ZF has announced that its new electric drive system, which is based on a modular approach, will start volume production with a European vehicle manufacturer in 2018. A drive system which is positioned at the centre of the axle is at the heart of the drive and can be used for various vehicle and performance categories. The system features an electric motor configured as an asynchronous motor (ASM).
With up to 150 kW, the unit sets new standards in terms of power-to-weight ratio. An electric motor, a two-stage single-speed transmission complete with differential as well as power electronics form a highly-integrated and extremely compact unit. The technology is suitable as an all-electric drive for battery-powered, fuel-cell or hybrid electric vehicles and is suitable for both front and rear axle applications. Production will take place in Schweinfurt, the headquarters of ZF's new E-Mobility Division; construction of the new production facility is already underway.
In an axially parallel electric drive module, ZF has integrated an electric ASM and a two-stage single-speed transmission with a differential, a housing and a cooling unit as well as power electronics including software. The motor and transmission share one housing, simplifying production and final assembly. The system generates up to 150 kW and the impressive maximum engine torque of 380 Nm is converted into a 3,500 Nm axle torque. Despite the integrated ASM, the compact drive system weighs a relatively light 113 kg. The entire unit has an axial length of around only 450 mm, a width of 380 mm and a height of 510 mm. Furthermore, the compact design naturally uses fewer materials and requires less installation space. This makes it easier for vehicle manufacturers to integrate the system into different vehicle architectures.
Unlike the permanent-field synchronous machine (PSM), the ASM does not require any magnetic materials and therefore does not need any rare earth elements such as neodymium or dysprosium.. The availability and price of such raw materials fluctuates vastly across the globe – which could entail a cost risk that is hard to assess for vehicle manufacturers and suppliers. The ASM also requires only a simple concept to sense speed which offers advantages in terms of safety and reducing electric drag loss. In addition, the ASM allows a broader range between constant and peak performance which therefore lends itself particularly well to short-term, high performance requirements.
Speeds of around 13,000 rpm place exacting demands on the transmission with regard to noise emission. ZF has opted for an axially parallel single-speed transmission ratio of 9.6:1 which reduces the speed level in two subsequent spur gear stages. The optimized gearing and the low-loss mounting of rotating parts enable not only noise reduction but also very high efficiency across a wide operating range. The innovative design is rounded off byan integral overall cooling concept, which includes both power electronics and the electric motor and also cools the electric motor's rotor. This results in very good acoustic and dynamic characteristics of the electric drive in the vehicle.
By integrating the power electronics into the electric drive ZF has been able to solve a problem that occurs in many all-electric vehicles regarding the interplay between the electric motor and power electronics. The power electronics convert the battery’s direct current into a three-phase alternating current required for the electric motor.
As part of this process, notable losses occur in certain driving cycles. These losses are reduced with ZF’s electric drive axle system: as the actuation voltage is increased with the help of special modulation processes, the motor current can be reduced while performance remains unchanged. With normal driving cycles, this process significantly increases range.