Derating of induction motors due to waveform distortion

The detrimental effects of nonsinusoidal voltage on induction motor performance are described. The derating of induction motors due to harmonic distortion is discussed in detail. Derating of NEMA Design B induction motors of different output ratings and for two types of enclosures (drip-proof and totally enclosed) due to different cases of harmonic distortions are discussed. IEEE Standard 519 suggests that no derating of the motor would be necessary for a harmonic content of up to 5%. However, no limit is specified in regard to the individual harmonic content. The conclusion is that a restriction for the second harmonic should be included on the harmonic distortion limits established by the IEEE Guide for Harmonic Control and React Compensation of Static Pow Converters (ANSI/IEEE Standard 519, Apr. 1981), and derating in some cases should be considered for less than 5% harmonic distortion. Drip-proof machines are found to be less affected by harmonic distortion than totally enclosed machines. Efficiency plays an important role in the degree of derating. Less efficient machines would require a higher derating. It is also clear that smaller machines (less than 5 HP) are affected more by the harmonics than are larger machines