1. Reducing Conducted Transients in Automotive Windshield Wiper Motors Robert Langdorf, Shuvra Das, Mohan Krishnan University of Detroit Mercy

2. 2 SAE 2006-01-0297 Project Objectives Study the causes of conducted transients and develop a low-cost design solution to reduce them Apply knowledge and skills obtained during other university coursework Gain additional understanding of automotive motors and their electrical/mechanical interrelationships

3. 3 SAE 2006-01-0297 Problem Description When an electric motor is switched off, a large amount of energy (measured as a negative voltage) can be emitted to the main power net and can often be damaging to other devices. For current design motor, transient emissions of >200V are possible. Customers desire no more than 100V (even less for some customers).

4. 4 SAE 2006-01-0297 Design Considerations Cost (there is already a very costly solution using varistors) Packaging/Space Constraints  Use of standard components Effects on Other Electrical Requirements  Other Conducted Emissions (radio interference)  Conducted Immunity  Radiated Emissions Effect on Motor Performance

5. 5 SAE 2006-01-0297 Problem-Solving Approach 1. Create a working circuit model 2. Perform some hand calculations on the 2 nd - order system 3. Perform PSPICE simulation 4. Apply DOE principles to find optimum solutions using PSPICE, Minitab & Excel 5. Build and test physical samples to validate results

6. 6 SAE 2006-01-0297 Background Information The current design: Capacitors Inductors Terminal Connections to Cover Assembly Printed Wiring Board

7. 7 SAE 2006-01-0297 The active components during a “switch-off” function are: –Two 0.47  F Capacitors –Two 5  H Inductor Coils –Motor (including inherent induction properties) Background Information

8. 8 SAE 2006-01-0297 Background Information The circuit used for simulation and analysis:

9. 9 SAE 2006-01-0297 Assumptions The high speed part of the circuit was neglected - there is no current flowing through it. Relay was assumed to have a switching time of 0.5  s. (Ford spec is <1  s) Motor armature inductance was measured at approximately 970  H. Motor resistance, including armature and brushes was measured at approximately 0.5 , but was assumed lower due to magnetic effects.

10. 10 SAE 2006-01-0297 Assumptions, cont.. The rotational load on the motor (~10Nm) was accounted for with a 25  resistance from motor ground to source ground.  There are 2 different grounds in the system Line resistance was assumed to be 1.25  between each side of the power source and the motor brush card terminals.  Note: these two assumptions were derived empirically by changing values until a solution was found that approximates the result of a typical experiment.

11. 11 SAE 2006-01-0297 Comparison of solution to test result Production part test result:

12. 12 SAE 2006-01-0297 Comparison of solution to test result PSPICE Result:

13. 13 SAE 2006-01-0297 Comparison of solution to test result The previous voltage responses exhibit:  Voltage peaks of similar magnitude  Similar dampening characteristics

14. 14 SAE 2006-01-0297 Ground-to-ground issue For a production motor, the motor ground to source ground was captured:

15. 15 SAE 2006-01-0297 Ground-to-ground issue The PSPICE model produces a similar result:

16. 16 SAE 2006-01-0297 Hand Calculations Hand calculations were done using the same model as used in PSPICE. The following calculation is done to find the approximate magnitude of the negative transient spike Finding the decay takes considerably more calculation

17. 17 SAE 2006-01-0297 Steady State Solution Current through motor at t=0 is 4.737A v c1 = 7.588V, v c2 = 5.921V

18. 18 SAE 2006-01-0297 Initial conditions di/dt = 9.184 A/s at t = 0 +

19. 19 SAE 2006-01-0297 2 nd Order Differential Equation The following equation can be derived: The following parameters can be calculated:

20. 20 SAE 2006-01-0297 2 nd Order Differential Equation The response is underdamped and the natural frequency can be expressed as: The natural response can be expressed as:

21. 21 SAE 2006-01-0297 2 nd Order Differential Equation The forced response, which will be neglected for now, is expressed as:  This is neglected because I do not have an expression for i L related to i R The parameters A & B in the natural response equation are calculated by applying the initial conditions:

22. 22 SAE 2006-01-0297 2 nd Order Differential Equation The expression for current with all of the constants applied becomes: The expression for voltage across the capacitor C1 becomes:

23. 23 SAE 2006-01-0297 2 nd Order Differential Equation Solution Plot of voltage across C 1 versus time:

24. 24 SAE 2006-01-0297 Simulation Result PSPICE Result: V = -219.2 V @ t = 23.5  s

25. 25 SAE 2006-01-0297 Experimental Design Comment on inductors, L 1 & L 2 :  Changing the values of the external inductors has very minimal effect on the transient solution. Inductors in series simply add and these 5mH coils are negligible compared to the 970mH motor inductance.  These coils only will significantly effect the RFI filtering.  For the purpose of these experiments, the coils will be left unchanged.

