Frequency domain evaluation of a traction PMSM drive with direct torque control including field weakening area

This paper presents a method for frequency analysis of traction drives supplied from a dc trolley wire with an input LC filter. The proposed method is suitable for frequency analysis of the traction drive in whole speed range and enables to find resonant or other observed harmonics in the operating drive circuit which could cause dangerous oscillations particularly at resonant frequencies of the power circuit. The method is used for detailed frequency analysis of a direct torque controlled traction drive with a surface mounted permanent magnet synchronous motor. The analysis is performed in a wide speed range including field weakening area and the dangerous resonant frequency is found and the behaviour of the drive in dependence on the stator frequency is observed. A solution based on demanded machine torque correction is used for the drive stability enhancement. The behaviour of the drive with this improvement is evaluated with the proposed method as well. The proper function of the frequency analysis in the wide speed range for the non-stabilized and stabilized drive is verified by simulations as well as by experimental measurements on a down scaled traction drive with rated power of 10.7 kW.