Electric gears to increase performance of future EVs
Current electric vehicles almost exclusively use permanent magnet motors, because they have high power density and excellent efficiencies. However, a disadvantage of their permanent magnets is that their magnetic flux cannot be adjusted. With such a fixed magnetic flux, the torque of the motor is approximately proportional to the current, while the motor voltage is proportional to the speed.
Low speed, high torque
At low speeds, there is no problem: the induced voltage in the motor winding is well below the voltage that the traction inverter can supply based on its battery voltage. The rest of the supplied voltage can be used to get the desired currents in the motor winding, and thus to achieve a sufficiently high torque. Such an operating point with low speed and high torque is typical for an acceleration from standstill.
High speed, low torque
At a certain speed, the induced voltage in the motor winding becomes the same as the maximum voltage that the traction inverter can supply. This puts a limit on the maximum speed. In order to achieve speeds above this limit, the magnetic field in the motor has to be reduced. This can be done by so-called field weakening: the motor currents can create an opposing field that partially compensates the field of the permanent magnets. The speed can then be further increased, but at the cost of significantly less achievable torque: the current that is used for field weakening, cannot be used for torque production anymore. However, this operating range typically corresponds to driving at an almost constant speed on the highway, so the strong reduction of the available motor torque is not a problem.
What does make field weakening undesirable, is that the transistors in the inverter must be significantly oversized to conduct the field weakening currents. The extra currents in the motor winding also give rise to additional losses, which reduces the driving range of the vehicle. These two limitations are tackled by the electric gear that was studied in one of our other SBO research projects.
Dynamic reconfiguration of the motor winding
The only requirement to make an electric gear from a standard electric motor drive, is that the phase windings of the electric motor need to be divided into multiple identical parts. Except from this, no adjustments need to be made to the electric motor drive. Or in other words, the specifications such as maximum torque and rated power of the motor do not change.
How does an electric gear work? At the low speed/high torque driving mode, there are no issues. Just like in the original motor, the individual parts of the motor winding can be connected in series. The inverter voltage is sufficiently high compared to the induced voltage in the motor, so that a sufficiently high current can be realized in the motor windings. This electric current flows through all the turns of the winding, so that the maximum motor torque can always be delivered.
At high speed, however, the induced voltage in the series-connected windings exceeds the inverter voltage. By reconfiguring the motor windings to a parallel connection, the induced voltage of the phase winding can be reduced. The speed range can hence be extended. But because the inverter current is now distributed over parallel branches, the maximum deliverable torque of the motor decreases. However, as mentioned earlier, this is not a problem for an electric vehicle application.
Further research
In the Horizon Europe follow-up project called HighScape, in which our consortium partner Bluways is also involved, we are currently examining how the electric gear can be combined with an on-board battery charger. The reconfiguration switches, the motor winding, and the inverter are reused in both functionalities. Because the motor winding is reused as a filter in the on-board charger, the big challenge is removing the motor torque during the charging mode.