26. 26 SAE 2006-01-0297 Experimental Design Using PSPICE & Minitab, a DOE was performed, modifying only the values of the capacitors, C 1 & C 2. Each capacitor was simulated at 5 levels:  0.047  F, 0.1  F, 0.47  F, 1  F, 4.7  F

27. 27 SAE 2006-01-0297 Experimental Design Using Minitab’s response surface feature, regression equations were formulated to help solve for the expected minimum and maximum peak voltages

28. 28 SAE 2006-01-0297 Main Effect Plots

29. 29 SAE 2006-01-0297 Main Effect Plots

30. 30 SAE 2006-01-0297 Interaction Plots

31. 31 SAE 2006-01-0297 Interaction Plots

32. 32 SAE 2006-01-0297 Regression Equations Minimum voltage peak: Maximum voltage peak:  Note: C 2 is insignificant in the min. voltage equation and the interaction C 1 xC 2 is insignificant in both equations.

33. 33 SAE 2006-01-0297 Regression Solution This yields as an optimum solution:  C 1 = 1.18  F, C 2 = 2.97  F  V min = 0V, V max = 84.7V When tested in PSPICE, the result is:  V min = 145.5V, V max = 122.7V ????? This means there must be some other relationship – try using the log of the capacitance values

34. 34 SAE 2006-01-0297 Log Regression Equations Minimum voltage peak: Maximum voltage peak:  Note: C 2 is insignificant in the min. voltage equation and the C 2 2 is insignificant in both equations.

35. 35 SAE 2006-01-0297 Log Regression Solution This yields as an optimum solution (with minimum peak-to-peak voltage):  C 1 = 3.81  F, C 2 = 4.7  F  V min = -85.6V, V max = 51.8V When tested in PSPICE, the result is:  V min = -86.6V, V max = 65.5V This is a much better model!!!

36. 36 SAE 2006-01-0297 Log Regression Solution Based on feedback from the supplier, it is not recommended to pursue use of 4.7  F capacitors due to the high cost of materials. 3.3  F capacitors are relatively less expensive. Using 3.3  F as a limit, the log regression model is re- optimized to yield:  C 1 = 3.3  F, C 2 = 3.3  F  V min = -90.4V, V max = 49.0V PSPICE yields:  V min = -93.8V, V max = 64.4V

37. 37 SAE 2006-01-0297 Other Possible Solutions Several other possible solutions exist to fix the transient spike problem:  Bridge capacitor (Y-type)  Voltage suppressor  Diode These devices are placed in the circuit in this location: Since these are much more capable of fixing the problem than only capacitors, the capacitance used in conjunction with these items can be reduced (thereby reducing cost).

38. 38 SAE 2006-01-0297 Experimental Design #2 Another designed experiment was run to simulate the effects of the various solutions:  No change  Bridge capacitor (0.47  F)  Voltage Suppressor (Vishay TPSMA27A)  Diode (D1N4184 from PSPICE library) Each option was run at 3 levels of matched C1 & C2 (matched may be better to suppress RFI):  0.47  F, 0.047  F, 4.7nF

39. 39 SAE 2006-01-0297 Dotplots of PSPICE Results

40. 40 SAE 2006-01-0297 Dotplots of PSPICE Results

41. 41 SAE 2006-01-0297 Analysis of Dotplots It is evident from these plots that one of the recommended solutions may have a major impact. Data means for each solution:  None - min = -974.2, max = 829.8  0.47  F Cap - min = -188.4, max = 175.6  TPSMA27A - min = -30.1, max = 1.0  D1N4148 - min = -3.7, max = 0.5

42. 42 SAE 2006-01-0297 PSPICE Result for 0.47mF Bridge Capacitor

43. 43 SAE 2006-01-0297 PSPICE Result for Voltage Suppressor

44. 44 SAE 2006-01-0297 PSPICE Result for Diode

45. 45 SAE 2006-01-0297 Motor Build and Test Plan 2 sets of parts have been built and tested:  Motors with the current capacitors (3x3 full factorial DOE)  C 1 = 0.47  F, 1  F, 3.3  F  C 2 = 0.47  F, 1  F, 3.3  F  Motors with smaller capacitors and 2 of the voltage reduction solutions previously mentioned (3x2 full factorial):  C 1 & C 2 = 0.47  F, 0.047  F, 4.7nF  C 3 = 0.47  F bridge capacitor, TPSMA30A Voltage Suppressor

