The electric motor is driven by a dedicated frequency inverter. This inverter is speed and torque controlled in order to provide the requested power to the propeller shaft depending on the operation mode. In the electric operation, the inverter controls the motor in speed mode. When the diesel motor take over, in order to provide a smooth transition, the speed of the electric motor is synchronised on the diesel motor speed and the drive is switched in torque control mode. During the diesel operation the drive is controlling the motor in torque providing the requested power to charge the batteries. Modern battery systems request a sophisticated charging curve the inverter must provide independently of the driven shaft speed.
The power range of the inverter is adapted to the motor power. Small inverters are air cooled and mostly provided in IP 55 enclosures. Bigger inverters up to 200 ... 250 kW are liquid cooled in order to reduce the size and optimize the heat transfer.