voltage (more about this a little later), and jammed V-belts are a few of the conditions that can cause a motor to draw more than normal current. Too much current results in the generation of too much heat within the windings of the motor. For every 10°C above the maximum temperature rating of the motor , the expected life of the motor is reduced by 50%. This is sometimes referred to as the “half-life rule.”
The Electrical Apparatus Service Association has a tremendous amount of technical literature available regarding electric motors and common failures.
Motor circuits can be simple or complex depending on factors such as the type, size, and characteristics of the motor or motors, how each motor is to be operated, and the electrical requirements. Given this information, the electrician usually begins by locating the power source, then planning the circuit between the source and the motor. The first component in this circuit is usually the branch-circuit short-circuit and ground-fault protective device.