ROTATING MACHINE AS A SOURCE OF HARMONICS

Rotating machines also act as source of harmonics in power system. Causes of harmonics generation in rotating electrical machines are classified into following categories:

1. MAGNETIC NONLINEARITIES OF THE CORE MATERIAL: Nonlinear magnetization characteristics of the core material cause harmonic generation.

2. NON UNIFORM FLUX DISTRIBUTION IN AIR GAP: often it is assumed that the air-gap flux distribution is uniform and the operating principles of rotating machines are discussed based on this assumption. But in most of the rotating machines, flux distribution in air-gap is not uniform which leads to harmonics production.

3. SLOT HARMONICS: slots are inevitable in rotating machines. Alternate presence of slot and teeth changes the reluctance of the magnetic flux varies in similar type of alternating fashion. This variation acts as a reason for harmonic generation. Harmonics produced due to pitch factor and distribution factor.

4. DESIGN PARAMETERS LIKE PITCH FACTOR AND DISTRIBUTION FACTORS: harmonic generation in synchronous generator depends on the different design factors like pitch factor and distribution factors.

5. ROTOR SALIENCY: rotor saliency brings the variation of reluctance in the magnetic path and reactance in electric path which contribute to the harmonic generation.

6. CRAWLING: it is a common problem faced by induction motors. During this fault, odd harmonics like 5th and 7th orders appear. Fifth harmonics rotates in the same direction as of the fundamental but 7th order harmonics rotate in opposite to this direction. It changes the operating characteristics of the motor. During crawling, 7th harmonics is dominating over the 5th harmonics and lowers the stable operating zone to one seventh speed of the rated value.

7. COGGING: It is a problem where induction motors fail to start at all. Harmonic production from the motor during this condition is different from the normal condition.

8. ROTOR MISALIGNMENT: rotor misalignment causes variation of flux linkage in each cycle of rotation contributing to harmonic generation.

9. MASS UNBALANCE: with the aging, mass unbalance is observed specially in the rotor side. This refers to the core property and adds in harmonic generation.

10. BAR BREAKAGE: bar breakage in an induction motor, either symmetrically or asymmetrically, is reflected in harmonic generation in rotor circuit as well as in stator side.

11. FRACTAL ERROR: fractal error in core material produces harmonics.

12. UNSYMMETRICAL FAULT: unsymmetrical fault is also a reason for harmonic generation related to negative sequence components.

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