I have a 6 pulse SCR drive, driving a large (1000KW, 750VDC) DC Separately excited DC Motor. In a typical SCR drive, the power factor improves with speed, and if the drive is to be run at 50% speed for a long time, then this reduced power factor puts a strain on the AC Generators.
I can replace the SCR drive, with a DC Fed Chopper Drive, such that incoming AC is fully rectified by a 6 pulse Diode Bridge and appropriate capacitor(s) is used to form the DC link. Then this is connected to the DC motor, via an IGBT switch, which chops the DC at either positive or negative or both ends. The same logic that determined, phase control angle for SCR shall now decide the Duty Cycle for the IGBT switch.
Will the above arrangement improve power factor? If Yes, what specific requirements need to be fulfilled in the control circuit, if any. If NO, why not.
My assumption here is that since the DC rectification is full wave, there is a higher chance that current drawn can be made sinusoidal. However, once the DC link capacitor charges up to the full DC voltage, any current draw will depend on the duty cycle of the PWM. I am trying to find out if there is any special consideration for this PWM, such that as far as possible, the current drawn from AC source remains in phase with the AC voltage. That would improve power factor, right?
Thanks and Best Regards, Vishal Sapre