As hybrid and electric vehicle systems continue to age after more than 12 years in the automotive market, determining electric transaxle or transmission state-of-health (SOH) and/or confirming that a catastrophic event has occurred is becoming a more important aspect of the aftermarket service industry and those that service electric transmissions. As the aftermarket continues to become more of an option to hybrid owners for service, it also becomes more important than ever to ensure that determining SOH or confirming an electric motor-generator unit (MGU) has reached its end-of-life (EOL) becomes a repeatable and reliable process.
For example; as engine compression, cylinder leak-down and rpm balance can change (deteriorate) with time or mileage (aging) resulting in the EOL phase of the product, an MGU can also experience aging effects that will ultimately result in EOL. However, just as the calendar time or mileage (aging) that an engine fails can vary dramatically and can be dependent upon many factors (drive cycle, geographic location, maintenance history, etc.). MGU aging also can be affected by numerous factors that statistically can help determine the general EOL expectations. During my 26 years of experience in developing, testing, and servicing electric drive and battery pack systems, rarely is there an opportunity to test MGUs that have a wide range of mileage and chronological ages in one location that can be tested in a single day.
The TVS value, as a comparison to a reference value, can measure the SOH of the stator and rotor.
So, when I was presented with the opportunity to acquire this type of test data, I immediately agreed to perform the testing. The tests were to be performed on a varied population of Toyota Prius transaxles and compile statistical data on the aged MGUs for the owner of the facility and for our company database.
Case Study Testing Background Information
The following information serves as background information on the population of Toyota Prius transaxle MGUs that were tested as part of this case study:
· Test date: Nov. 15, 2012
· Test and Data Acquisition Engineer: Mark Quarto
· Test Location: Midwest recycling/salvage business that specializes in hybrid electric vehicle components
· Toyota Prius Transaxle Type: Generation II
· All transaxles tested had been removed from the vehicle and stored in a warehouse on pallets. (Note: The MGUs could have been tested with the transaxle installed in vehicle or out of vehicle)
· Number of Transaxles in test population: 20
· Mileage ranges of test population: 28,000 to 148,000
· Model Year ranges of test population: 2004 to 2009
· MGUs tested: MG1 and MG2
· Number of tests on each MGU to acquire data: One (1)
· Testing temperature: 5.5°C (42°F)
· Humidity: 58 percent
Case Study Test Instrumentation and Data Acquisition:
· MS Excel - for entering/compiling test data and calculating statistical values
· All Test Pro 33EV (AT33EV) – Motor Circuit Analysis tool to acquire motor test data
· IEEE 56, 118 and 120 cover Motor Circuit Analysis testing methods, including how data is collected by instrumentation.
The DV percent figure is a measurement of the insulating losses in the motor-generator.
· Rationale for instrument selection: AT33EV scored the highest of five (5) MGU testing methodologies in a General Motors (GM) internal study of MGU test instrument prognostic and testing capabilities. It also scored the highest in a study performed by an external GM electric motor testing supplier when the study was replicated to ensure repeatability of test results and instrument performance. The study results were summarized in two articles and can be downloaded and reviewed at http://www.autoresearchanddesign.com/techinfo.php.
· Instrument testing parameters acquired by instrument to determine MGU SOH: Direct Current (dc) Resistance (milliohms), Inductance, Impedance, Capacitance, Phase Angle, Current-Frequency Ratio, Dissipation Factor (contamination), and Insulation Resistance.
· Test results provided by instrument:
o Phase winding dc resistance expressed in Ohms of Resistance – this data will be utilized to test for internal motor connections (i.e., corrosion, loose crimp connections, etc.). The dc resistance can also be used to indicate very severe internal coil (copper-to-copper) shorting or more severe phase-to-phase shorting (intra or inter phase winding failures). The dc resistance test is ineffective in identifying less invasive copper to copper shorting and will not assist in determining winding/stator slot aging.
o Insulation Resistance (IR) Testing expressed in Ohms of Resistance - the IR test is observed and compared to the elapsed time to achieve its highest resistance level to determine the insulation to ground resistance barrier.
o Dissipation Factor (DF) – expressed in percentage (derived from inductance, impedance, and phase angle and capacitance measurements) is the measure of the dielectric (insulating) losses in an electrical insulating material in an alternating (current) field and the resulting energy dissipated as heat. DF is used as a means of measuring changes in MGU phase winding wire coating (dielectric varnish or enamel) quality state, wire-to-wire and phase-to-phase dielectric quality state, and stator slot liner insulation (dielectric) quality state to identify any insulating losses due to contamination and/or deterioration (aging).
