1. A Back-emf Based Method to Detect Magnet Failures in PMSMs 報告學生：蔡秉旂 指導教授：龔應時 教授 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 1, JANUARY 2013 Julio-César Urresty, Jordi-Roger Riba, and Luís Romeral Electronic Engineering Department, Universitat Politècnica de Catalunya Edifici Gaia, Rambla Sant Nebridi, 08222 Terrassa,Barcelona, Spain Electrical Engineering Department, Escola d’Enginyeria d’Igualada, Universitat Politécnica de Catalunya Plaça Rei 15, 08700 Igualada, Barcelona, Spain

2. Outline INTRODUCTION SPMSM ANALYZED FAULT SEVERITY INDEX ZSVC FOR HEALTHY AND PARTIALLY DEMAGNETIZED SPMSMS EXPERIMENTAL VALIDATION OF ZSVC-BASED METHOD CONCLUSION REFERENCES

3. INTRODUCTION Demagnetization faults are troublesome because they have a profound impact on the overall performance of permanent magnet synchronous motors (PMSMs). This work presents and verifies experimentally a system to detect such faults which is based on the measure of the zero sequence voltage component (ZSVC). A fault severity index which allows quantifying the harshness of such faults is also proposed and its behavior is analyzed from experimental data. Features of the proposed method include low computational burden, simplicity and high sensitivity.

4. SPMSM ANALYZED The analyzed SPMSM has symmetrical concentrated stator windings connected in series and the rotor has skewing. Additional parameters of the analyzed SPMSM are given in Table I.

5. SPMSM ANALYZED It is supposed that the faulty SPMSM has two poles 50% partially demagnetized as shown in Figs. 1 and 2.

6. SPMSM ANALYZED

7. Experiments were carried out by means of healthy and faulty SPMSMs according to the scheme presented in Fig. 4.

8. SPMSM ANALYZED The SPMSM acting as a motor was modified according to Fig 1. The neutral point of its stator windings was made accessible as well as the two terminals of a pair of poles of phase winding (coil in Fig 5).

9. SPMSM ANALYZED It can be clearly appreciated that every time the removed magnets pass in front of the studied pole of the phase winding, the back-emf voltage is reduced.

10. SPMSM ANALYZED An experimental acquisition of the back-emf voltage induced in a pair of slots is presented in Fig 7. It shows a close similarity with the simulation results.

11. SPMSM ANALYZED Fig 8 shows the total back-emf induced in a phase of the stator windings of a healthy and a faulty SPMSM.

12. SPMSM ANALYZED In order to illustrate this phenomenon an FEM simulation using the parameters in Table I has been carried out where the SPMSM was simulated with a full pole removed. These results are shown in Fig 9, which proves that the entire back-emf induced in phase has a lower amplitude.

13. SPMSM ANALYZED Fig 10 proves that as in the case of the back-emf, when the machine operates as a motor, the stator currents only present odd harmonics due to the own geometry of the machine but they do not contain fractional harmonics. Hence, under the assumptions made in this work, the analysis of the current spectrum does not allow diagnosing these types of faults.

14. FAULT SEVERITY INDEX First,the rotor of a healthy SPMSM is supposed to haven identical magnets with the same remanence Br. Second, it is assumed that the back-emfvoltage in a phase winding Ephaseis proportional to the number of magnets and also to the angular speed of the rotor. As detailed in Fig 1, the studied SPMSM has four removed magnets over a total of 24 ( Ntotal = 24 and Neffective = 20 ). Hence, the theoretical value of the factor k = 20/24 = 0.833

15. FAULT SEVERITY INDEX However, when the SPMSM acts as a motor the back-emf is not available. Therefore, another system to measure the factor k is required. Additionally, the back-emf induced voltage matrix due to the permanent magnets can be written as

16. ZSVC FOR HEALTHY AND PARTIALLY DEMAGNETIZED SPMSMS Fig 11 shows a diagram of the experimental setup proposed to measure the ZSVC.

17. According to Fig. 11, the currents through the three balanced resistors must satisfy Rma = Rmb = Rmc = Rm ZSVC FOR HEALTHY AND PARTIALLY DEMAGNETIZED SPMSMS

18. Equations of SPMSM In the case of a healthy machine, k = 1 the fault severity index proposed in this work to measure demagnetization faults may be obtained as in ZSVC FOR HEALTHY AND PARTIALLY DEMAGNETIZED SPMSMS

19. EXPERIMENTAL VALIDATION OF ZSVC- BASED METHOD Fig 12 shows the fundamental harmonic amplitude of the ZSVC (third harmonic of the supply frequency) for a healthy and partially demagnetized SPMSMs.

20. Fig 13 plots the experimental relation between the fundamental harmonic amplitude of the ZSVC and the speed when operating under rated load. EXPERIMENTAL VALIDATION OF ZSVC- BASED METHOD

21. Table II summarizes the experimental results of the fault severity index proposed in (12) when the partially demagnetized SPMSM operates at different speeds under rated load. EXPERIMENTAL VALIDATION OF ZSVC- BASED METHOD

22. CONCLUSION Demagnetization faults may severely deteriorate SPMSMs performance, thus leading to a premature ageing or an irreversible fault condition. The proposed fault severity index can be obtained during normal operation of the machine, since with the accessible neutral point, it is not necessary to stop the machine to measure the ZSVC and to perform the analysis. Features of the proposed method here include low computational burden, simplicity and high sensitivity. Additionally, simulated and experimental data prove that the value of the fault indicator provides reliable information about the severity of the fault.

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