# Improvement on Flux Weakening Control Strategy for Electric Vehicle Applications
###### Abstract
Several FW control strategies [11], have been proposed recently; nevertheless, even if they are extensively tested in motor operation, few studies [12; 13] and papers investigate on the effects of abrupt torque setpoint transition from motor operation to braking one in deep flux weakening region [14] as a consequence of a suddenly let go of the throttle. In the case of electric vehicle, the control strategy is usually carried out with a torque feedback control, while the driver adjusts the vehicle speed with the throttle. When a substantial torque transition is required (e.g., during braking), the torque setpoint moves from its maximum value to zero and, if the motor is operating in the FW region, the d-axis current component must be properly controlled to counteract the flux linkage of PMs. This working condition is critical, since the current vector must be kept ideally very close to the d-axis position 1, see Figure 3. In this operating point, even a small error in the electrical angle (e.g., due to the position sensor on the motor or to the delay introduced in the PWM period) could move the current vector from the motor operating region, position 1 of Figure 3, to the generator operation region, position 2 of Figure 3, producing unwanted additional braking torque or vibrations. Figure 3: Current vector position at high-speed operations where: 1. motor operating region, 2. generator operating region. Moreover, during fast torque setpoint transitions, the error of the current angle due to iron losses changes its sign [15], introducing additional issues in the managing of the energy-flow from the motor to the battery pack. The latter issue has to be addressed to avoid hazardous operation or overcharge of the battery pack. In [16] author proposed a FW control scheme based on the detection of the \(i_{sd}\) current error. The \(i_{sd}\) axis error is used to decrease the q-axis voltage setpoint. Figure 4 shows essentially a modification of the classical feed forward (FF) vector control method. This control scheme allows a smooth and fast transition from the MTPA strategy to FW control strategies, without the need of an accurate knowledge of the motor parameters or look-up tables (LUT) of the machine magnetic model. The FW control scheme of [17] calculates a suitable \(i_{sd}\) setpoint by means of the outer voltage loop. As a consequence the voltage vector is forced within the limits represented by the Voltage DC BUS. This control scheme of Figure 5 allows the maximum voltage exploitation, nevertheless in this approach no specific strategy is deeply investigated to manage a possible fast torque transition at high speed during FW operation. A control strategy suitable for traction, in case of power train based on high speed permanent-magnet synchronous motor, is proposed in [18]. This control performs in a wide speed range, adding some additional blocks and the equations of the MTPV strategy, it mimics the same approach of the outer voltage loop solution. Nevertheless, even in [18], the issues related to torque transition from motoring to braking are not investigated. Finally, no tests on a real vehicle are shown [18]. Similar control strategies are presented in [19; 20]. Improved MTPA and flux weakening control strategies that take into account magnetic saturation and the need of MTPV to obtain a full exploitation of the short term current rating can be found in [21]. Figure 4: Traditional control scheme proposed in [16] based on \(i_{sd}\) current error. Figure 5: Traditional FW control scheme proposed in [17] based on outer voltage. Given the aforementioned scenarios, this paper presents two control schemes to properly address the torque transitions at high speed in FW operation: the former is based on the outer voltage loop [17] and the latter on [16]. Simulation and experiments of the proposed solutions are presented in the following paragraphs.