Field-weakening or flux-weakening is a technique for increasing the speed of an electric motor above its rating at the expense of reduced torque. Field-weakening is used for motor control in automation applications and traction motor control for electric vehicles and locomotives to achieve higher motor speed when lower torque is acceptable.

The永磁同步电机（PMSM) is popular in these applications because of its high power density, high speed, and fast dynamic response. However, PMSM speed is limited when the stator terminal voltage reaches the inverter output limit. Therefore, a PMSM requires field-weakening to increase its shaft speed above its design rating. One approach to achieve higher motor speed is to regulate the inverter power electronics to manipulate the stator d- and q-axis currents to counter the magnetic airgap flux generated by the rotor magnets.

Field-weakening control involves reducing the resulting d-axis flux,\(\lambda_{d}\), by lowering the effect of the resulting air-gap flux linkage associated with the permanent magnets,\(\lambda_{pm}\). This is done by driving the component of the magnetizing d-axis stator current negative in the PMSM as shown in Figure 1 below.

图2中的扭矩速度特性曲线表明，电动机的反电动机（定子电压）与电动机速度成比例地上升。此行为发生在PMSM的恒定扭矩区域中，其中field-oriented control(FOC) is an accepted way to regulate the motor. However, when the speed continues to rise, the applied voltage reaches maximum and the back EMF voltage exceeds the applied voltage, preventing the motor speed from increasing. To increase the motor speed above its base speed, field-weakening mode is used while maintaining a constant output power, which is the product of torque and motor speed. During field-weakening, the motor can rotate faster at the maximum available voltage, at the expense of reduced maximum torque.

图3示出了作为电压限制椭圆和定子电流左侧左侧的电流限制圆的交点的现场弱化控制操作（i_d， 一世_q) plane.

为了了解场弱化，可以使用绑定场弱区OABC的轨迹来评估当前的矢量轨迹。轨道I沿OA是每安培的最大扭矩（MTPA.) curve, where MTPA can be achieved by manipulating the current vector trajectory to match the OA curve. Trajectory II follows the current limit circle from A to B. The current limit is defined by the constraints of the DC bus and power electronics. Trajectory III represents deep field-weakening along BC, the maximum torque per volt (MTPV) curve. During MTPV operation, the motor generates the maximum speed and torque allowed within the voltage constraint ellipse, which is bounded by the DC bus. Regardless of the torque transient response, the optimized field-weakening trajectories or operating points are always located within the grey area.

图4显示了用于PMSM的现场弱化控制的系统级块图万博1manbetx^®。外速控制回路产生扭矩命令作为MTPA场弱化控制块的输入。内部电流环由克拉克和公园变换A.空间矢量发电机。

电机控制块集™提供参考例子显示field-weakening control，和代码生成部署，以帮助您使用现场弱化控制万博1manbetx。

To learn more about how to design and implement motor control algorithms, see电机控制块集和Simscape Electrical™。