Austin Bonnett
EASA Education and Technology Consultant
Electrical Apparatus Service Association, Inc.
Operation of AC induction motors at voltages and frequencies other than their nominal values can significantly affect motor operating costs, performance characteristics, and life expectancy. This paper explores these changes using the NEMA Motor-Generator standard MG-1 1998 as the benchmark for acceptable performance and variations. The paper updates a 1999 IEEE Pulp & Paper Conference presentation, expanding its scope to commercial and general industrial applications.
Motor reliability, performance, and life cycle cost are crucial elements of successful motor applications from the user's perspective. Industrial motor specifications typically address bearing life, vibration, geometry, and efficiency in detail to achieve desired results. When defining the power supply, the nominal voltage is stated with the understanding that NEMA MG-1 will apply. This standard allows for variations in voltage and frequency, along with voltage unbalance. However, these variations can adversely affect motor performance and life, a fact often overlooked due to the limited distribution of MG-1.
The paper reviews NEMA MG-1 standards and their impact on motor performance and life. It highlights that alternating-current motors should operate successfully under running conditions at rated load with variations in voltage or frequency within specified limits. However, performance within these variations may not meet the standards established for operation at rated voltage and frequency, potentially reducing motor life significantly.
Small variations in voltage and frequency can cause notable changes in motor performance. For instance, a 10 percent increase or decrease in voltage can increase heating at rated horsepower load, accelerating insulation system deterioration. Voltage variations also affect power factor, locked-rotor and breakdown torque, slip, and speed. Higher frequencies improve power factor but decrease load rotor torque and increase speed and friction losses. Conversely, lower frequencies decrease speed, increase locked-rotor torque, and decrease power factor.
An industrial study conducted by Dr. P. Pillay in 1995 examined voltage variations in a petrochemical application and their impact on motor performance. The study revealed that actual operating voltages can be higher than the nominal values, affecting motor performance and life. Variations in voltage significantly impact motor starting torque, which varies as the square of the flux density.
Motor insulation life is affected by temperature changes resulting from voltage and frequency variations. For every 10°C increase in winding temperature, the expected thermal life of the winding is halved. Voltage unbalance also has a drastic impact on winding temperature rise and insulation life. NEMA MG-1 provides guidelines for derating motors operating under unbalanced voltage conditions to prevent damage.
The paper concludes that controlling voltage and frequency as close as possible to nominal values is essential for maintaining motor performance and life. Failure to prioritize this can result in significant reductions in motor performance and life.
Key Points Covered:
- NEMA MG-1 standards for voltage and frequency variations
- Impact of voltage and frequency variations on motor performance
- Effects of small variations in voltage and frequency
- Industrial study on voltage variation in petrochemical applications
- Impact of voltage variations on motor starting torque
- Motor insulation life affected by temperature changes
- Guidelines for derating motors under unbalanced voltage conditions
Key Takeaways:
- Voltage and frequency variations can significantly affect motor performance and life.
- Small variations in voltage and frequency can cause notable changes in motor performance.
- Controlling voltage and frequency close to nominal values is essential for maintaining motor performance and life.
- Industrial studies reveal actual operating voltages can be higher than nominal values, affecting motor performance and life.
- Motor insulation life is drastically affected by temperature changes resulting from voltage and frequency variations.
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