*4.3.3. Proposal of novel flux-weakening control based on nonlinear current compensation*

Figure 10(a) illustrates the block diagram of the novel flux-weakening control based on nonlinear current compensation. Part 1 is the torque controller in the constant torque region. The current command *i s* \* is the output of the P1I speed controller, and then decomposed into d- and q-axes components, *i dx* \* and *i qx* \* according the maximum torque-per-ampere trajectory until the current regulator begins to saturate.

**Figure 10.** (a) The block diagram of the novel flux-weakening control based on nonlinear current compensation. (b) daxis current modification

The proposed flux-weakening controller includes Parts 2 and 3. The proposed flux-weakening control algorithm uses the output reference voltage of the synchronous PI current regulator to identify the onset of the flux weakening. As the speed of the IPMSM increases, the output of the current regulator approaches to the boundary of the pulse-width modulator. With the reduced margin of the voltage, the performance of the current regulator gets worse and finally loses its controllability. As shown in Figure 10(b), the controller ensures the margin of the voltage by compensating the d-axis current toward the negative direction to prevent saturation of the current regulators. But at the low-speed region, the output voltage of the current regulator is usually less than *V*max and thus Part 3 is not activated. The function of Part 2 is to adjust the q-axis current command *i qx* \*, as the d-axis current command *i qs* \* increased toward the negative direction, because of the current limit *Is*max.
