Method for operating a motor vehicle drive train

A method for operating a drive train of a motor vehicle, the drive train utilizing at least one permanent-magnet electric machine (EM) as the motor vehicle drive source, the at least one permanent-magnet electric machine (EM) including a rotationally fixed stator (ST) and a permanent-magnet rotor (RO), a transmission (G) arranged in a power flow between the electric machine (EM) and driving wheels (DW) of the motor vehicle, the transmission (G) configured for selectively implementing different gear ratios between an input shaft (GW1) and an output shaft (GW2) of the transmission (G), a power converter (LE) associated with the electric machine (EM) and configured for converting a direct current voltage of an electrical power supply (BAT) into an alternating current voltage for operation of the electric machine (EM), the method comprising:

• selecting one of the gear ratios of the transmission (G) depending on a driving strategy of the motor vehicle;
• operating the electric machine (EM) in a field weakening condition with the power converter (LE), a magnetic field of the stator (ST) counteracting a magnetic field of the permanent-magnet rotor (RO) in the field weakening condition in order to reduce a voltage induced by the rotor (RO) in the stator (ST);
• determining a temperature value of the electric machine (EM) and/or of the power converter (LE);
• and
• depending on the temperature value, adjusting a changeover of the one of the gear ratios of the transmission (G) specified by the driving strategy via prematurely carrying out, delaying, or preventing the changeover in order to reduce, not increase, or increase with delay a rotational speed of the rotor (RO).