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BMW’s hybrid motor design seeks to deliver high efficiency and power density with lower rare earth use

Drawing from a 2012 BMW patent application showing one embodiment of a proposed method to increase the efficiency of an electric motor. In addition to magnetic layers, the rotor has two magnetic flux influencing groups comprising a number of air-filled recesses. Source: BMW patent application Nº 2012/0267977. Click to enlarge.

With its introduction of the eDrive motor—a proprietary hybrid synchronous motor designed to exploit both permanent magnets and the reluctance effect—in both the BMW i3 (earlier) and i8 (earlier post), BMW is advancing what it sees as an answer to achieving the highest possible power density and efficiency coupled with minimum possible use of magnets containing rare earth materials.

The 50 kg electric motor powering the BMW i3 generates a maximum output of 125 kW for a specific power (SP) of 2.5 kW/kg. The motor offers linear power delivery extending into high rev ranges, with maximum revs of 11,400 rpm. As one point of comparison, the permanent magnet motor in the 2011 Nissan LEAF was rated at 80 kW from a 58 kg motor, for a SP of 1.38 kW/kg.

Background. Although the improvement of batteries is a major focus of development for reducing the cost and improving the efficiency of electric drive vehicles, improvements in electric traction drives (motor, power electronics, transmission, thermal management) also play a critical role.

As one example of that interest, the US Department of Energy (DOE) Vehicle Technologies Office (VTO) Advanced Power Electronics and Electric Motors (APEEM) program is focusing its research on developing new power electronics (PE), electric motor (EM), thermal management, and traction drive system technologies that will leapfrog current on-the-road technologies. A key element in this is attaining weight, volume, efficiency, and cost targets for the PE and EM subsystems of the traction drive system using novel traction motor designs that result in increased power density and lower cost.

There are a variety of types of electric motors that can be applied to electric drive vehicles—e.g., DC, AC induction, permanent magnet, switched reluctance, and axial flux. Currently, most of the major vehicle manufacturers are using permanent magnet motors in their hybrids and EVs (e.g., Chevy Spark, Ford Focus Electric, Honda Fit EV, Nissan LEAF, Toyota Prius), with the major exception of Tesla Motors, which uses an AC induction motor in the Model S (as it did in the Roadster as well), and Toyota, with its Tesla-designed electric powertrain. Notably, in its first experimental dip into electrical vehicles with the Mini-E, BMW also used an AC induction motor, provided by AC Propulsion.

Very broadly, an induction motor uses AC current fed to the windings of the stationary outer stator to create a rotating magnetic field. Current is induced in bars in the rotor, which in turn generate magnetic fields that are attracted to the stator. The rotor’s induced current and magnetism cause it to follow the field generated by the stator, producing rotation and torque. AC induction motors contain no permanent magnets.

In general, induction motors have the advantage of being the most widely manufactured and used, but they do not currently meet the US DRIVE (Driving Research and Innovation for Vehicle efficiency and Energy sustainability) combined requirements of cost, weight, volume, and efficiency.

Permanent magnet motors, on the other hand, use magnets mounted on or embedded in the rotor to couple with the motor’s current-induced, internal magnetic fields generated by electrical input to the stator. Permanent magnet motors tend to offer a compact design with high torque density, and can take advantage of a lower current that induction motors. However, the cost of permanent magnet materials is an issue, as is the potential for thermal damage to the magnets.

Switched reluctance motors are potentially the lowest-cost candidates (no magnets), and are simple and robust. However, they have serious problems of high torque ripple, high noise, and a low power factor, and are significantly less efficient.

Thus, work is ongoing to devise new motors with lower loadings of permanent magnets, or different materials, or, in some cases, hybrid designs combining aspects of different motor technologies. In a 2012 paper published in the Journal of Electrical Engineering & Technology analyzing a hybrid motor structure, Beser et al. noted that:

Parallel to the growing technology, the demand for electrical motors with different characteristics has been gradually increasing in the industry. Therefore, the research concerning improvement of the electrical motors has been widely studied in the literature. New motor types have been proposed and existing motor types have been developed in these studies.

