Stator Turn Fault Tolerant Strategy for Interior PMSynchronous Motor Drives in Safety Critical Applications

1. A STATOR TURN FAULT TOLERENT STRATEGY FOR INTERIOR PMSYNCHRONOUS MOTOR DRIVES in SAFETY CRITICAL APPLICATIONS Youngkook Lee Professor Thomas G. Habetler School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta Georgia

2. Part 1. Introduction Background of the Research Problem Statement and Research Objective Survey on Previous Work Part 2. Proposed Work Modeling of an IPMSM with Stator Turn Faults Turn Fault Tolerant Strategy Part 3. Conclusions and Future Work Outline

3. Cross-Sectional View Interior PM Synchronous Motors (IPMSMs) Permanent Magnet Stator Rotor • Features • Having large power (torque) density, wide constant power-speed ratio, and high efficiency • Creating special challenges under any fault condition due to the presence of permanent magnets that cannot be turned off at will • Requiring special care in safety critical applications where any failure can result in serious accidents

4. Defined as a performance characteristic that a fault in a component or sub-system does not cause the overall systems to malfunction Quantified in terms of “reliability and availability” Fault Tolerance • Increased conventionally by “conservative design and redundancy”;however, these approaches increase the cost and complexity of the system • Recently, increased via “fault diagnosis and tolerant strategies”; however, focusing on how to detect a fault, while the research on how to increase the availability remains uncharted area

5. Referring to the insulation failures in several turns of a stator coil within one phase Stator Turn Faults • Generating excessive heat in the shorted turns due to a large circulating current • Developing rapidly into the catastrophic failures • Initiating a large portion of stator winding-related failures that attribute to about 35~37% of induction machine failures

6. Part 1. Introduction Background of the Research Problem Statement and Research Objective Survey on Previous Work Part 2. Proposed Work Modeling of an IPMSM with Stator Turn Faults Turn Fault Tolerant Strategy Part 3. Conclusions and Future Work Outline

7. Problem Statement and Objective Research • The primary objective of this research is to develop a stator turn fault tolerant strategy in IPMSM drives satisfying the following requirements: • Preventing a turn fault from developing into the destructive phase • Not resulting in the complete loss of the availability of the drive under a turn fault condition • Not requiring any change in the standard IPMSM drive configuration • The ultimate goal of this research is to develop a complete solution for high turn fault tolerance of IPMSM drives in safety critical applications including modeling, detection method, and tolerant strategy

8. Part 1. Introduction Background of the Research Problem Statement and Research Objective Survey on Previous Work Part 2. Proposed Work Modeling of an IPMSM with Stator Turn Faults Turn Fault Tolerant Strategy Part 3. Conclusions and Future Work Outline

9. Approaches in Previous Work • Since it is generally accepted that there is no way to prevent turn faults from developing destructive phase except for stopping the machine completely, a small amount of work has been done in the following three ways: • Redundancy • Development of fault tolerant machines • Post-fault operations for stopping the faulty machine without further damage

10. Redundancy Approach Back up Controller Controller 1 Controller 2 DC source 2 DC source 1 Gate signal Gate signal Inverter 1 Inverter 2 Motor 1 Motor 2 Load Position sensor 2 Position sensor 1

11. 1 Fault Tolerant Machines • Requirements for Fault Tolerant Machines • Complete electrical isolation between phases • Implicit limiting of fault currents • Magnetic isolation between phases • Physical isolation between phases • More than 3-phases • Switched Reluctance Motors (SRMs) • Coming close to achieving the requirements • Having inherently large acoustic noise and vibration, and low efficiency • Requiring a different converter topology from the standard 6-switch full bridge inverter

12. 2 Fault Tolerant Machines • Converter Topologies for Conventional 3-phase motors and SRMs (b) SRMs (a) Conventional 3-phase Motors

13. 3 Fault Tolerant Machines • Modular Fault Tolerant PM Motors • Combining the advantages of PM motors and SRMs • Being subjected to stator turn faults due to the presences of the permanent magnets • Requiring the same converter topology as SRMs

14. D5 D3 D1 VDC A B C D4 D6 D2 O ic ib ia eb ea ec n 1 Post-Fault Operations • Free-Running Mode • Not being an appropriate post-fault operation for IPMSMs • Possibly being subjected to a critical damage on the dc-link due to unregulated generating power in high speed ranges • Resulting in the loss of the control over the speed and torque

