## ESKAM finishing electric drive axle module for commercial vehicles, new production technologies; vehicle testing this year

##### 02 February 2015
 Electric drive axle module with two motors and integrated power electronics. Groschopp AG. Click to enlarge.

The ESKAM (Electric Scalable Axle Module, Elektrische SKalierbare AchsantriebsModule) consortium in Germany, sponsored by the German Federal Ministry of Education and Research (BMBF), is completing the development of an optimized electric drive axle module for commercial vehicles, consisting of two motors, transmissions and power electronics. All components fit neatly and compactly into a shared housing, which is fitted in the vehicle using a special frame construction also developed by the project engineers.

The individual modules developed by the various partners are complete, as are new manufacturing techniques developed by the partners. The consortium is now putting the individual parts together to make a demonstrator. After that, they want to fit the axle module into a real vehicle for testing by the end of 2015.

 The modules—with dual motors and integrated power electronics—are scalable for various vehicle types. © Fraunhofer IWU. Click to enlarge.

Eleven partners are in the ESKAM consortium: Ebm Erich Büchele Maschinenbau GmbH; Technical University of Dusseldorf, Electrical Engineering and Electrical Machines; Groschopp AG; Hirschvogel Automotive Group GmbH; University of Applied Sciences Aalen, General Engineering; Metal foundry Wilhelm Funke GmbH & Co. KG; REFU Elektronik GmbH; Salzgitter Hydroforming GmbH & Co. KG; University of Stuttgart, Institute for Power Electronics and Electrical Drives (ILEA); Wilhelm Vogel GmbH Antriebstechnik; and Fraunhofer Institute for Machine Tools and Forming Technology (IWU).

The axle module presents numerous advantages, such as a high power density and a very high torque. For drivers, this means very fast acceleration. While the speed of most electric motors is approximately 10,000 to 15,000 rpm, the ESKAM motor (from Groschopp) achieves speeds of 20,000 rpm, with maximum torque of 45 N·m (33 lb-ft) and power of 32 kW (43 hp).

When we started on the project three years ago, we were the only ones who could obtain such high speeds. In the meantime, others have been attempting similarly high speeds. But our head-start in accumulating development experience has given us a technological edge, which we intend to further extend.

—Dr. Hans Bräunlich, project manager at IWU

As well as designing the axle module, the project researchers and developers simultaneously developed the required series production technologies. IWU had the lead role in this work as well as being the technological lead for the overall project. Series production brings economic advantages, with reductions in production costs of up to 20 percent, according to Bräunlich.

Spin extrusion. As an example, gearbox shafts are usually manufactured from expensive cylinders or by means of deep-hole drilling. In both cases, the excess material is unused. By contrast, researchers at IWU have chosen new, short process chains together with methods that allow greater material efficiency. One such method is spin extrusion, which was developed by IWU.

Although it also uses a block of material, in this process the blank is shorter than the finished shaft.

To help visualize the process, think of pottery. The material is extruded during the shaping process, and pressed outward in a longitudinal direction. This allows us to use virtually all the material, cutting material costs by approximately 30% and reducing the overall weight of components.

—Hans Bräunlich

Until now, there have been only initial ad-hoc approaches for this method. Now the scientists have made the technology fit for series production. The toothed gear wheels are also made using a different process. Instead of milling them from the material, they are now manufactured using a special forming process called gear-rolling, which was also developed at IWU. This method does not produce any metal chips, and effectively no material is lost.

All-purpose module. The flexibility of the axle module is not limited to batch sizes either, but also extends to geometry. Because the module is scalable, it be applied in vehicles ranging from small vans and municipal vehicles to buses and trucks, said Bräunlich.

Could this become a competitive alternative to in-wheel motors?

I favor motors opposite the bearing mount. In wheel motors take up space for brakes and inboard required half shafts and CV joints.

I am surprised that no one has come out with an after market add that attached to a rear differential that adds a Kers type system. Could it not increase mileage by 20%. That would mean a 17mpg old ICE car could get 21mpg.

You can make a 20 mpg car get 30 mpg, but if it costs $10,000 and takes 10 years to pay back, few would do it. I have direct experience of comparing transverse electric motor mounts versus those using longitudinal coupling. Dyno results showed conclusively that turning power through 90 deg from a longitudinal motor, as is shown here, was proven to be the inferior choice. That aside, the architecture of using two motors, despite the incremental cost factor of the dual drive, looks to be an excellent idea. The ability to dispense with the differential components as done here, may avoid a problem that Tesla seems to be having with their two stage reduction box. Incidentally any visit to the Tesla website soon reveal it to be inhabited by moderators who are extremely sensitive to any fault details being released with regard to their gearbox, naturally speculation is inclined to be rampant. My take on the problem is that moving from motoring to regen braking is so fierce that the differential spider gears are being given a hard time. Fast changes in slew rate aren't problematic until you pass through the dead band and hear the clunk signature as pressure is suddenly exerted at the reverse face on the gear teeth that are already engaged. In fairness it should be pointed out that very few vehicles have exhibited this symptom straight out of the gate. So it could be exacerbated by driver style as much as anything. Many of the owners are A-types who don't appreciate they are confronting bleeding edge technology and while$140k of their money is on the line they resent even the slightest suggestion that perhaps they should not do pretty much as they damn well please.