46. 46 SAE 2006-01-0297 Comments on Build Plan Cost is a serious consideration:  0.047  F ~ $0.025  0.47  F ~ $0.046  1  F ~ $0.092  3.3  F ~ $0.13  TPSMA30A ~ $0.16  Diode ~ too expensive to seriously consider

47. 47 SAE 2006-01-0297 Motor Test Plan All motors were subjected to CE 410 (conducted emissions) DOE principles are applied to analyze testing results They will also be subjected to CE 420 (RFI emissions). However, timing did not allow such testing to be completed during the scope of this project

48. 48 SAE 2006-01-0297 Test Results Following shows how Minitab outputs analysis results: General Linear Model: Min versus C1, C2 FactorTypeLevelsValues C1fixed30.47 1.00 3.30 C2fixed30.47 1.00 3.30 Analysis of Variance for Min, using Adjusted SS for Tests SourceDFSeq SSAdj SSAdj MSFP C12 1980.01980.0990.0 2.21 0.138 C2 2 16958.516958.5 8479.3 18.96 0.000 C1*C2 4 3101.1 3101.1 775.3 1.73 0.187 Error18 8050.3 8050.3 447.2 Total 26 30090.0 P = 0 translates to virtually 100% confidence that the factor is significant.

49. 49 SAE 2006-01-0297 Test Results – Experiment #1 Assumptions of normality, independence of the testing order, constant variance and independence from other variables are deemed adequate based on analysis of residuals. For the minimum peak voltage:  The value of C1 is ~86% significant  The value of C2 is 100% significant  The interaction is ~81% significant The effect plots (next slide) show that the optimum condition is when both capacitors are 3.3  F, similar to the simulation results.

50. 50 SAE 2006-01-0297 Test Results – Experiment #1 Optimum Settings at C1, C2 = 3.30

51. 51 SAE 2006-01-0297 Test Results – Experiment #1 For the maximum peak voltage:  The value of C1 is 100% significant  The value of C2 is ~80% significant  The interaction is ~99.4% significant The effect plots (next slide) show that the optimum condition is when both capacitors are 3.3  F, similar to the simulation results.

52. 52 SAE 2006-01-0297 Test Results – Experiment #1 Optimum Settings at C1, C2 = 3.30

53. 53 SAE 2006-01-0297 Test Results – Experiment #1 Using the optimum settings from this experiment (C1 & C2 are 3.3  F):  The negative peak is about -110V  The maximum peak is about +50V Compared to the current capacitor design:  Negative peak ≈ -200V  Positive peak ≈ +85V  This is a significant improvement

54. 54 SAE 2006-01-0297 Test Results – Experiment #2 Assumptions of normality, independence of the testing order, constant variance and independence from other variables are deemed adequate based on analysis of residuals. For the minimum peak voltage:  The effect of the matched capacitors is not statistically significant.  The effect of the suppression device is 100%  The interaction effect is ~93% significant. Based on the effect plots (next slide), the ideal solution is the combination of 4.7 nF capacitors and the voltage suppressor.

55. 55 SAE 2006-01-0297 Test Results – Experiment #2 Optimum Settings at C3 = TPSMA30A & C1C2 = 0.0047

56. 56 SAE 2006-01-0297 Test Results – Experiment #2 For the maximum peak voltage:  The effect of the matched capacitors is ~92% significant  The effect of the voltage suppression devices is 100% significant  The interaction is ~97% significant The effect plots (next slide), show that the optimum solution is the combination of 4.7 nF capacitors and the voltage suppressor, similar to the simulation results.

57. 57 SAE 2006-01-0297 Test Results – Experiment #2 Optimum Settings at C1C2 = 0.0047 & C3 = TPSMA30A

58. 58 SAE 2006-01-0297 Test Results – Experiment #2 Based on analysis, the overall optimal solution includes:  Matched Smaller Capacitors ~4.7nF  Vishay Voltage Suppressor (TPSMA30A or similar) Using these optimum settings:  The negative peak is ~ -90V  The maximum peak is ~ +15V. Compared to the current capacitor design:  Negative Peak ≈ -200V  Positive peak ≈ +85V  This is a significant improvement, even better than the optimized design as determined in Experiment #1.

59. 59 SAE 2006-01-0297 Conclusion Cost comparison:  Optimal solution  3 x $0.025 + $0.16  $0.235 per motor.  Current cost  3 x $0.046  $0.138 per motor  Varistor solution   $0.38 per motor

60. 60 SAE 2006-01-0297 Conclusion Questions