A new electrified powertrain—integrating the motor, inverter, reducer, and power delivery module (PDM) into a smaller and lighter package—has been developed for the MY2013 Nissan Leaf. This integration made it possible to reduce the number of high-voltage harnesses and connectors and cooling water circulation pumps needed between components, while also increasing luggage capacity at the rear of the vehicle and optimizing the torque output of the motor and the reduction gear ratio.
Integrating the motor and inverter eliminated the three-phase high-voltage wiring harnesses and connectors previously used to connect them. For connecting the three-phase wiring between the motor and inverter, a terminal block is positioned on the motor, and three-phase busbars from the inverter are connected directly to the terminal block. Elimination of three-phase wiring harnesses, inverter members, and other parts, along with the downsizing of each component, reduced the weight and volume of the motor-inverter alone by 11.7 kg (25.8 lb) and 5.1 L (311 in3), respectively, compared with the 2011 model.
To facilitate installation in the motor compartment, the overall size and weight of the integrated electrified powertrain had to be reduced. The inverter was downsized and made lighter in weight by optimizing its internal layout and part shapes.
To integrate and position the inverter on top of the motor, pillar-like structures were needed to connect the box-shaped inverter to the cylindrical shape of the motor. To accomplish that, the front and rear motor covers were extended upward like pillars to form a concave space between the inverter and the motor. That space was used effectively by locating the large condenser of the inverter there, which helped to hold down the system’s overall height.
The internal parts of the inverter were also optimized. The three-phase terminals were positioned facing the three-phase terminal block on the motor to shorten the length of the three-phase busbars. The busbar between the power module (PM) and the condenser was integrated with the condenser to minimize inductance. The PM busbars were also designed with the shortest length possible, and the position of the connection to the PDM was adjusted.
As a result, the total length of the five busbars (dc, three-phase) on the 2013 model was successfully reduced by 55% compared with the 2011 model.
Other measures taken included the adoption of waterproof low-voltage connectors integrated with the printed circuit board (PCB), discontinuation of the four sub-covers used previously, and a significant reduction of the types and numbers of bolts used, thereby reducing the part count considerably. As a result, the time needed for installing the new system in the vehicle was reduced by 26% compared with the 2011 model.
Connecting the motor and inverter via a three-phase busbar means that the heat generated by the motor coils is transferred via the busbar to the inverter. It is necessary to suppress the heat transfer from the motor coils because the allowable temperature around the busbar on the inverter side is lower than that of the motor coils. The heat release performance of the terminal block used to connect the busbar was improved to inhibit this heat transfer. Specifically, the area of contact between the plastic and metal (aluminum and steel) parts of the terminal block were increased to improve the heat transfer characteristic. The aluminum part of the terminal block was constructed so that it comes in direct contact with the cooling water supplied to the motor.
The inverter was designed such that the current sensors provided on the three-phase busbars could be mounted as close to the PM as possible. That was done to obtain a temperature distribution for the three-phase busbars where the temperature decreases from the motor toward the PM. Specifically, the vertical position of the PM was reversed from that on the 2011 model, and the water jacket was also positioned on the top side, making it possible to locate the current sensors at positions removed from the motor. Positioning the water jacket on the top side and the water-cooled terminal block on the motor allows efficient cooling of the heat transferred from the busbar connection. As a result, the overall temperature distribution inside the inverter on the 2013 model has been reduced by several K compared with that of the 2011 model.
This positioning of the water-cooled terminal block on the motor and the improvement of the internal layout of the inverter make it possible to keep the temperature rise due to the integration of the motor and inverter within the allowable temperature range.
The motor produces electromagnetic excitation forces when it generates torque. These electromagnetic forces are transmitted through the structures of the connected parts and resonate with the natural frequencies of the parts they pass through to produce noise. To address this issue, the electromagnetic circuit of the motor was optimized to reduce electromagnetic forces, and the shapes of the parts the forces pass through were also optimized, thereby reducing noise and vibration.
The geometries of the water jacket and the inverter case were optimized to reduce the radiation noise caused by excitation forces input from the motor. Measures were also taken to prevent PCB-mounted components from falling out due to motor vibration. Specifically, a vibration analysis was conducted on the PCB to identify areas where the stress induced by motor vibration was low. The pattern of mounting components on the PCB was then improved such that heavy components like the electrolytic condenser and others are mounted in areas of low stress. This improved mounting pattern suppresses the level of vibration applied to such components.
This article is based on SAE tech paper 2013-01-1759 by Hirofumi Shimizu, Takahito Okubo, Izuho Hirano, Shigeaki Ishikawa, and Makoto Abe of Nissan presented during the “Electric Motor & Power Electronics (Part 2 of 2)” SAE 2013 World Congress session.