Magnet type motors and reluctance motors are among the popular subjects for the electrical motors in the literature. Various structures have been discussed and realized for these motors. Particularly, the interest in magnet type motors has been increased along with the improvements in the features of the magnet materials. The applications of the magnet type motors can be classified as permanent magnet synchronous motors (PMSM) and brushless dc motors (BLDC). Although performance of the magnet type motors is perfect at base speed, their speed range is quite narrow.

Reluctance motors include no magnet material on their rotors. This feature removes the demagnetization risk and makes the motor suitable for high temperatures. Reluctance motors operate according to the reluctance principle. They can be separated into two groups as switched reluctance motors (SRM) and synchronous reluctance motors (SynRM).

…Permanent magnet assisted reluctance motors (PMa-SynRM) have been designed to combine the advantages of magnet type and reluctance motors. Since these motors have high power density, high power factor, high efficiency and wide speed range, they become a considerable and popular topic. Because the produced torque is a combination of magnet and reluctance torques, this motor type can be called as a hybrid motor. The rotor can be designed in many different structures to provide both magnet and reluctance torques.

BMW’s approach. BMW notes that because there is usually only a limited amount of installation space inside a vehicle, as a rule, motors with high power output and torque and low weight are preferred; the efficiency of the drive system correlates directly to the electric range of the vehicle. As high-voltage batteries are expensive, BMW says, drive modules should be able to attain the maximum possible vehicle range from the available battery power using the highest degree of efficiency.

Permanent magnet motors deliver reluctance torque as well as permanent magnet (PM) torque. In a 2012 patent application, BMW inventors note that synchronous permanent magnet motors generate a difference between a series inductance in the direction of the magnets (that is, in the direction of the pole) and a cross inductance transverse to the direction of the pole, producing reluctance torque when the drive motor is suitably actuated. This additional torque acts in addition to the magnetic moment generated by the permanent magnetic flux.

BMW’s motor is primarily a synchronous permanent magnet motor, but with a specific arrangement and dimensions for the components used to produce the self-magnetizing effect only otherwise induced by reluctance motors. This additional excitation causes the electromechanical field formed by the current supply to remain stable even at high revs. The maximum revs of the motor developed for the BMW i3 are 11,400 rpm.

As known from the prior art, the current, flowing in the stator of the drive motor, in the field weakening range has a substantial d component. The magnetic field, which is generated by this current component, acts against the field of the permanent magnet. However, as a rule the flux density in the magnet material is not reduced; rather the magnetic flux is expelled from the stator. The magnetic flux seeks a path along an air gap, formed between the stator and the rotor, in the rotor iron of the drive motor. The bottlenecks that the magnetic flux finds in its path—the so-called magnetic pockets—induce in connection with the current of the stator the path of the magnetic flux to switch back and forth multiple times between the rotor and the stator. As a result, the flux density fluctuates in the teeth of the stator, so that the frequency of the flux density fluctuation significantly exceeds the base frequency of the electric drive motor. This situation leads to significantly higher iron losses and, thus, to a considerable reduction in the efiiciency of the motor. This effect occurs especially in the field weakening range, for which the motor should be optimized based on its operating characteristics.

A well-known possibility of suppressing the flux density fluctuations between the rotor and the stator consists of making the bottlenecks, which occur in the path of the magnetic flux, more uniform. This feature can be implemented by increasing the number of magnetic layers. However, such a design is impractical to manufacture, because then the magnets have to be very thin. Inherent in such an approach is the high risk that the magnets will break when they are inserted into their recesses. The result is a steep increase in the production costs.

The object of the present invention is to provide an electric drive motor that is intended for a vehicle, in particular a motor vehicle, as a traction drive, which has a higher efficiency when running in the field weakening range…This electric drive motor has a stator and a rotor having at least one pole pair. Each pole of a respective pole pair comprises a magnet arrangement comprising at least one buried magnetic layer. According to the invention, each pole has a number of magnetic flux influencing groups, each of which has a number of air-filled recesses, which are not assigned to a magnet of a respective magnetic layer for purposes of flux conductance.

