Method for performing rotational speed synchronisation

An electrical powertrain system for a vehicle, the electrical powertrain system comprising:

• an electrical storage system,
• an inverter,
• a three phase AC electric propulsion motor drivingly connected to a wheel of the vehicle via a stepped gear transmission having a first transmission component and a second transmission component, wherein the AC electric motor is rotationally connected to the first transmission component, wherein the first transmission component of the stepped gear transmission is a gear wheel, wherein the second transmission component of the stepped gear transmission is an axially displaceable shift sleeve, and wherein the inverter is arranged for controlling the voltage and current supplied from electrical storage system to the three phase AC electric propulsion motor,
• a switching mechanism with a set of electrical switches for enabling the stator windings of the AC electric motor to be selectively connected in star or delta configuration, and
• a control unit for controlling operation of the switching mechanism and the inverter, wherein the control unit is configured for:
• obtaining a command, or determining, that a gear switch should be performed,
• setting the stepped gear transmission in a neutral state,
• shifting the stator winding configuration of the AC electric motor to delta configuration, or ensuring that the stator winding configuration of the AC electric motor is in delta configuration,
• controlling operation of the AC electric motor for accomplishing rotational speed synchronisation of the first and second transmission components,
• displacing the shift sleeve axially for providing a torque transfer connection between the first and second transmission components, and
• shifting the stator winding configuration of the AC electric motor to star configuration when the rotational speed synchronisation of the first and second transmission components is accomplished.