Motor rotor temperature is a key factor that has an important impact on motor performance and safety. However, many electric drives currently do not have the function of rotor temperature evaluation. Under the background that OEMs have increasingly higher requirements for control accuracy, rotor temperature estimation is particularly important.
Rotor structure
1, why rotor temperature estimation is needed
(１) Rotor temperature affects torque
The relationship between permanent magnet synchronous motor rotor temperature and torque can be simply described as:
Torque (T) = Current (I) × Magnetic Flux (Φ)
Among them, the current (I) is the input of the motor, and the magnetic flux (Φ) is related to the rotor temperature. When the rotor temperature increases, the Br of the magnet will decrease and the magnetic flux (Φ) will decrease, thus affecting the output of torque (T). Therefore, there is a negative correlation between rotor temperature and torque, that is, as the rotor temperature increases, the torque output decreases.
Motor temperature and torque relationship diagram
(２) Estimating rotor temperature helps protect rotor from demagnetization
Water-cooled motors have limited cooling effect on the rotor, so the rotor temperature often remains high after the stator temperature cools down. Excessive rotor temperature will cause demagnetization of the magnets and affect the performance of the motor. If the temperature of the rotor can be effectively known, some protection strategies can be implemented for the rotor to prevent the demagnetization effect caused by overheating of the rotor.
Magnetic steel demagnetization curve
2, difficulties in estimating temperature with electronic control
Motor torque control diagram
Increased calibration requirements: As mentioned above, if a temperature variable is added, the rotor temperature needs to be known very accurately. At present, the method used to measure the rotor temperature is infrared remote sensing. This method must be shielded very carefully, otherwise the temperature will not be accurate, which means it will be white calibration.
The control requirements are high: the controlled flux linkage is written in advance, but due to differences in each motor, the flux linkage of each motor must be different. If the redundancy of the control algorithm is not high or the consistency of the raw materials cannot be guaranteed, it does not make much sense because the estimated temperature is also inaccurate.
3, summary
In order to improve motor performance and safety, the controller needs to have the ability to monitor and regulate the rotor temperature. By accurately monitoring and controlling the temperature of the rotor we can protect the permanent magnets from high temperature damage and maintain stable operation of the motor. In addition, by monitoring the rotor temperature, we can also predict the performance changes of the motor and find and solve potential problems in time.
(Source: Electric Drive Home)