KEYWORDS – Active discharge, Bus voltage regulation, Crash safety, Electric Vehicle (EV), Permanent magnet synchronous machine (PMSM) High-voltage electric drive systems are commonly used in electric vehicles (EVs). When there is an emergency situation (e.g. crash event), the dc-link capacitor voltage is required to decrease to a safe level to prevent electric shock injury hazards. A dynamic break circuit is extensively adopted to discharge the dc bus in EVs at the price of the compactness and cost of the drive system. Machine winding-based discharge method is considered one of the most effective solutions for EVs crash safety. In this paper, a simple and straightforward winding-based dc-bus capacitor discharge strategy is proposed for PMSM drive in EVs. The proposed discharge process at high speed can be divided into two stages, namely the voltage regulation phase and the slow discharge phase. When the back-electromotive force (back-EMF) of the PMSM at high speed exceeds the safe voltage level, the dc-link voltage should be carefully regulated during the active discharge process. In the voltage regulation phase, the dc-link voltage is well maintained under the safe voltage level (e. g. 60 V) with the vector control algorithm. The d- and q-axis current references are obtained based on their maximum achievable limits and the voltage regulation factor, and this factor is derived from the relationship between the measured dc-link voltage and its lower and upper limits. The voltage regulation phase ends once the motor speed decreases to a certain threshold. In the slow discharge phase, the d- and q-axis currents are ramped to zero. Several simulations are carried out to validate the performance of the proposed active discharge method. Once an emergency occurs, the relay between the dc-link capacitor and the high-voltage battery is disconnected, and the proposed winding-based active discharge method is activated. A large d-axis current can be observed when the dc-link voltage is much higher than the safe voltage. When the dc-link voltage is lower than its upper limits, some negative q-axis current can be also observed. The dc-link voltage is regulated within its upper and lower limits until the motor speed decreases to a certain threshold. There are some requirements to implement the proposed active discharge method. Since the method is based on the vector control algorithm, the current feedback and rotor position information are necessary for the implementation. Besides, high control frequency is preferred to achieve good voltage regulation performance. Furthermore, the power supply for the gate drivers of the switching components should be functioning well during the active discharge process. Unlike the conventional winding-based active discharge method which uses two PI controllers to derive d- and q-axis current references, the proposed method uses a voltage regulation factor to calculate the current references, which is computation-efficient and straightforward. The voltage regulation factor is derived from the relationship between the measured dc-link voltage and its lower and upper limits, so the dc-link voltage can be well regulated within its predefined bandwidth. In this paper, a simple and straightforward winding-based dc-bus capacitor discharge strategy is proposed for PMSM drive in EVs. Compared to the existing methods, the proposed method derives the d- and q-axis current references in a more computation-efficient way. The simulation results showed that the proposed method exhibits a fast discharge rate and good voltage regulation performance.
Dr. Zekun Xia, Staff Engineer, NIO Inc.