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ELECTRIC AXLE DRIVE MODULE FOR HIGH SPEEDS A compact, efficient electric axle drive module by ZF for small and mid-size passenger cars sets new standards. The module’s asynchronous motor is designed for 21,000 rpm speed and does not require any rare earth elements. From an acoustic viewpoint, this high-revving concept places particular demands on the integrated transmission with an axially parallel input transmission ratio of 16:1. Specific adjustments to the gearing parameters and the system interfaces paved the way for further noise improvements.
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AUTHORS
DR. HANS-JÖRG DOMIAN is Head of Corporate Advanced Engineering and Design at ZF Friedrichshafen AG in Friedrichshafen (Germany).
DR. KARL-HERMANN KETTELER is Development Engineer in the area of Electric Motors and Energy-storage Devices at ZF Friedrichshafen AG in Friedrichshafen (Germany).
DIPL.-ING. STEPHAN SCHARR is Development Engineer in Advanced Engineering Driveline at ZF Friedrichshafen AG in Friedrichshafen (Germany).
CREATED FOR CITIES
Despite continual progress in battery technology, energy storage remains a limiting factor on the range of all-electric vehicles. Consequently, the number of electric vehicles will increase primarily in urban conurbations in the first instance where passenger cars cover on average less than 60 km a day. The demand for locally zero-emission driving is growing constantly as urbanisation and the associated population growth translate into increasing traffic volumes which, in turn, push up pollution and noise. Consequently, ZF designed its electric passenger car axle drive module for subcompact and compact vehicles, which are designed essentially for urban usage. The developers’ primary aim was to increase the limited battery-related range by means of a low-cost electric drive which is as efficient as possible – instead of opting for a more powerful battery which would make 12I2013
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the e-car much more expensive. The holistic approach adopted includes a new electric motor concept as well as transmission, power electronics, and control software. The electric axle drive module is particularly suitable as a supplementary rear-axle hybrid module for front-wheel drive cars with a combustion engine, which account for more than 75 % of all passenger cars produced worldwide. In contrast to combustion engines, electric motors can call upon a high torque level over a wide range of speeds. Developers of electric vehicles therefore have to address the basic question of whether you need a transmission in the driveline and what benefits it brings. ZF has opted for a transmission with its electric axle drive module to provide a gear ratio that is as high as possible. After all, an electric vehicle needs a transmission for the conversion of speed and torque if you are to come up with an affordable solution. In case of the electric motor, the mass increases with the torque. Thus, the power (power = torque times speed) can be achieved either with a high torque and low speeds or with high speeds and moderate torque. The advantage of the second approach relates to the electric motor’s lower weight. However, you then need higher transmission ratios and may have to increase transmission weight. Nonetheless, this downside is more than offset by the lighter electric motor and the need for less installation space. And in terms of costs, one kilogram of the electric motor is more expensive than one kilogram of the transmission. During development of its electric axle drive module, ZF also focused on factoring in the changed differentiating features of battery-electric vehicles compared with a conventionally driven passenger car. Noise characteristics proved the crucial element: Since the electric car no longer has a combustion engine, noise generated by the transmission and driveline becomes more prominent. In particular, the electric axle drive module’s high-revving concept adopted by ZF places specific demands on the transmission with regard to noise emission. If the electric components are too noisy, customer satisfaction could quickly suffer. The same applies to the efficiency of the overall system; after all, any shortcoming in this area would be noticeable to the driver in terms of reduced range. ZF
also focused on keeping production costs low for the electric axle drive module – ultimately the target segment of subcompact and compact vehicles is extremely price-sensitive. EFFICIENT ON THE ROAD
Extensive research and simulations are required to optimise the electric motor as the key component in the drive system in terms of size, weight, efficiency, and technology. After all, the motor converts electrical energy into mechanical energy. The focus was initially on comparing the concepts behind the asynchronous machine (ASM) and the hitherto often used permanent-magnet synchronous machine (PSM), ➊. As part of this process, the ZF engineers did not focus exclusively on the criteria of the general New European Driving Cycle (NEDC), but on customer-specific driving cycles for this passenger-car category which tend to reflect real-usage scenarios: It involved optimising the efficiency of the motor in those rpm and power ranges which drivers commonly use as part of everyday motoring, ➋. ASM AS BEST SOLUTION
Following extensive calculations, an ASM with an innovative high-revving concept emerged as the best solution: Under the motto “Power from rpm” the drive reaches its target output of a maximum of 90 kW (30 kW continuous output) and the maximum torque of 1700 Nm on the output shaft from a high motor speed – with 21,000 rpm at maximum vehicle speed. As a result, the electric axle module is light − weighing just 45 kg − and compact − the entire drive unit is just 460 mm long, 325 mm wide, and 253 mm high. At the same time, the compact design uses fewer materials and thus costs less to build. The often cited weight drawbacks compared with the PSM have been overcome thanks to the high-revving design, ➌. Instead, the ASM offers other benefits: Unlike the PSM, the technology used does not require any rare earth elements such as neodymium and dysprosium. The availability and price of these raw materials on global markets fluctuate wildly – which entails a cost risk that is virtually impossible to assess for OEMs and suppliers of elec-
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C OVER STORY TR ANSMIS SIONS
60 ASM
PSM
50
Motor weight [kg]
40 30 Design focus 20 10 Equivalent motor speed at 150 km/h [rpm]
0
8000
13,000
21,000
26,000
40,000
6
10
16 Final electric drive ratio i [–]
20
30
➊ Comparison of concepts regarding speed and weight: lightweight and compact solution due to high speed approach of ASM
1500
90 kW
1000 Wheel torque [Nm]
100 km/h Brenner Pass
150 km/h
500 0 -500 85 % of operation points Area of operation points of seven cycles and routes
