Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
The current source inverter fed vehicles are less frequently used than the voltage source inverter fed vehicles. The main feature of the current source inverter is that the DC link contains a high inductance smoothing choke, instead of a smoothing capacitor. The line side converter – can produce continuous positive and negative DC voltage – generates controlled i e DC current. The three phase current source inverter switches this DC current cyclically altering to two appropriate phase of the motor. At thyristor current source inverter the number of the switching states is six, electrically the conducting states replace each other with 60° (Fig.5.15.b.). Heavy duty commutating capacitors are required for switching between the conducting states.
The field oriented control of the induction motor is achieved through that the current conducting periods are timed to the rotor flux position. While the current vector direction can take six discrete directions, the field orientation can be fulfilled only on average in one 60° period. This causes that the motor torque ripples with electric 60° periods. Fig. 5.15.a represents the schematic main circuit diagram of the thyristor current source inverter and induction motor driven BDVmot multiple unit’s drive system of one bogie.
The line side converter is an economical version of a double bridge circuit. To avoid the former mentioned torque ripples in this vehicle such i e DC current control is applied that compensates the torque ripples with varying the current vector magnitude in a 60° period at low frequency control (at low vehicle speed).
Besides the thyristor current control inverter, e.g. IGBT current source inverters also exist that consist of commutating elements. Besides the six current vector presented in Fig.5.15.b, the ī=0 vector can be switched with this current source inverter. The six current vectors and the 0 vector can be varied by PWM pulse width modulation with multiple frequency compared to the thyristor current source inverters, i.e the field orientation can be “smoothly” achieved. Commutating capacitors are not required; despite this a three phase capacitor bank should be connected to the motor terminals, the motor current can close through this bank at ī=0 vector switching state.