Owing to the magnetic flux influencing groups the magnetic resistance along the air gap in the rotor iron of the motor can be homogenized in a simple and economical way. By introducing a number of magnetic flux influencing groups it is possible to suppress or at least minimize the fluctuations of the magnetic flux density in the teeth of a stator. This approach significantly reduces any iron losses that may occur, especially in the field weakening range. The result is the suppression of the switching back and forth of the magnetic flux between the rotor and the stator of the drive motor.

—US Patent Application Nº 2012/0267977

In the patent doc, BMW inventors note that the method is flexible for a number of different designs of electric drive motors, and that such motors could be produce at low cost, because the recesses can be introduced during the punching process.

The method, they note, leads to an increase in efficiency especially at high speeds, and that the result from using such a motor would be an extended cruising range using an existing battery having a specified energy content.

On the other hand, the required battery capacity can be reduced while still retaining a given cruising range of a battery-powered vehicle. In this case a vehicle can be made available at a reduced cost, because at the present time the electric battery accumulator represents the largest cost factor of the vehicle.

—US Patent Application Nº 2012/0267977




Most future EVs will certainly benefit from improved reduced weight e-motors, controls, batteries, wheels, tires, lighter more aerodynamic bodies, improved more efficient e-ancillaries etc.

A redesigned, light weight 1500 to 2000 lbs EV could carry 4-5 passengers, 500 to 700 Kms per full charge by 2020 or so.

Will one of the current major like BMW do it or will Tesla be the leader again?


AC induction motors don't meet the "combined requirements of cost, weight, volume, and efficiency" so Tesla got it all wrong ? I have seen the motor of the Tesla S and it is barely bigger than a big water melon, and Tesla offers the best electric range at the moment. I don't understand or there is something I miss


What does the Tesla motor weigh?
I haven't managed to dig out figures.


Treehugger - of course you "missed it". In reality you are right. Although SRM is the best (better than induction motor) in terms of ...everything people have a problem dealing with torque ripple. High degree of sofistication of a motor drive is required to extract all advantages of this electric machine. Guys, it is "only a software". However, power stage of motor drive is also not simple in order to meet the software sofistication requirements. Controlling the current shape in the stator allows elimination of torque ripple but ... well it is a long story and subject of university course.


Permanent magnet motors are more suitable for low powered applications, while AC induction motors are more suitable for high powered applications.

Permanent magnet motors are susceptible to overheating; over-cooking the neodym can ruin it.


In this MIT paper from 2011 (, where induction and permanent magnet motors were compared, it's said in conclusion:

"The induction motor presented here, designed with the fundamental requirement that it be able to deliver a
peak power of 50 kW over a relatively narrow speed range, turns out to be nearly the same size as the
permanent magnet motor designed for the same purpose. In application with a typical driving cycle, the
induction machine turns out to be more efficient. This conclusion is robust, even if the permanent magnet
machine is more efficient at its peak efficiency operating point."

It's known fact that induction motor is significantly cheaper to build, perm. magnets are expensive. Unless some scientific breakthrough is made to make them from inexpensive materials.

IMHO, the most probable reason car companies (except Tesla, original GM-Ev1, and current GM eAssist mild hybrid) avoid using induction motors is the lack of engineers able to write control software for them. It's said they're much more complex to program, the vector control. Due to lack of demand, few e-motor makers offer them.



Don't know the weight of Model-S' motor, but do know for Tesla Roadster.
Tesla Roadster initially used motor of 70 lb (~ 32 kg), 180 kW peak, maybe more KWs. They found out that the powerful, air-cooled motor would quickly overheat, so they increased mass to have higher thermal capacity (so it can run at higher power for longer, still air cooled).
The newer motor was 110 lb (50 kg) heavy, 225 kW peak, air cooled. So it's 225/50 = 4.5 KW/kg, peak power, perhaps 30 second peak, not sure.
The sport model of roadster, with hand-wound motor may have a little higher value for max power.

Induction motor can easily be overloaded by 150% (ie 2.5 times continuous power) for short time. Passenger car accelerations need for peak power is usually below 10 sec, for acceleration, so induction motor is very suitable for that purpose.

Tesla Model-S uses water/liquid cooled motor, I don't know its motor weight.