Contamination is/can be, a cumulative effect and is derived from micro elements of aluminum, steel, friction material, oil contaminants, plastics, moisture, etc. that provides a medium in which energy can transfer between phase wires, phase windings, between phase windings and stator slot liner insulation, or between phase winding wires the slot liner insulation and MGU back-iron (i.e., stator lamination stack) that is electrically common with the vehicle chassis. This results in weakened/aged phase winding coating and/or slot insulator (dielectric) materials. Since a (final) catastrophic failure of an MGU can be the result of cumulative contamination during the course of its service life, DF test data results are an important prognostic/diagnostic metric to the user to assist in determining MGU insulation SOH.
o Test Value Static (TVS) – a dimensionless number comprised of a sub-set of the aforementioned instrument testing parameters. The 3-Phase winding parameter test data is then calculated by software algorithms that provide a resulting numerical value to the user for determining 3-Phase MGU stator and rotor electrical and magnetic performance. The user compares the dimensionless number to a reference number (provided with the tester) for determining numerically how far the tested MGU data has drifted (or not) from new MGU test data of the same type or generation of transmission. The TVS value also eliminates the need for rotating the MGU by rotating a wheel or pushing a vehicle to test the 3-Phase stator windings, rotor magnets or rotor bars, and shorting rings.
o MGU sub-system testing: AT33EV is capable of testing MGU rotor and stator SOH without rotating (spinning) the rotor
o Connection of AT33EV to MGU cables was accomplished by using three (3) 0.375” diameter pure copper adapters with resistance in the low micro-Ohm range, knurled surfaces, and external threads (two adapters 3 inches in length and one adapter 4 inches in length) to permit repeatable instrumentation connection to MGU cables.
Case Study Data Presentation
The MGU test data is presented in the two tables provided in this article. The first table provides test data for transaxle MG1 (generator) while the second table provides test results on transaxle MG2 (drive motor). The table columns provide the following data (from left to right):
· Transmission sample number
· Vehicle Odometer reading from which transmission was removed
· Resistance 3-2 / 2-1 / 1-3: Resistance values when measuring Phases 3 to 2, 2 to 1, and then 1 to 3. The results of the resistance test are the comparison of the phase winding values to determine the overall resistance balance. The Institute of Electrical and Electronic Engineers (IEEE) Standards Document 1415-2006 states “the three (resistance) values are compared – all readings should be within 3 percent to 5 percent of the average of the three readings.” The standard values ensure that there is electrical dc (resistance) balance between all of the MGU phase windings.
· DF percentage (Dissipation Factor) - Is a number derived from AT33EV software algorithms that provide resulting contamination test data in a percentage (%) format for the user. The data utilized to determine MGU DF is capacitance (the primary element for DF testing), Inductance, Impedance, phase angle, and current-to-frequency ratio as additional electrical elements used by the software to scrub the data. In the data, DF is presented in percentage and capacitance units (%). However, to simplify data reporting in this article percentages will be utilized in three ranges:
o ≤ 6 percent = Good (OK) – contamination within acceptable limits
o 6 percent to 10 percent = Warning (W) – contamination is high but not out of limits
o ≥ 10 percent = Failed/Failure (F) – contamination is excessive, out of limits
MGU winding contamination testing is covered in IEEE Standard 43-2000. IEEE 56, 118 and 120 cover Motor Circuit Analysis testing methods, including how data is collected by instrumentation.
· TVS (Test Value Static) - The TVS value permits testing of MGUs by comparing the test data to a reference (new) unit. (Editor’s note: The base reference values for the Gen II units tested are 5.80 for MG1 and 13.30 for MG2). By utilizing a qualified reference number any MGU SOH can be determined by using this comparison method. Specially, the TVS value can assist in determining the level of MGU aging (deterioration) of windings, stator slot insulation, rotor/stator magnetic condition, etc., or if the unit itself has already failed. The key concept of using the TVS metric is being able to test a transaxle/transmission on the vehicle whether it uses direct connection to the final drive, single or multiple planetary gear sets or internal hydraulic clutch systems. However, TVS data will not determine if a 3-Phase MGU problem is the stator or rotor. It can only determine whether there is an electrical or magnetic unbalance in the rotor or stator.
· TVS data results are reported and compared to the reference value as follows:
o ≤ 3 percent variance from reference = OK - Good Stator and Rotor balance
o ≥ 3 percent but ≤ 5 percent variance from reference = WARNING that Stator or Rotor is beginning to become electrically or magnetically out of balance
o > 5 percent variance from reference = FAILURE - Stator or Rotor electrical or magnetic properties out of balance
TVS values are used for determining 3-Phase MGU stator and rotor electrical and magnetic performance.
There seemed to be no correlation between the age and/or mileage of the tested units and TVS or DV percent.
In automotive systems, it is irrelevant whether the problem is the rotor or stator because the transaxle/transmission must be disassembled in the vehicle or removed from the vehicle. In either case, the rotor and stator are removed and a new or known good stator and rotor assembly (tested prior to use) can be used to replace units that have failed testing or indicate data consistent with advanced aging. Replacing both the stator and rotor would alleviate a possible misdiagnosis or more costly testing.
Case Study Data and Results Discussion
There were a total of 20 Generation II (2004-2009) electric transaxles as part of this study. Although not reported in the data, each transaxle was tested for insulation resistance (IR) at 500Vdc. There were zero (0) transaxles that failed the IR test. However, two of the transaxles (sample 12 and 16) were slow to achieve the 500Vdc IR level @ > 10 seconds) which, from testing experience, indicates the commencement of above normal insulation leakage and a weakness in the MGU winding insulation or stator slot insulation materials. This type of data result will eventually evolve into a MGU failure but, predicting time to failure is not in the scope of this article. However, there are quality statistical methods available (such as Wiebull analysis or using Reliability statistics) that can assist in predicting time to failure of the MGU based on the results of electrical test properties, operating environment, etc.
