Hybrid & Electric Vehicle Corner
by: Curt Ward
Professor at Joliet Junior College
Exploring the Toyota Electronic On-Demand All-Wheel Drive System
As I write this article, the fall semester is under way, and it is great to have students back in the classroom and the lab. In my quest to have a wide variety of power flow demonstrators for my EV lab space, I recently had the opportunity to disassemble a Toyota electronic rear drive axle assembly. In this article I will share some of the items I learned and how I plan to implement them in the curriculum.
Figure 1: Drive Unit
The drive unit that was disassembled came out of a 2023 AWD Corolla Hybrid. The electronic drive unit has been used in the United States market since 2022 in the Camry, the Carolla, and the CH-R models. Toyota service information refers to the drive unit as motor/ generator 3 (MG3). MG1 and MG2 are in the transmission. The assembly is relatively small when compared to other rear drive units because it does not contain its own inverter. Instead, the three-phase current is directed to the drive motor from the main inverter located on top of the transmission. The differential is in the left side of the assembly and the drive motor is in the right side (See Figure 1 – Drive Unit).
Figure 2: final Drive
The first step in the disassembly process is to remove the drive axle flanges and external electrical harnesses. Once these components are removed, it was easy to unbolt the end covers from the central housing. Doing so reveals the final drive assembly and the drive motor (See Figure 2- Final Drive and Figure 3- Drive Motor). The final drive assembly consists of a drive motor output gear, an intermediate shaft with an input and output gear, and a differential ring gear. The motor-to-tire gear ratio is approximately 7.14 to 1.
Figure 3: Drive Motor
The entire assembly is lubricated and cooled with Toyota Genuine e-Transaxle Fluid TE via splash lubrication, there is not an internal or external oil pump. This fluid differs from ATF in reduced viscosity and is designed primarily for the cooling of motor-generator.
Figure 4: Resolver
The drive motor is a permanent magnet motor that is rated at 30kW or just over 40 hp. The stator windings are a hairpin design, and the permanent magnets in the rotor are arranged in a “V” pattern for greatest electrical efficiency. The motor speed, position, and direction of rotation is determined by a standard resolution resolver (See Figure 4 – Resolver on the facing page). The temperature sensor is in the stator windings. The normally dormant AWD system is designed to provide up to 50% of the torque when front wheel slip is detected.
Like many of the other drive units that have been disassembled, this unit will be used to help our students understand the electrical operation and mechanical power flow. The student will be expected to identify the components and calculate the final drive ratios. The student will be able to measure the resistance of the temperature sensor, and the stator windings. The students will also be able to practice loss of isolation tests on the stator windings. All these tests will help the student to determine if a failure is internal or external to the drive unit. In the North American sales market, except for axle flanges and seals, the assembly is replaced as a unit for all internal failures.
I will finish this article with the same offer I make after each of my presentations. If you are interested in getting started in the process of adding hybrid and electric vehicles to your curriculum or want more information, please feel free to reach out. I am more than willing to sit down in-person or online and share my experiences. Are you looking for a classroom textbook? Reach out to Pearson and ask for a review copy of the all-new Electric and Hybrid Electric Vehicle text that Jim Halderman and I co-authored. It is a comprehensive text covering all the latest information on the subject.