Engineers are occasionally asked to predict average motor life or mean time between failures for a given three-phase induction motor. A reasonable answer can be obtained if the numerous variables that affect motor life are quantified, such as:

1. Supply voltage and frequency: Although NEMA MG1-12.44.1 stipulates that motors must tolerate +-10% voltage variation and +-5% frequency variation; the motor should be operated at nameplate voltage and frequency for optimum life.
2. Supply source: Supply voltage waveforms should be sinusoidal, balanced within 1%, per NEMA MG1-12.45, with rise times greater than 2 milliseconds and contain less than 2% total harmonic voltage distortion (typical of ‘clean’ utility power). This reduces turn-to-turn, coil-to-coil, and phase-to-ground voltage stress and prolongs motor insulation life. Hard-switching solid-state inverters must be equipped with output-side inductive filters with >5% impedance to provide a comparable waveform.
3. Motor environment: The motor must be installed in an environment that is appropriate for the motor enclosure. Ambient temperature should remain between (0 and 40)°C and elevation at 3300 ft or less. Motor ventilation must not be obstructed and mechanical shock and vibration must be held to a minimal level.
4. Bearing maintenance: Motor bearings must be properly lubricated per IOM published guidelines, with water and contaminants kept to a minimum. Bearing loads should be controlled to provide a minimum B-10 life of 25,000 hours and bearings must be replaced periodically to prevent permanent motor damage. Shaft voltages (caused by unbalanced line voltage or operation from inverter power) must be limited to 0.5 volts peak.
5. Motor Starting: Across-the-line starts should be kept to a minimum and not exceed NEMA MG1-12.54 guidelines, since the life of a motor is affected by the number of across-the-line starts. When the above conditions are met, the insulation life of the motor can be estimated with reasonable accuracy. In 1980 the US Department of Energy published report DOE / CS-0147 to define the average expected life of electric motors, which is summarized in the table below.
HP Range Ave Motor Life (years) Life Range (years)