-1000 -1500
0
200
400
Driving resistance Constant power operation
600
800 1000 Wheel speed [rpm]
1200
1400
1600
80
80
60
60
40
40
Moment [Nm]
Moment [Nm]
➋ Analysis of the relevant operation modes: optimising the efficiency of the e-motor in those speed and power ranges which drivers commonly use as part of everyday motoring
20 0 -20
20 0 -20
-40
-40
-60
-60
-80 0
0.8
-80 5000
0.82
0.84
10,000 n [rpm]
0.86
0.88
15,000
0.9
20,000
0.92
0.94
0
0.8
5000
10,000 n [rpm]
15,000
0.82 0.84 0.86 0.88 0.9
20,000
0.92 0.94 0.96
➌ Comparison of PSM and ASM motor efficiencies: following extensive calculations of relevant operating modes, an ASM with a high-revving concept emerged as the best solution
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tromobility. Not least, the ASM just needs a fairly simple, conventional speed sensor for control purposes, whereas the PSM requires a sophisticated angle-of-rotation sensor to record the position of the rotor. The overall advantages are reflected in the performance of the ZF innovation prototype [1], a test vehicle based on a subcompact car, which has already clocked up several thousand testing kilometres with the electric axle drive module and is still going strong for testing purposes, ➍: The subcompact car weighing around 1250 kg including driver and partial load accelerates from 0 to 100 km/h in well under 9 s, with zero emissions and without any perceptible driveline noise, before going on to a top speed of over 150 km/h. The drive’s outstanding dynamic handling is compelling. At the same time, the electric car can negotiate inclines of up to 35 %. The whisper-quiet drive also provides a new driving experience. ZF’s innovation prototype is testimony to the potential inherent in particular of combining electro mobility and lightweight design strategies. With the concept, lightweight chassis components supplement the electric axle drive including inverter, thus increasing both the range and the driving dynamics of the electric vehicle. NOISE OF THE TRANSMISSION
Speeds of 21,000 rpm place exacting demands on the transmission with regard to noise emissions. ZF has opted for an axially parallel input transmission ratio of 16:1 for the electric axle drive module which reduces the speed level in two subsequent steps: : In the first step a planetary gearset is installed. This transfers the torque very smoothly thanks to gears arranged symmetrically around the input shaft, which effectively minimises high-frequency transmission noise. : Only in the second step, at a considerably lower speed range, does ZF use a pair of spur gears. Apart from the effective noise reduction, the transmission is highly efficient − achieving a figure of over 97 % in most ranges. The upshot is the drive’s outstanding acoustic characteristics in the vehicle, with noise from the electric drive being imperceptible in the test vehicles. This
result was possible thanks to targeted, consistent input from simulation. Right from an early design stage, specialists from the area of acoustics and gearing discussed their experiences and devised optimisation potential on the basis of established models. The simulation tools were honed continually throughout the project. Specific adjustments to the gearing parameters and the system interfaces, taking into account the interaction and factors influencing the integrated system, paved the way for further improvements, ➎. Thus, for instance, a great deal of effort went into designing the transmission path between planetary gearset and housing. The excitations produced in the gearing were thus specifically reduced. The drive housing − acting as a “loudspeaker” in the broadest sense − simply attenuates the relevant vibration patterns and hence noise, even eliminating both entirely in certain areas. LOW-LOSS CONVERSION
The electric axle drive module also allows ZF to solve a problem which occurs in many all-electric vehicles from the interplay between the electric motor and inverter. The inverter converts the battery direct current into the threephase alternating current required for the electric motors. As part of this process, notable switching losses occur in certain driving cycles with the usual switching frequency of 10 kHz. These losses are reduced with the electric axle drive module thanks to a special topology: By increasing the design voltage the motor current can be reduced while maintaining the output figures. The design voltage is increased by means of a boost converter, which is, however, only run where necessary. Taking into consideration normal driving cycles, the range can be increased by up to 6 %.
➍ On-road testing with the ZF innovation prototype car: high power density and low weight result in emissionfree, efficient vehicle dynamics; using the example of an innovation prototype, ZF shows the potential of the combination; the electric drive system and the lightweight design chassis components were built in the ZF innovation prototype to do real testing; basis is the small car Suzuki Splash
with integrated power electronics at the IAA 2013: The output power here is 120 kW, with the axle torque increased to 2500 Nm. Thanks to the high-revving concept and integrated inverter, the reconfigured enhanced version also boasts a high power-to-weight ratio. The axially parallel design is, however, retained, along with the two-speed transmission and the integrated differential. An electric vehicle drive modular system with various installation lengths is planned to meet the requirements of various customers and models. In the
uprated variants, the electric axle module is no longer the preserve of subcompact and compact cars: Used as an axle hybrid module, the electric drive innovation is also suited to front-wheel drive vehicles through to the mid-size luxury segment. REFERENCE [1] ZF: Systematically Looking into the Future: ZF Innovation Prototype Combines Electromobility and Lightweight Design Approaches. http://www.zf.com/ corporate/en/press/press_releases/products_press/ products_detail.jsp?newsId=21997672, press release, 4 September 2013
MORE POWER IN ALL SITUATIONS
The ZF electric drive is a compact, efficient electric passenger car axle drive module, which sets new standards in terms of power-to-weight ratio with a high-revving asynchronous motor, twospeed transmission, and integrated differential. ZF is working hard on getting the drive ready for volume production. ZF presented a re-configured enhanced version of the electric axle drive system 12I2013
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➎ Early simulation optimises acoustics: speeds of 21.000 rpm place exacting demands on the transmission with regard to noise emission; ZF has opted for an axially parallel input transmission ratio of 16:1 for the electric axle drive module which reduces the speed level in two subsequent steps
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