Avoidance of the gear lash within the differential requires consideration of the differential system being replaced by a dual drive system. It will become apparent that the amount of gear hobbing remains the same. The left/right wheel axles are terminated in gears which are standard helicals rather than bevels. If it is assumed that a two stage reducer layout is to be maintained then the final differential gear containing two, perhaps three spider gears is eliminated entirely with the placement of three new gears, one of which would be the motor pinion for the additional motor.

The High RPM motor presents another interesting design avenue. Higher rpms effectively increase machine power density and permits the use of lighter motor frame sizes. Notice the stipulation on lighter rather than smaller frame sizes since it facilitates the options of either higher power or increased affordability. BTW Tesla has tested one of their induction motors for 24 hrs @ 24000 rpm. What is not known is the frame size and whether it was performing under load.

Finally the "shared housing" may not be the best approach. Again Tesla uses this highly integrated approach and in my opinion it has been a disaster as far as maintenance costs go. Of course solid failures justify dropping out the transaxle but some complete transaxles were exchanged when just a shim needed replacement. Automotive training usually governs that area of how things are supposed to work, the experience to identify problems in these early years will take time to acquire by a dispersed group of people. In the meantime the ability to disassemble major components in situ is going to be a consideration if the cost of electric vehicle maintenance is not going to go through the roof.

@T2,
There is similar electric-axle for trucks, but with transverse e-motors:
http://wrightspeed.com/products/the-route/#features

They say there it's "electronic differential". Both motors are with 2 speed (in specs they list 4 speeds). They claim gearchange speed of 80 ms.
I guess they don't shift gears at the same time for both wheels, so they can have asymetric torque fill during shifts. Also it's possible that they don't change gear unless opposing wheel (providing torque fill) has at least 40% of traction .

As for suspected issues with differential in Tesla, there is another type of differential from Schaeffler, using planetary gears. They claim it's about 30% lighter.

Do you think this differential would be less susceptible to problem you anticipated in Teslas?

@T2,
Regarding 20,000+ rpm motors:
In the text it is not said if the motor is induction or a PM one (made by Groschopp). On their sites (US or German) there are only lower power motors, lower rpm, both IM and PM.

A year or two ago ZF announced an inexpensive transverse electric drive using a 21,000 rpm induction motor paired with 2-stage planetary reducer (16x). Peak power 90 kw, constant output 30 kW.
They didn't say if it was 2 pole or 4 pole machine.

If 4 pole (IM), it would mean it should be driven with 700 Hz (3 phase) inverter, if 2 pole (IM), it would be 350 Hz (3 phase).

As losses for (regular) iron core rise quickly as frequency goes above 500 Hz, isn't it natural they use 2 pole design (up to 350 Hz)?

Do you think they may be using 5-Phase system, at 350 Hz to get higher torque?
Five-phase motors are used in some designs, or at least considered in some papers.

There was a 4-pole motor here at GCC, going up to 21,000 rpm for F1 Honda KERS. But it used very expensive, low loss, cobalt steel (some sources say 10x more expensive than regular iron), definitely not used in ZF low-cost design.

@Alex _C interesting links BTW. On the electrical side I can answer some of your questions directly. Going forward maybe the GCC discussion pages should be used since topics that fall off the Home page have less accessability to the general user base as time goes by unless you have happened to have saved the exact URL of course.

A discussion on AEVA (motor & controllers) entitled 14,000 rpm machines considers PM versus Induction. It was brought up in the first post with the expectation that PM would be summarily dismissed. As it turns out there was considerable interest perhaps as long as the Toyota Prius and Nissan Leaf continue to use them I suspect. For tow trucks and forklifts which are mostly used as mobile positioning systems they will always be a good choice.

My experience is that 5-phase PM machines manufactured by VEXTA are mainly confined to applications known to be sensitive to normal torque ripple.

The robust nature in both temperature and overcurrent points to the 4-pole per phase induction motor as optimal for automotive usage. The 6-pole per phase may produce better torque/mass ratio but needs research. Greater than 6-pole may be OK for the industrial market but has slotting issues which obviates its use with with smaller automotive frame sizes.

For 4-pole motors, excitation frequency = RPM/30 Hz

Copper loss is not a consideration in the stator. Electrical loss in the rotor -since that's where all the electrical power ends up after it crosses the annular gap- is key so copper rotors are mandatory but most industrial motors (premium effcy) are now using them today, see the Eurotherm catalog.

Magnetic losses generated by hysterisis and eddy currents become less a consideration when manufacture includes the use of thinner laminations and the finest electrical steel that Accelor-Mittal can make.

Simply put, the Tesla Model S with its latest \$140k flagship P85D 5 seat sedan with insane 3.1 secs to 60mph exemplifies the induction motor as the go to solution for BEVs.