Probably the highest value KW/kg have some axial flux PM motors, some may have 4 KW/kg.
Check out sites of EVO Electric and Oxford YASA Motors.
They don't spin very fast, have large diameter.
EVO Electric motor was used in a series hybrid racing car in Jan 2013 Dakar rally, coupled to 5 (or 6) speed transmission (climbing dunes requires lots of torque). It did finish the race.

Henry Gibson

In recent years ABB introduced a new more efficient form of motors; this they call the synchronous reluctance motor. It is more efficient than the standard squirrel cage motor and is more powerfull with the same weight. It requires an electronic drive to operate but it might be possible to operate from the mains after starting it. The rotor has certain similarities to the one in this article.

ABB has a line of large motors that operate on thousands of volts so that the costs and inefficiencies of step down transformers are eliminated. It is conceivable that such motors equipped with reluctance rotors can be started with electronics and operated from the mains. At least two such inductance motors are operated from 30,000 volt direct current undersea cables through electronic drives.

It is now logical and economic to start eliminating Alternating current distribution from buildings. AC wires running through metal conduits loose a lot of energy. Many CFLs can run on direct current now. Most chargers for phones and computers can run on direct current as can most most desk computers and electronic light ballasts.

It is amusing that motors for automobiles are required to be efficient but automobiles and their operation is not required to be efficient. High speed operation eliminates all advances in motor or engine design as does poor aerodynamics.

Low power cheap electric automobiles as plug in hybrids can do most of the work required of personal automobiles and even tiny range extender generators are adequate for unusual longer trips. Regular high speed automobile travel is well done with ordinary ICE engines. The cheapest most efficient and lowest weight drive system seems to be the Hydraulic hybrid. Artemis now has an operating standard size hydraulic hybrid windmill and may soon have the largest windturbines of the world. No electrical transmission can best the size and weight advantages of the hydraulic system. The high efficiency switched valve motors of Artemis reduces greatly the losses of low power operation. The free piston hydraulic engine of NOAX type can be combined with Artemis motors for even higher efficiencies.

No engine oil changes are ever needed for Bladen jets or Capstone turbine powered automobiles. The Bladen jets version uses switched reluctance generators and motors.


Many thanks for the info.
It appears then that BMW's claims for the superiority of their engine are unwarranted.

william g irwin

As stated above, no matter the drive train type or efficiency, the vehicle efficiency is the limiting factor at high speeds. Air resistance is related to the cube of speed as I recall. Slippery is very important.
It seems like most driving today is done on expressways around inner/outer loops and interstates even to get to the local mall. It seems like another large portion is done on 'side' roads at ~40 to 50mph. Not too much done in the city at low speeds unless stuck in accident or rush hour traffic.
Anyhow, I have read that a fair amount of drive train efficiency is lost in the drive components themselves - like trannys and diffys etc. Not so much a problem when the ICE is low efficiency already, but crank up the motor efficiency and the drive train becomes more important. Doesn't this call for simpler or no diffy or tranny - direct drive? This implies a trade off between gear reductions, motor design speeds, and dynamic range. Seems like lots of room for o/a drive train design variations/optimizations. How much of this fancy motor's power/efficiency gets to the road?

Kit P

Careful reading of the above material including linked material reinforces my prediction that BEV will be museum pieces in 20 years rather than a common part of our transportation system. The reason is simple. The ICE works so well for transportation.

Most of the reason for promoting BEV are just bogus. Some politicians think that we are running out of fossil fuels, AGW is catastrophic, and the environment is going to hell in a hand basket. For these reasons they pour tax dollars into R&D. I actually think that is great because you never know what the benefit might be.


The people who designed and built this engine are looking to electric as the future for cars.

ICE is fine.
So were horses, but that didn't make them the ultimate in transport.

Kit P

"So were horses ...."

And? There are many examples of technology replacing something rapidly because it was clearly better.

BEV have been around a long time but it was the ICE that became dominate.


What part is too difficult for you to understand about BMW making this to assist their electric cars?

Try reading the facts for a change, instead of inventing them.

Kit P


I did not have any problem understanding the BMW press release and remaining skeptical. Generally speaking I am skeptical of of all car makers press releases.

My first questions what does it cost. I will pay more for a quality product but when it comes to 5 passenger cars it is hard to beat a Civic or Corolla. I am sure Davemart does not understand the concept of value for your money. A case of substance over marketing.

