At the beginning of the 1990s, politicians and the general public called for the use of electric drives in motor vehicles. It is, in principle, possible to operate these vehicles without emissions at the place of use. The challenge is to provide these vehicles with sufficient electrical energy on board.
The state of the art for electric drives for road vehicles has been - and still is - the three-phase induction motor, whether with a synchronous or asynchronous machine. Although the efficiency achieved with three-phase drives can be described as very good - depending on the operating point of the machine the efficiency is between 80 and 90% - there is still an potential for improvement here. This also applies to the transverse flux motor (TF) and the switched reluctance machine (SR).
The following project objectives have been defined:
- Demonstration of the functionality of SR technology for road vehicles
- Reaching the technical characteristics of synchronous / asynchronous drives
- Showing the cost advantages of SR technology.
- Demonstration of the functionality of TF technology for road vehicles
- Validation of the improved efficiency against existing synchronous / asynchronous drives
- Minimization of the costs by using powdered iron materials in the stator
- Validation of improved efficiency in generator operation versus existing generators
- Demonstrating the resulting emission and fuel consumption advantages for serial hybrid vehicles
In order to achieve these project objectives, the following project content was agreed:
- Development, design and testing of a prototype
- Testing the prototype on the test bench
- Installation and testing of the prototype in a vehicle
- Development, design and testing of a propulsion prototype
- Testing the propulsion prototype on the test bench
- Installation and testing of the propulsion prototype in a vehicle
- Development, design and test of a generator prototype
- Construction of an APU (Auxiliary Power Unit) with TF generator
- Test of the APU
The OKOFEH project examined possible solutions of applying SR technology and TF technology in an electric car. However, during the implementation of the project, there were unexpected technical problems for both technologies which could not be satisfactorily solved over the course of the project.
In the case of the SR technology, a prototype was constructed that could essentially meet the specifications of torque and performance. This prototype was successfully tested on a motor test bench. Together with a transaxle gearbox, it was also tested on a transaxle test bench.
For the TF technology, it was not possible to build a drive according to the specifications of the car manufacturer. This problem has emerged during the course of the project after the design and testing of a motor prototype. The reason for this is that the previously known calculation methods for the design of a machine as complex as the TF machine were found to be insufficient during the course of the project.
In summary, it can be said that both technologies can not be used for electric road vehicles in the passenger car sector at this time and in the near future. Before this can happen, significant research and development efforts must be made for both technologies:
In the case of SR technology, it has to be examined whether the high noise level is a specific feature of this technology and therefore makes it unsuitable for use in the vehicle or whether there are measures to remedy this defect satisfactorily. Furthermore, it is necessary to examine the extent to which the cost advantages of the SR machine are maintained in series production for the entire drive if the production of special semiconductor bridges makes cost optimization possible for the SR inverter.
In the case of the TF technology, significant improvements in the powder material have to be achieved in order to be able to build a TF machine which meets the required performance and efficiency and the limited available installation space in the passenger car.
Findings of the study are published in detail by a final report (German only) which is available online via https://www.tib.eu/de/suchen/id/TIBKAT%3A50646217X/Optimierte-Komponenten-f%C3%BCr-Elektro-und-Hybridfahrzeuge/?tx_tibsearch_search[searchspace]=tn