Determination of the rotor temperature of a permanent magnet synchronous machine

1. A method for determining a rotor temperature of a permanent magnet synchronous machine in an electrically driven vehicle, comprising:

in a driving state in which no torque is demanded of the permanent magnet synchronous machine, operating a power electronics system supplying the permanent magnet synchronous machine with energy in a current regulation mode;

regulating the power electronics system such that an alternating current on an AC side of the power electronics system is zero;

while the alternating current on the AC side of the power electronics system is zero, measuring a direct voltage on a DC side of the power electronics system and an electrical angular velocity of the permanent magnet synchronous machine;

calculating an alternating voltage applied to the AC side of the power electronics unit from the measured direct voltage and a duty cycle of the power electronics unit; and

calculating the rotor temperature based on the measured direct voltage and the measured electrical angular velocity, wherein the rotor temperature is calculated from the calculated alternating voltage applied to the AC side of the power electronics unit and the measured electrical angular velocity of the permanent magnet synchronous machine according to rotor temperature≈alternating voltage divided by the measured electrical angular velocity.

2. The method according to claim 1 , wherein the driving state, in which no torque is demanded of the permanent magnet synchronous machine, is a coasting mode.

3. An apparatus for determining a rotor temperature of a permanent magnet synchronous machine, comprising:

a power electronics unit that feeds the permanent magnet synchronous machine with alternating current;

at least one current sensor arranged on an AC side of the power electronics unit, which is configured to measure an alternating current in at least one phase supplying the permanent magnet synchronous machine;

at least one voltage sensor arranged on a DC side of the power electronics unit, which is configured to measure a direct voltage applied to the DC side of the power electronics unit;

at least one rotational speed sensor, which is configured to measure a rotational speed of the permanent magnet synchronous machine; and

at least one device for controlling the power electronics unit and for receiving and processing the measured direct voltage and the measured rotational speed from the sensors, for calculating an alternating voltage applied to the AC side of the power electronics unit from the measured direct voltage and a duty cycle of the power electronics unit and for calculating the rotor temperature of the permanent magnet synchronous machine based on the measured direct voltage and the measured rotational speed, wherein the rotor temperature is calculated from the calculated alternating voltage applied to the AC side of the power electronics unit and the measured electrical angular velocity of the permanent magnet synchronous machine according to rotor temperature≈alternating voltage divided by the measured electrical angular velocity.

4. The apparatus according to claim 3 , wherein the at least one device for controlling the power electronics unit and for receiving and processing the measured data from the sensors and for calculating the rotor temperature of the permanent magnet synchronous machine is configured to regulate an alternating current on the AC side of the power electronics unit to zero.

5. The apparatus according to claim 3 , wherein the at least one device for controlling the power electronics unit and for receiving and processing the measured data from the sensors and for calculating the rotor temperature of the permanent magnet synchronous machine is configured to determine an electrical angular velocity of the permanent magnet synchronous machine from the rotational speed of the permanent magnet synchronous machine measured by the at least one rotational speed sensor.