15. VDC A B C D4 D6 D2 O ic ib ia eb ea ec n 2 Post-Fault Operations • Symmetrical Short-Circuit Operation • Being a good choice for post-fault operation for IPMSMs • Resulting in the loss of the control over the speed and torque

16. Part 1. Introduction Background of the Research Problem Statement and Research Objective Survey on Previous Work Part 2. Proposed Work Modeling of an IPMSM with Stator Turn Faults Turn Fault Tolerant Strategy Part 3. Conclusions and Future Work Outline

17. Demands for Modeling An accurate model is required to develop an effective detection method or tolerant strategy A test-bench for confirming any fault detection scheme or tolerant strategy is required since even a minor deficiency can result in a serious damage to the drives 1 Modeling of an IPMSM with Stator Turn Faults • Approaches in Modeling of Electric Machines • Finite element analysis (FEA) based models • Accurate, but take long time for simulation and require detail specification of the machine • Equivalent circuit-oriented models • Simple, but less accurate and difficult to consider non-linearity in the magnetic system

18. A Circuit-Oriented Model of IPMSMs with Turn Faults Being derived in phase-variables Being integrated with a vector-controlled drive model since almost IPMSM applications utilize current-controlled inverters Being used to investigate the behaviors of a stator turn fault in an IPMSM drive 2 Modeling of an IPMSM with Stator Turn Faults • Basic assumptions for the henceforth analysis • Each phase winding consists of turns connected in series, and the 3-phase windings are Y-connected with a floating neutral • A stator turn fault occurs on the a-phase winding

19. a a ia as1 ia ia- if Rf as2 if ic ib b ic ib b c c Schematic Diagram of an IPMSM drive with a turn Fault Q3 Q5 Q1 D5 D1 D3 Q6 Q4 Q2 D2 D6 D4

20. Stator Line-Neutral Voltages Developed Torque 1 Machine Equations under Fault-Free Conditions (1) (2) where, , , is the number of poles, represents the rotor position in electrical radians.

21. Stator Self- and Mutual Inductances Flux Linkages Contributed by Permanent Magnets 2 Machine Equations under Fault-Free Conditions (a) Self inductances (b) Mutual inductances

22. Stator Line-Neutral Voltages Developed Torque 1 Machine Equations under Turn Fault Conditions (3) (4) where,

23. 2 Machine Equations under Turn Fault Conditions * Rearranging (3) and (4) yields • Stator Line-Neutral Voltages • Developed Torque (5) (6)

24. Machine Equations under Turn Fault Conditions 3 • Voltage Equation at the Healthy Turns • Voltage Equation at the Shorted Turns • Summation of the Line-Neutral Voltages (7) (8) (9)

25. Implementation of a simulation model • Block Diagram of the Simulation model Speed controller Current controller Phase voltage generator IPMSM Load Discrete PI controller S-function (anti-windup PI with feed- forward controller) Pole to phase Phase- variable Model Motion Equation

26. Key Parameters for Specifying the Model

27. Simulation under Various Rotating Speeds • Simulation Conditions and Summary of the Results

28. Simulation under Various Loads • Simulation Conditions and Summary of the Results

29. Simulation under Various Fault Fractions • Simulation Conditions and Summary of the Results

30. Characteristics of Turn Faults in Current-Controlled Inverter-Driven Applications A Stator turn fault induces a large circulating current in the shorted turns that has the following characteristics : 1 • The fundamental frequency is the same as the synchronous frequency • The amplitude is strongly related to the amplitude of the stator line-neutral voltages, while fault fraction has very little effect • The current is mainly limited by the stator resistance and leakage inductance • The current generates magnetic flux that acts against the main air-gap flux. In the case of a stator turn fault where a large number of turns are shorted, the additional flux can be large enough to demagnetize the permanent magnets

31. Characteristics of Turn Faults in Current-Controlled Inverter-Driven Applications A Stator turn fault in current-controlled inverter-driven applications induces … 2 • Reduced positive sequence impedance, and increased negative sequence and coupling impedances as the same as those in line-fed applications • Decreased positive sequence and increased negative sequence voltages since the inverter tries to control the currents so as to follow their references by reducing positive sequence voltage and compensating negative sequence voltage • A circulating current that decreases as fault fraction increases, because the amplitude of current is nearly proportional to the amplitude of the stator line-neutral voltage and negative sequence voltage is much smaller than positive sequence voltage

