BMW’s hybrid motor design seeks to deliver high efficiency and power density with lower rare earth use
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
US Patent application Nº 2012/0267977: Electrical Drive Motor for a Vehicle
Esra Kandemir Beser, Sabri Camur, Birol Arifoglu and Ersoy Beser (2012) Analysis and Application of a Hybrid Motor Structure Convenient to Modify the Magnet and Reluctance Torques on the Rotor. Journal of Electrical Engineering & Technology Vol. 7, No. 3, pp. 349-357 doi: 10.5370/JEET.2012.7.3.349