What Is the Typical Life Expectancy of an AC Motor?
Engineers often face the question: "what's the average life of an AC motor?" (Or: can you predict the mean time between failures for a three-phase induction motor?) There's no exact answer to these questions. But you can determine a reasonable answer by quantifying the variables affecting motor life. These variables include:
- Supply Voltage and Frequency. NEMA MG1-12.44.1 stipulates that motors must tolerate +-10% voltage variation and +-5% frequency variation. For optimum life, run the motor should at nameplate voltage and frequency.
- 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. Equip hard-switching solid-state inverters with output-side inductive filters with >5% impedance. This will provide a comparable waveform. - Motor Environment. Install the motor in an environment appropriate for the motor enclosure. Ambient temperature should remain between (0 and 40)°C and elevation at 3300 feet or less. Do not obstruct motor ventilation and keep mechanical shock and vibration at a minimal level.
- Bearing Maintenance. Lubricate motor bearings per IOM published guidelines, keeping water and contaminants to a minimum. Bearing loads should be controlled to provide a minimum B-10 life of 25,000 hours. Replace bearings 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.
- Motor Starting. The number of across-the-line starts will affect the life of a motor. Keep across-the-line starts to a minimum and do not exceed NEMA MG1-12.54 guidelines.
Meet the above conditions and you can estimate the insulation life of your motor with reasonable accuracy. In 1980 the US Department of Energy published report DOE / CS-0147 to define the average expected life of electric motors. The following table summarizes the report:
| HP Range | Average Motor Life (years) | Life Range (years) |
|---|---|---|
| <1 | 12 | 10-15 |
| 1-5 | 17.1 | 13-19 |
| 5.1-20 | 19.4 | 16-20 |
| 21-50 | 21.8 | 18-26 |
| 51-125 | 28.5 | 24-33 |
| >125 | 29.3 | 25-38 |
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