Alan Parker

I hope Kit P visits here in 20 years (if gcc is still running) to see his prediction about BEVs being relegated to museums being proven false. Every trend is now pointing towards the fact that the ICE will not be the dominant form of propulsion for light duty vehicles in 20 years. For new light duty vehicles it probably won't even be true for new sales in 15 years. The ICE is horrible for propulsion, the only 'good' thing about an ICE car is the energy density and practicality of the fuel, everything else is just working around all the problems. Here we have facts.

Kit P

So Alan what do you drive?

The trend for ICE is that they are lasting longer and longer. I drove my '89 Ford Ranger to work today as I do almost everyday. I have not changed to spark plugs in 15 years since I bought it. At the time I was thinking that I would get a few years service for my $1200 investment.

It runs like new, how horrible is that?

"everything else is just working around all the problems"

What problems are you talking and why would you think a BEV will solve them?

At this point a few people buy BEV to be different. The silance of BEV is deafening. When BEV owners start talking about 20 years of relaible service, you will see more BEV on the road instead of the junk yard. In that case you will not need to go to museums to see them.

Those are the facts Alan and you may want to consider that there is a difference between prdeictions and facts.

Alan Parker

Kit P I drive a 2012 Nissan LEAF. My other half has a Prius and there are about 1% of journeys in the last six months that couldn't be done in the LEAF. I'm never buying another pure ICE again, I would consider something like the BMW i3 with Rex, it depends on how fast the batteries develop.. BEV has massive scope for improvement whereas ICE is pushing against the law of diminishing returns. That's why I'm willing to bet real money with anyone that still thinks the ICE will be the dominant force in the next few decades.

So you don't like the silence of a BEV? I guess you've driven one for a few months then and really decided you don't like it? If so then fair enough, if not then you're really in no position to compare the two technologies. Since I've driven an ICE for decades prior to the beginning of this year I'm qualified to comment and I'm 100% sure you'll live to be proven wrong. But hey who knows maybe a new source of mega cheap oil will appear soon and the ICE will stop needing gears and sounding like a tractor.


I agree with AP. California is already getting close to 10% electrified vehicles.

Others will follow California's lead soon.

Kit P

“So you don't like the silence of a BEV? ”

That is it? That is the big problem BEV solve. My ICE are very quite.

“BEV has massive scope for improvement ”

Certainly the Leaf has a lot more room for improvement than my '89 Ranger. Common sense says that Alan should have waited to buy a BEV until the improvements were made.

“I'm willing to bet real money ..”

How much did the leak cost?

“no position to compare the two technologies ”

Two things here. First, I did not have to have personal experience in every 5 passenger car when I bought my wife's 2007 Corolla. I went down to the library to check out Consumer Reports. I could rule out cars with poor reliability and mileage, and high cost. Then I could rent a couple of cars for the weekend to see if my wife liked it. Second, there is no data to support BEV. I even asked the Toyota dealer if he has some data to support the higher cost of the pious.

“I'm qualified to comment ”

There is no qualification to blog on the internet. I was 'qualified' to supervised the operation of my ships reactors. I still had to justify what I did with reason for what I did.

“mega cheap oil will appear soon ”

This might be a poor example of a economic reason. Maybe someday there may be a good economic reason. Common sense says that Alan should have waited to buy a BEV until that day.

“I'm 100% sure you'll live to be proven wrong. ”

Alan would be 100% wrong. First, I am an old guy. Second ICE would have to get worse. My first car was an '60 Ford Falcon. The Leak is a better car.

Alan Parker

We seem to have gone from arguing whether or not the ICE will still be dominant in 20 years to whether or not I jumped too early by getting a LEAF. Well maybe I did, but someone has to. Anyway I'm leasing it until the beginning of 2015. The monthly cost including electricity is really very attractive, if it wasn't I would not have done it.

After the lease is up I really want a BMW i3. Originally I wasn't a fan of leasing but since this technology is moving so fast I'm thinking that leasing might be the thing to do for the next 10 years or so. We shall see.

Anyway this is too many comments for gcc, so if you want the last word go ahead.

Kit P

"monthly cost including electricity is really very attractive"

What is the cost?

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