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## Reducing Conducted Transients in Automotive Windshield Wiper Motors

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**Reducing Conducted Transients in Automotive Windshield Wiper**Motors Robert Langdorf, Shuvra Das, Mohan Krishnan University of Detroit Mercy**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 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). 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 SAE 2006-01-0297**Problem-Solving Approach**• Create a working circuit model • Perform some hand calculations on the 2nd-order system • Perform PSPICE simulation • Apply DOE principles to find optimum solutions using PSPICE, Minitab & Excel • Build and test physical samples to validate results SAE 2006-01-0297**Background Information**The current design: Inductors Printed Wiring Board Capacitors Terminal Connections to Cover Assembly SAE 2006-01-0297**Background Information**• The active components during a “switch-off” function are: • Two 0.47 mF Capacitors • Two 5 mH Inductor Coils • Motor (including inherent induction properties) SAE 2006-01-0297**Background Information**• The circuit used for simulation and analysis: 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 0.5ms. (Ford spec is <1ms) • Motor armature inductance was measured at approximately 970 mH. • Motor resistance, including armature and brushes was measured at approximately 0.5W, but was assumed lower due to magnetic effects. SAE 2006-01-0297**Assumptions, cont..**• The rotational load on the motor (~10Nm) was accounted for with a 25W resistance from motor ground to source ground. • There are 2 different grounds in the system • Line resistance was assumed to be 1.25W 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. SAE 2006-01-0297**Comparison of solution to test result**• Production part test result: SAE 2006-01-0297**Comparison of solution to test result**• PSPICE Result: SAE 2006-01-0297**Comparison of solution to test result**• The previous voltage responses exhibit: • Voltage peaks of similar magnitude • Similar dampening characteristics SAE 2006-01-0297**Ground-to-ground issue**• For a production motor, the motor ground to source ground was captured: SAE 2006-01-0297**Ground-to-ground issue**• The PSPICE model produces a similar result: 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 SAE 2006-01-0297**Steady State Solution**• Current through motor at t=0 is 4.737A • vc1 = 7.588V, vc2 = 5.921V SAE 2006-01-0297**Initial conditions**• di/dt = 9.184 A/s at t = 0+ SAE 2006-01-0297**2nd Order Differential Equation**• The following equation can be derived: The following parameters can be calculated: SAE 2006-01-0297**2nd Order Differential Equation**• The response is underdamped and the natural frequency can be expressed as: The natural response can be expressed as: SAE 2006-01-0297**2nd 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 iL related to iR • The parameters A & B in the natural response equation are calculated by applying the initial conditions: SAE 2006-01-0297**2nd Order Differential Equation**• The expression for current with all of the constants applied becomes: The expression for voltage across the capacitor C1 becomes: SAE 2006-01-0297**2nd Order Differential Equation Solution**• Plot of voltage across C1 versus time: SAE 2006-01-0297**Simulation Result**• PSPICE Result: • V = -219.2 V @ t = 23.5 ms SAE 2006-01-0297**Experimental Design**• Comment on inductors, L1 & L2: • 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. SAE 2006-01-0297**Experimental Design**• Using PSPICE & Minitab, a DOE was performed, modifying only the values of the capacitors, C1 & C2. • Each capacitor was simulated at 5 levels: • 0.047mF, 0.1mF, 0.47mF, 1mF, 4.7mF 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 SAE 2006-01-0297**Main Effect Plots**SAE 2006-01-0297**Main Effect Plots**SAE 2006-01-0297**Interaction Plots**SAE 2006-01-0297**Interaction Plots**SAE 2006-01-0297**Regression Equations**• Minimum voltage peak: • Maximum voltage peak: • Note: C2 is insignificant in the min. voltage equation and the interaction C1xC2 is insignificant in both equations. SAE 2006-01-0297**Regression Solution**• This yields as an optimum solution: • C1 = 1.18mF, C2 = 2.97 mF • Vmin = 0V, Vmax = 84.7V • When tested in PSPICE, the result is: • Vmin = 145.5V, Vmax = 122.7V • ????? • This means there must be some other relationship – try using the log of the capacitance values SAE 2006-01-0297**Log Regression Equations**• Minimum voltage peak: • Maximum voltage peak: • Note: C2 is insignificant in the min. voltage equation and the C22 is insignificant in both equations. SAE 2006-01-0297**Log Regression Solution**• This yields as an optimum solution (with minimum peak-to-peak voltage): • C1 = 3.81 mF, C2 = 4.7 mF • Vmin = -85.6V, Vmax = 51.8V • When tested in PSPICE, the result is: • Vmin = -86.6V, Vmax = 65.5V • This is a much better model!!! SAE 2006-01-0297**Log Regression Solution**• Based on feedback from the supplier, it is not recommended to pursue use of 4.7mF capacitors due to the high cost of materials. 3.3mF capacitors are relatively less expensive. • Using 3.3mF as a limit, the log regression model is re-optimized to yield: • C1 = 3.3 mF, C2 = 3.3 mF • Vmin = -90.4V, Vmax = 49.0V • PSPICE yields: • Vmin = -93.8V, Vmax = 64.4V 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). SAE 2006-01-0297**Experimental Design #2**• Another designed experiment was run to simulate the effects of the various solutions: • No change • Bridge capacitor (0.47mF) • 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): • 0.47mF, 0.047mF, 4.7nF SAE 2006-01-0297**Dotplots of PSPICE Results**SAE 2006-01-0297**Dotplots of PSPICE Results**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 • 0.47mF Cap - min = -188.4, max = 175.6 • TPSMA27A - min = -30.1, max = 1.0 • D1N4148 - min = -3.7, max = 0.5 SAE 2006-01-0297**PSPICE Result for 0.47mF Bridge Capacitor**SAE 2006-01-0297**PSPICE Result for Voltage Suppressor**SAE 2006-01-0297**PSPICE Result for Diode**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) • C1 = 0.47mF, 1mF, 3.3mF • C2 = 0.47mF, 1mF, 3.3mF • Motors with smaller capacitors and 2 of the voltage reduction solutions previously mentioned (3x2 full factorial): • C1 & C2 = 0.47mF, 0.047mF, 4.7nF • C3 = 0.47mF bridge capacitor, TPSMA30A Voltage Suppressor SAE 2006-01-0297**Comments on Build Plan**• Cost is a serious consideration: • 0.047mF ~ $0.025 • 0.47mF ~ $0.046 • 1mF ~ $0.092 • 3.3mF ~ $0.13 • TPSMA30A ~ $0.16 • Diode ~ too expensive to seriously consider 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 SAE 2006-01-0297**Test Results**• Following shows how Minitab outputs analysis results: • General Linear Model: Min versus C1, C2 • Factor Type Levels Values • C1 fixed 3 0.47 1.00 3.30 • C2 fixed 3 0.47 1.00 3.30 • Analysis of Variance for Min, using Adjusted SS for Tests • Source DF Seq SS Adj SS Adj MS F P • C1 2 1980.0 1980.0 990.0 2.21 0.138 • C2 2 16958.5 16958.5 8479.3 18.96 0.000 • C1*C2 4 3101.1 3101.1 775.3 1.73 0.187 • Error 18 8050.3 8050.3 447.2 • Total 26 30090.0 • P = 0 translates to virtually 100% confidence that the factor is significant. 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. SAE 2006-01-0297**Test Results – Experiment #1**Optimum Settings at C1, C2 = 3.30 SAE 2006-01-0297