Odometer Data
The odometer data in this case study is very wide and has been rounded to the nearest 1000 miles for ease of reporting. The transaxle with the lowest odometer data point is 23,000 miles and the highest data point of 148,000 miles.
Phase Resistance Measurement Data
Phase resistance data is reported in units of dc milliohms (mOhms). Transaxle MG1 sample 8 reported the lowest resistances of 96.40 – 96.90 mOhms. Transaxle samples 3, 4, 7, and 8 reported the lowest resistances for MG2 of 120.00 – 121.00 mOhms. All transaxle sample phase resistance measurements were < 3 percent resistance variation for phase resistance balance and, therefore, were within the IEEE 1415 to 2006 standard for dc resistance balanced electric machines.
Dissipation Factor Measurement Data
Dissipation Factor percent (DF%) data indicates that transaxle MG1 sample 13 was the only unit not scoring in the normal range with 7.06 percent (WARNING range). Transaxle MG1 sample 16 data at 5.99 percent nearly placed it in the WARNING category with sample 13. None of the Transaxle MG2 unit data resulted in a DF percent resulting in a WARNING or FAILED test result. However, Transaxle sample 16 DF percent of 5.66 is within the confines of acceptable test but, is on the border of WARNING data. Both MG1 and MG2 data for sample 16 nearly place it in the WARNING data category for both MGUs.
Test Value Static (TVS) Measurement Data
The TVS (dimensionless number) measurement is the most complex measurement data numerical value to report. The target TVS reference value for a Generation II MG1, as noted earlier, is 5.80 while the TVS reference value for MG2 is 13.30.
Rarely is the opportunity presented to test such a variety of units in a single day.
Data acquired for the MG1 transaxle samples indicated that samples 12 and 5 were in a WARNING state (≥ 3 percent but ≤ 5 percent variance from the reference data target), while sample 11 indicated a FAILED state (≥ 5 percent variance from the reference data target). Data acquired for the MG2 transaxle samples indicated that sample 1, 2, 4, 10, 11 and 13 were in a WARNING state (≥ 3 percent but ≤ 5 percent variance from the reference data target), while sample 15 indicated a FAILED state (≥ 5 percent variance from the reference data target).
Case Study Conclusions
Although this case study involves a small sample size of 20 the data is consistent with testing that has been completed on hundreds of MGUs (whether Toyota product or their competitors) using the methodology outlined in this article. It is understood that if there was a Generation II vehicle population in the field of ≈1.5M vehicles, the sample size necessary to provide a 95% data confidence (with +/- 3 percent Confidence Interval) level would be ≈1100 transaxles.
The sample size in this study is far from the number necessary to attain a reliable statistical modeling of 2004-2009 MG1 and MG2 MGU electric machines. A large enough statistical population pool (sample size) coupled with a high confidence level and confidence interval to achieve statistical numbers that are reliable to drive a statistical conclusion were outside of this case study scope. However, the preponderance of the testing evidence from this case study (and others like it) have been consistent with other case study results for providing prognostic and diagnostic value to field technicians in testing the SOH of an MGU. This data assists the technician by indicating the onset of conditions that would lead to a catastrophic failure.
The data further indicates that, based on the data provided in this case study, it can be concluded that winding resistance data does not trend (or track) with other MGU SOH failure modes. All phase resistance testing on the MGUs in this case study indicated that there was balance between all of the MGU phases and each complied with the IEEE 1415-2006 standard. The DF data acquired from each of the MGUs did not trend or track with dc resistance testing data nor did it trend to TVS data. Therefore, an MGU can contain balanced phase resistances and DF percent data that is within the tolerance bands but fail TVS testing.
This testing complies with IEEE 56, 118 and 120 covering Motor Circuit Analysis testing methods, including how data is collected by instrumentation. Also, based on the case study data, it is possible to contain balanced phase resistances, TVS data that is within the tolerance band but, acquire warning levels for the DF percent. This testing complies with MGU winding contamination testing contained in IEEE Standard 43-2000.
In summary, resistance data, DF percent data, and TVS data are decoupled in the failure modes or SOH of an MGU. By using fundamental electrical engineering principles, combined with advanced math and software algorithms to scrub the data, a total picture of MGU SOH or the confirmation of a catastrophic failure IS possible. This is good news for technicians in the field because, in the past, many of the MGU operational/performance problems, winding or slot insulation aging measurement or, trying to identify difficult intermittent conditions has been unreliable or impossible.
Unreliable diagnostic techniques such as using only a milliohmmeter, use of only an milliohmmeter and IR, or using a combination of a milliohmmeter, IR, and impedance meter are unable to detect the subtle changes in phase winding or stator slot insulation and therefore, cannot detect the onset of electrical or insulation failure modes. These methods may be acceptable to identify a narrow band of failure modes or confirm a catastrophic condition but none are able to deliver advanced MGU winding and slot liner SOH testing.
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