Method and apparatus for controlling an electric machine

An apparatus controlling an electric machine by means of an inverter, wherein the inverter has a plurality of controllable power switches which are configured to convert a direct voltage of a voltage source, coupled to the plurality of controllable power switches, into an alternating voltage for supplying the electric machine with electrical energy, the apparatus comprising a control unit configured to

• switch the electric machine into an open-circuit mode, wherein in the open-circuit mode all of the plurality of controllable power switches of the inverter are opened,
• detecting a phase current of the electric machine,
• compare one of the phase current or a first variable derived from the phase current to a first reference value,
• switch over the electric machine from the open-circuit mode into a short-circuit mode based on the comparison of the one of the phase current or the first variable derived from the phase current to the first reference value, wherein in the short-circuit mode a portion of the plurality of controllable power switches that are assigned to a first potential of the voltage source are closed, and wherein a second portion of the plurality of controllable power switches that are assigned to a second potential of the voltage source are opened,
• compare one of the phase current or a second variable derived from the phase current to a second reference value, and
• switch over the electric machine from the short-circuit mode into the open-circuit mode based on the comparison of the one of the phase current or the second variable derived from the phase current to the second reference value.

A motor vehicle drive train comprising:

• an electric machine configured to make available drive power;
• and
• an apparatus for controlling the electric machine, the apparatus including a control unit configured to
• switch the electric machine into an open-circuit mode, wherein in the open-circuit mode all of the plurality of controllable power switches of the inverter are opened,
• detecting a phase current of the electric machine,
• compare one of the phase current or a first variable derived from the phase current to a first reference value,
• switch over the electric machine from the open-circuit mode into a short-circuit mode based on the comparison of the one of the phase current or the first variable derived from the phase current to the first reference value, wherein in the short-circuit mode a portion of the plurality of controllable power switches that are assigned to a first potential of the voltage source are closed, and wherein a second portion of the plurality of controllable power switches that are assigned to a second potential of the voltage source are opened,
• compare one of the phase current or a second variable derived from the phase current to a second reference value, and
• switch over the electric machine from the short-circuit mode into the open-circuit mode based on the comparison of the one of the phase current or the second variable derived from the phase current to the second reference value.