Introducing three temperature correction compensation models for stators of new energy vehicle motors in engineering applications

First of all, some colleagues may have a question: since there is a stator temperature sensor, why do we need to make a stator temperature correction model? Here we discuss its necessity as follows.

• The temperature sensor measurement value of the motor stator is not necessarily the highest temperature point in the winding. Correction and supplementation are needed to determine the maximum temperature of the stator winding to avoid motor damage under any operating conditions.

• When there is a problem with the stator temperature sensor, the stator temperature needs to be estimated so that the motor can support the vehicle limp home.

The following introduces three stator temperature correction compensation models used in engineering applications.

1. Model I

The higher the rotor speed, the more uneven the cooling effect of the surrounding air on the entire stator winding. Therefore, the temperature measured by the stator temperature sensor deviates greatly from the actual winding temperature. Model I corrects the stator temperature based on the stator temperature measurement value by correlating it with the motor speed. As shown in Figure 1.

Figure 1. Stator temperature correction model based on stator temperature measurement and rotor speed (T is temperature in the figure)

2. Model II

Based on the thermal equivalent loop model, the stator temperature compensation value is calculated by the stator current and the rotor speed to correct the stator measurement temperature.

Figure 2. Stator temperature correction model based on stator temperature measurements, phase currents, and rotor speed.

According to the thermal equivalent circuit model, referring to the formula derivation process of "New Energy Vehicle Motor Controller Temperature Calculation and Its Model - DC Capacitor", it can be obtained:

According to the above formula, the ST-Filter model can be obtained as shown in Figure 3

Figure 3. ST-Filter model

3. Model III

Based on the thermal equivalent loop model, the stator temperature is estimated from the motor controller coolant temperature, phase current, and rotor speed.

Figure 4. Stator temperature calculation model based on coolant temperature measurements, phase currents, and rotor speed.

Taking into account the sudden change of coolant temperature, in order to avoid the calculated value of stator temperature changing too quickly, the coolant measured temperature value in model III is also filtered by the ST-filter module, which is the same as the ST model of model II.

4. Engineering application analysis

1) Model I has a simple structure and does not consider the influence of phase current. The calibration workload in engineering applications is small. Compared with Model II, its dynamic accuracy is lower.

2) The time constant for thermal capacity filling, i.e. Tau, varies with the rotor speed and phase current, and can be obtained through FEA simulation. Its value varies for different motors.

3) The △T_offset lookup table in the ST-Filter module changes with the rotor speed and phase current. The highest temperature position point of the stator winding under different working conditions can be obtained through FEA simulation, and then the △T_offset lookup table can be calibrated and corrected on the test bench or the whole vehicle.

4) Considering the initialization value of T_offset in the ST module, after the motor controller is powered off and shut down for more than n τ, it is considered to be in steady state, that is, thermal equilibrium, and the temperature of the motor stator is the same as the temperature of the coolant. For the ST-Filter module, the following points need to be considered when modeling:

-. Read the power-off time to determine whether the temperature of the motor stator has reached thermal equilibrium;

-. If the motor controller is restarted after power failure, and the stator temperature reaches thermal equilibrium, the initialization value of T_offset in model II is set to 0, and the initialization value of T_offset in model III is set to the coolant temperature value.

-. If the motor controller is restarted after power failure and the stator temperature has not reached thermal equilibrium, it is necessary for the motor controller to store the latest T_offset in NVM before power failure and consider the temperature cooled down within n τ, for example: use the time length to look up the table to correct the T_offset value in NVM, and finally this value is used for the initialization value of T_offset.