Novel electric motor design

A full-size electric motor dedicatedly designed for long-haul trucks have been prototyped and prepared for tests at Chalmers. The motor with continuous power at 200kW is specified for uphill climbing in a hybrid powertrain in Volvo long-haul trucks.

The most energy of a long-haul truck is consumed in its cruising phase with high speed and low torque. To order to reach higher efficiency for an electric motor and its inverter in the cruising phase, magnetic field in the motor must be optimally controlled. If the core losses and reactive power is minimized, the losses in both the motor and its inverter will be minimized. The highest drivetrain efficiency will be achieved.

The selected topology of the electric motor is an Electrically Excited Synchronous Machine (EESM) because its field current can be controlled. The field current can be set to an optimal value to keep high power factor and enough toque. In the cruising phase, only a limited field current is required due to its lower torque.

To eliminate extra mechanical friction and regular maintenance, brushes and slipring for the field current injection are replaced by a wireless connection. The wireless power transfer is undertaken in high frequency. Therefore, the size of the device can be small enough compared to machine size.

In order to increase the motor power density and efficiency, direct oil cooling is applied for the stator winding and the rotor winding. It is necessary measure to keep the rotor temperature low during high torque operation.

The simulation results show the electromagnetic losses can be reduced by 50 % compared to a commonly used Permanent Magnet Synchronous Machine (PMSM) at the cruising operating point. The power factor can be increased by 45 %.

The prototype EESM is installed on a back-to-back test bench and coupled to a same size PMSM. Two “home-made” inverters using silicone-carbide power modules will drive both machines. Test results will help us to verify the findings in the simulations.