32. Part 1. Introduction Background of the Research Problem Statement and Research Objective Survey on Previous Work Part 2. Proposed Work Modeling of an IPMSM with Stator Turn Faults Turn Fault Tolerant Strategy Part 3. Conclusions and Future Work Outline

33. Theoretical Foundations 1 • Relation between and (Stator Voltage) • Rearranging the voltage equation (8) at the shorted turns yields • Generally, the asymmetry introduced in the stator voltages due to a stator turn fault has a small effect on the overall stator voltage; thus 1 (11) where, represents the instantaneous value of the line-neutral voltage at the faulty winding. (12) where, represents the stator voltage vector.

34. Theoretical Foundations 2 • Relation between and (Stator Voltage) • The amplitude of the circulating current in the shorted turns is • Three Options for (1) Increasing the fault impedance (2) Increasing the resistance and leakage inductance of the stator winding (3) Reducing the stator voltage vector 2 (13)

35. Theoretical Foundations 3 • Machine Equations in the qd-variables in a steady State Condition under Fault-Free Condition • Stator Voltage • Developed Torque (14) (15)

36. Maximum Torque-per-Amp Trajectory (Motoring) Maximum Torque-per-Amp Trajectory (Motoring) Constant torque hyperbola, Voltage Ellipse, in the case that Constant torque hyperbola, A Voltage Ellipse, in the case that Current circle, in the case that A Current circle, in the case that B Current circle, in the case that Voltage Ellipse, in the case that Theoretical Foundations 4 • Representation in Circle Diagram

37. Development of the Proposed Strategy • From the torque equation, the q-axis current is expressed as a function of the d-axis current under a given torque condition as, • Inserting into the stator voltage vector equation yields, • The specific combination of the d- and q-axis currents minimizing , consequently, minimizing the circulating currents can be determined by solving (16) (17) (18)

38. Extension to Induction Motor Drives • From the induction motor torque and slip equations, • By inserting (19) and (20) into voltage equation, • By solving the following equation, (19) (20) (21) (22)

39. Simulation for Comparing with MTPA Operation 1 • Simulation Conditions : • Optimal d- and q-axis Current trajectories for reducing the stator voltage (a) d-axis current (b) q-axis current

40. Simulation for Comparing with MTPA Operation 2 • Comparison of (b) In the case of the proposed strategy (a) In the case of MTPA operation

41. Simulation for Comparing with MTPA Operation 3 • Comparison of Available Operating Areas with Limiting within 3 Times the Rated Current Normalized Torque MPTA operation : red circle marked Proposed strategy : blue x marked Normalized speed

42. Machine Equations in the qd-Variables under Symmetrical Short-Circuit Operation Stator Voltages Stator Currents Developed Torque Simulation for Comparing with Symmetrical Short-Circuit Operation 1 (23) (24) (25)

43. Simulation for Comparing with Symmetrical Short-Circuit Operation 2 • Comparison of In the case of the proposed strategy Current (A) In the case of symmetrical short circuit operation Time (sec) Circulating current in the shorted turns

44. Simulation for Comparing with Symmetrical Short-Circuit Operation 3 • Comparison of the a-phase Currents In the case of the proposed strategy Current (A) In the case of symmetrical short circuit operation Time (sec) a-phase current

45. Simulation for Comparing with Symmetrical Short-Circuit Operation 4 • Comparison of the a-phase Line-Neutral Voltages In the case of the proposed strategy Voltage (V) In the case of symmetrical short circuit operation Time (sec) a-line to neutral voltage

46. Simulation for Comparing with Symmetrical Short-Circuit Operation 5 • Comparison of the Developed Torque In the case of the proposed strategy Torque (Nm) In the case of symmetrical short circuit operation Time (sec) Developed torque

47. Effects of the Machine Specifications 1 • Parameter Lists of Different Machine Designs

48. Effects of the Machine Specifications 2 • Torque-Speed Characteristic Curves Blue solid line : #1 Red dotted line : #2 Black dashed line : #3 Torque (Nm) Speed (rpm)

49. Effects of the Machine Specifications 3 • Comparison of under MTPA Operation (a) In the case of Design #1 (b) In the case of Design #2 (c) In the case of Design #3

50. Effects of the Machine Specifications 4 • Comparison of under the Proposed Strategy Operation (a) In the case of Design #1 (b) In the case of Design #2 (c) In the case of Design #3