hormonicsinim-171105105222.pptxdbdhdjddj

Electric energy conservation

1. IDEAL INSTITUTE OF ENGINEERING NAME :- SHREYASI HALDAR
DEPT. :- ELECTRICAL ENGINEERING
ROLL NO. :- 27901623035
REG. NO. :- 232790120142 (OF 2023-24)
YEAR :- 3 rd ( 5 th SEM.)
SUBJECT :- ELECTRIC MACHINE –II
SUBJECT CODE :- PC-EE-501
TOPIC NAME :- HARMONICS AND CRAWLING EFFECT ON INDUCTION MACHINES

2. contents: Introduction Harmonics in induction motor Time harmonics Space harmonics Cogging Crawling Remedies

3. Introduction: With increasing the number of harmonics generating devices in power systems, the problem of their impact on the performance of system components like induction motors needs further consideration. In attempting to understand the performance of an induction machine, we consider that the air-gap flux wave is purely sinusoidal. It is from that assumption the analysis of induced emf , sinusoidal currents, the expressions for generated torque etc. proceed. In practice, there are deviations from this idealistic picture.

4. Types of harmonics in induction motors: Time harmonics Space harmonics

5. Time harmonics: The first non-ideality is the presence of harmonics in the input supply given to the three phase machine. The source may contain 3rd , 5th , 7th. . . harmonics. Vr = V1m sin(ω1t + φ1) + V3m sin(3ω1t + φ3) +V5m sin(5ω1t + φ5) + V7m sin(7ω1t + φ7) + − − − Vr3 = V3m sin(3 ω1 t + φ3) Vr5 = V5m sin(5 ω1 t + φ5) and so on…

6. Effects of time harmonics The actual effect of these harmonics on the induction machine would depend on the reactance of the machine since at high frequencies, it is the reactance component that dominates the resistance. Excitation voltage waveforms with considerable harmonic content may result when induction machines are controlled through inverters. Apart from the effects on torque, these harmonics cause considerable heating in the machine and are hence a cause for concern.

7. Space harmonics: These harmonics are due to mechanical construction of 3-phase induction motor. Space harmonics are created due to distribution of the winding, hence called ‘phase belt harmonics’. Harmonics created due to slot opening and closing are called ‘slot harmonics’. These harmonics will lead to misbehaviour of induction motor.

8. Cogging and Crawling Proper numbers of rotor slots are to be selected in relation to number of stator slots otherwise undesirable effects will be found at the starting of the motor. Cogging and Crawling are the two phenomena which are observed due to wrong combination of number of rotor and stator slots.

9. Cogging Induction motors have a series of slots in the stator and in the rotor. These slots should not be equal in number because if they are, there is a good chance that the motor will not start at all due to a characteristic known as cogging. The slots will align like a stepper motor.

10. Cogging For this reason, there are an unequal number of slots in the rotor and in the stator, but there can still be situations where the slot frequencies coincide with harmonic frequencies and this can cause torque modulations. The slots are skewed to keep an overlap on all slots to reduce this problem.

11. Crawling Another characteristic of induction motors, is crawling. There are harmonic fluxes developed in the gap due to the magnetics of the motor. These harmonics create additional torque fields. It has been observed that in induction motors especially the squirrel cage type motors, there is a tendency of the motor to run stably as speeds which are nearly one-seventh of the synchronous speed and this phenomenon is known a s crawlin g .

12. FIG: EFFECT OF SPACE HARMONICS ON PERFORMANCE OF IM

13. To avoid cogging and crawling (i) To avoid cogging and crawling: (a)Ss ≠ Sr (b) Ss - Sr ≠ ±3P (c) adding external resistance to SCIM. (ii) To avoid synchronous hooks and cusps in torque speed characteristics Ss-Sr ≠ ±P, ±2P, ±5P. To avoid undesirable heating effect due to time harmonics supply should be balanced. We can place some filters to reduce harmonic content in the supply. Remedies: