Traction drives preferably operate with motor-inverter combinations that use the field weakening function for the higher speed. In this speed range, which often covers 2/3 of the entire speed band, the motor control of the inverter is based on the reliably available voltage, e.g. from the battery. Compared to classical control, with the HighSpeedBoost function in practical applications we achieve e.g. 8% higher peak power and at the same time 0.5% improved efficiency.
When operating in the field weakening range, the limiting factor is the voltage, especially the phase voltage that can be generated with the inverter from the DC voltage, e.g. from the battery. This DC voltage can also fluctuate dynamically, e.g. it drops noticeably when high power is taken from the battery for an acceleration process. In real applications, we therefore have to deal with both: slow voltage fluctuations (battery state of charge) and very dynamic processes in the range of a few milliseconds.
In realizing the closed loop control, therefore, a safety reserve must be built in between the maximum phase voltage that can theoretically be generated for the motor and the voltage that is actually generated. This safety reserve is at the expense of power and efficiency, but without it we have the risk an uncontrolled shutdown of the system, which would not be acceptable.
For our HighSpeedBoost function, we take advantage of the fast response of our FPGA-based control. This enables us to bring the voltage at the motor terminal closer to its theoretical limit without risking a shutdown. In practical applications, we have thus been able to increase power by up to 8% while improving real overall efficiency by up to 0.5%.