165 - Creating an Efficiency Map for an IPM Motor
Module: DP, LS, RT
IPM motors use rare earth sintered permanent magnets because they have strong magnetic energy. They can use the magnetic torque from the magnet's field and the rotating magnetic field in addition to the reluctance torque that originates from the difference in inductance between the d-axis and q-axis, so they have a wide drive range and are highly efficient.
Their efficiency changes with their rotation speed and their load, so it is beneficial to create an efficiency map when designing the motor and its controls. However, the calculations required to create an efficiency map are typically huge, so it takes time to organize the results as well. Though is possible to estimate the efficiency by using the motor's voltage equation and torque formula to calculate the torque, voltage, and current, one cannot use this method to estimate the iron loss or account for the effects of the nonlinear magnetic properties of the motor's iron core. The main problem is the difficulty of correctly calculating the efficiency. To help with this problem, an efficiency map that accounts for influence from iron loss and nonlinear magnetic properties can be easily obtained by creating a JMAG-RT model of the target and using JMAG-RT Viewer's efficiency map calculation function.
This Application Note presents the use of the JMAG-RT Viewer to create an efficiency map for an IPM motor.
Speed-Torque curve / Efficiency map
Fig. 1 shows the Speed-Torque curve, and fig. 2 shows the efficiency map.
As seen in fig. 1, in the low speed region the torque becomes constant. This is due to the current controls. The torque also decreases as speed increases. This is because the current flowing to the motor coil decreases due to back electromotive voltage.
In fig. 2 it is possible to get a grasp of the efficiency at each drive condition. The range that maps the torque's positive value and efficiency also becomes the possible range of output for this motor. The range in which the torque has a negative value becomes the possible range of output and efficiency for the generator mode.