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Toyota Prius Transaxle Group Case Study

Determining Electric Motor Generator State-of-Health
Friday, July 19, 2013 - 10:35

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.

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