# Motor Vector Control Based on Speed-Torque-Current Map
## 3 Motor Vector Control Strategy Based on Speed-Torque-Current Map
### Motor Vector Control Strategy  
In order to solve the nonlinear problem of the PMSM current in the weak magnetic region, a vector control strategy is designed for the motor based on the speed-torque-current map, as shown in Figure 8. When the motor runs in the weak magnetic region, the d and q-axis currents are obtained using the lookup table method. This method avoids the control problems caused by the nonlinear current, which in turn extends the weak magnetic working region of the motor. It is convenient to obtain the current in the deep weak magnetic region by using the lookup table. Besides, the following problems can also be solved: it is difficult to obtain the current simply by using the weak magnetic PI controller, and the dynamic response of the motor is slow. Figure 7: Current map of the integrated control strategy. (**a**) d-axis current map; (**b**) q-axis current map. As shown in Figure 8, a speed-torque-current map link is added to the vector control strategy between the speed control link and the current control link. The output reference torque T\({}_{\text{ref}}\) of the speed loop and the actual motor speed are the inputs to this link. The reference currents of the d and q axes corresponding to the current speed and the target torque can be obtained by a lookup table. Since the speed-torque-current map contains the information of the weak magnetic region (including the deep weak magnetic region), the currents of the d and q axes are constrained in the voltage limit circle and current limit circle. It solves the transition problems of current control in different working ranges and the problem of torque fluctuation caused by current or voltage over-saturation. Charging and discharging are always present in the driving process of pure electric vehicles, which leads to the change of battery SOC (state of charge). The relationship between the battery voltage and SOC is shown in Figure 9. The change of the battery voltage causes the dynamic change of bus voltage amplitude [25], which in turn affects the d-axis current and q-axis current, as well as the speed and torque characteristics of the motor. The external characteristics of the motor under different bus voltages are shown in Figure 10.  
It is shown in Figure 10 that when the bus voltage decreases, the control boundary and the maximum torque boundary of the motor move to the left, which reduces the motor working range. If the influence of bus voltage variation is ignored, the operation of these areas cannot be controlled. When the voltage increases, the bus voltage cannot be fully utilized. When the voltage decreases, the working point may be outside the allowable motor working range, which leads to the over-current or over-voltage protection. The influence of bus current on the motor can also be seen from the perspective of current control of d and q axes, which is shown in Figure 11. When the bus voltage is U1, the motor works at points A1 and A2 to realize torques T1 and T2, respectively. If the bus voltage increases from U1 to U2, then the motor working points are B1 and B2, which improves the utilization ratio of the bus  
Figure 8: Vector control strategy based on the speed–torque–current map. Figure 9: Relationship between battery voltage and state of charge (SOC). voltage. On the contrary, if T1 and T2 are the required torques, the motor works at points B1 and B2. If the bus voltage decreases from U2 to U1 and the control strategy still takes B1 and B2 as working points. In this situation, the current regulator saturates and loses its ability to regulate the current. Considering the fluctuation of bus voltage, the relationship between the actual bus voltage U\({}_{\text{dc\_act}}\) and the currents of the d and q axes is as follows:  
\[\left(\frac{\text{U}_{\text{dc\_act}}}{\omega_{\text{s}}}\right)^{2}=(\text{ L}_{\text{q}}\text{i}_{\text{q}})^{2}+(\text{L}_{\text{d}}\text{i}_{\text{d}}+ \varphi_{\text{f}})^{2}. \tag{15}\]  
Figure 11: Influence of bus voltage change on the value of the d-q axis current.