Whether Tesla has found the optimal mechanical transmission layout where durability at high power will not be an issue remains to be seen. Tesla has seen problems because it is the only company pushing out sufficient quantities that even one in a hundred can no longer be considered as an inconsequential occurrence.

@T2, nice that you replied.
We can discuss here, I saved the link.

To me, 4 pole (per phase), 3 phase motor, in terms of torque producing ability is something between 2-pole 5-phase and 2-pole 7-phase motor, for the same max RPM and same motor dimensions.
You may want to see the following patents, same inventor, for "High phase order motor":
6351095; 6657334; 6831430; 7075265; there are some more for the same assignee.
They tested some of this above in a in-wheel motor for large commercial aircraft, high torque motor, I read it a few years ago, probably some links are around.
Looks like some Nr. of phases is not suitable because of certain harmonics.

With regard to inverter design (21,000 RPM, induction machine assumed), 5-phase 2-pole (per phase), it seems it would have more power switching elements (10), than 3-phase motor 4 pole one (6).
For the same power, probably those transistors could be of lower power handling ability for 5-phase design. Would they need to be for the same max voltage, lower max current or some other rule, I don't know. It may depend on motor wirings.

It would be interesting if you could shed some light on differences in requirements and costs for these 2 inverters.

Regarding earlier mentioned ZF 21,000 rpm motor, these 2 links provide some info, from designers, you may have read it already.
Both article are behind pricewall, but low res. previews are available:

Best do this way:
See first two pages, out of 4 (pgs 10 & 11) here:
Click on: Look Inside Icon (top right)

Pages 3 & 4 (12 & 13 in the mag) find at:
http://www.atzonline.com/Article/17668/Electric-Axle-Drive-Module-for-High-Speeds.html
(here it's at lower res, first 2 pages are better at first link).

On chart at bottom of third page there (or p13), there are efficiency diagrams for IM and PM motors.

Looks like IM would benefit more from 2 speed transmission than PM motor for two reasons:
1- to avoid very low rpm where IM is quite inefficient
2- IM has lower constant power range, less than 4 in almost all designs (incl Tesla Roadster motor). Experts say that for automotive use that range is desired to be at least 4. PM motors have wider range of constant power (IPM ones).

The 2 speed gearboxx doesn't need to be 2:1, it could be say 1.5 - 1.8:1. Lower value makes motor easier to sync quickly to new speed, when shifting.
Off course higher L/D rotor geometry helps, as moment of inertia is proportional with D^2 (or D^4).

Thinner laminations would reduce eddy currents. But I don't think it would reduce hysteresis losses.
So 350 Hz (5-phase) seems to be a more efficient system, but with some more switching elements.

Alex , I ask that perhaps you could take some of these OT ideas to other places where they may find a more enthusiastic setting. Have you tried Endless Sphere ? Also DIY Electric.

These will be my final comments here. I have tried to register in the discussion board. There is no register button in my browser so no go there.

5-Phase although of theoretical interest does require some inverter to motor topology changes since there are obvious ramifications to having legs separated by 72 deg.

Also I have no idea what ZF's planetary twin ratio device is about. Assuming an automotive drive is under consideration, then unlike an ICE there never should be the need to have to spin down an IM in order to enter a coarser gear ratio when you want to go faster and thus avoid an RPM range that may exceed the motor's mechanical limits. Such a condition can only mean that the motor specified was seriously undertorque for the application.

The max rpm of any machine should be just below the point at which the machine's rotor is likely to grenade. The reason is that motor power is proportional to rpm and since aerodynamic losses do not diminish with increasing vehicle speed it is preferable to gear the motor for its max rpm to coincide with the top road speed.

That said, some electrical designs experience torque roll off at high speed. This is because the stator winding pattern has been deliberately arranged to present a high V/F (Volts per Herz) ratio to the inverter. Motor torque is proportional to the product of motor current and V/F ratio. Clearly a larger V/F means an equivalent torque can be still be produced but with a smaller current. It is a popular trick that is used to enable the current rating of the inverter transistors to be reduced. The drawback is that a large V/F ratio will have the motor present a high back EMF at relative low rpm such that a period of constant power is entered followed by another area where the torque begins to drop inversely as the square of the speed. To maintain some semblance of performance the ability to select a gear ratio that will slow the motor rpms relative to road speed will bring the motor out of voltage saturation and generally the restored motor torque will temporarily compensate for the coarser ratio of gear now employed.

There is an elegant solution so that all this trouble can be avoided in the first place if the motor is wound for a low V/F and used along with an inverter with oversized transistors which is standard practice at Tesla.

As a point of fact whereas a general purpose 480V 60Hz motor will have a V/F of 8.0, the corresponding frame size Tesla motor is estimated to have a V/F of 0.25, meaning that for equivalent torque the Tesla motor will have to draw as much as 32 times more current than the previous motor. A situation which is not problematic since the Tesla inverter is well equipped with no less than 850 amp transistor ratings.

Hi T2 "I have direct experience of comparing transverse electric motor mounts versus those using longitudinal coupling."

Would you have a measure of the difference in efficiency, or an estimate?

Thanks.

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