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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

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    \$\begingroup\$ NO, the 3 phase rectifier will not provide a unity power factor. You need a 3 phase switching power supply to get anywhere close to unity PF. \$\endgroup\$
    – Jack Creasey
    Commented Jun 22, 2019 at 14:30
  • \$\begingroup\$ @JackCreasey, Does that mean, I need to have a 6 pulse IGBT rectifier...like 3 phase synchronous rectification? Is there a particular PWM pattern that I need to have to make this arrangement reach unity power factor? \$\endgroup\$
    – Vishal
    Commented Jun 23, 2019 at 15:49
  • \$\begingroup\$ You are NEVER going to achieve anywhere near unity PF per phase. With uncontrolled 3 phase rectification your PF will be at best 0.92 - 0.95 against the generator (closely coupled field/phase) so the generator should be just fine. Phase controlled rectification will make this worse. Perhaps this might help: libvolume3.xyz/electrical/btech/semester7/… Trying to get up to 1MW is always going to be a serious challenge (that's a LARGE motor). \$\endgroup\$
    – Jack Creasey
    Commented Jun 23, 2019 at 17:33
  • \$\begingroup\$ Thanks. For the application at hand, 0.92-0.95 is good enough. Is there a specific PWM pattern for switching the IGBTs that I need to employ to get to this power factor? \$\endgroup\$
    – Vishal
    Commented Jun 23, 2019 at 19:00
  • \$\begingroup\$ I'm not sure that you need PWM for the 3 phase rectification you should be investigating a DC-DC convertor for you rectified DC output IMO. \$\endgroup\$
    – Jack Creasey
    Commented Jun 23, 2019 at 19:12

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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.

That is not exactly true. At 50% speed, the power required from the generators will be 50% while the total RMS value (including harmonics) of the current remains constant. However the effect of the harmonics may be increased somewhat.

Will the above arrangement improve power factor?

The displacement power factor will be near 1.0, but there will still be harmonic content that will cause the total power factor to be less that 1.0, considerably less under some conditions. However the total RMS current at 50% speed is likely to be less than you are experiencing with the SCR drive. Also the SCR line notching will be eliminated.

You can reduce the harmonic content of a PWM drive in several ways. The best thing to do would to determine from the details of your power distribution system how much harmonic distortion to expect. Make a detailed study of the problems of your current installation. Then determine the cost and benefits of the various alternatives.

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  • \$\begingroup\$ Thanks for the viewpoint. I really appreciate your inputs. I have a couple of questions: (1). In the chopper arrangement, what are the conditions under which, the total power factor can go considerably less than 1.0? (2). Since the rectification is being done via 6 pulse diode bridge, which is unidirectional, and the DC Link capacitors will continuously absorb charge, as the current is drawn by the motor, Should the harmonic content still travel backward into the AC Line? (3). Will adding appropriate line reactors on the AC Line, improve the distortion factor? \$\endgroup\$
    – Vishal
    Commented Jun 23, 2019 at 15:40
  • \$\begingroup\$ (1) The total power factor can go considerable below 1.0 when the converter is lightly loaded and the source impedance is very low. (2) The harmonic current is caused by the nature of the rectifier with capacitor filter. The load controls the current drawn from the source. It is not a matter of the harmonic content "traveling backward." (3) Adding reactors to the AC line will improve the distortion factor if the line impedance is very low. I may be able to illustrate and quantify those points to some extent later. \$\endgroup\$
    – user80875
    Commented Jun 23, 2019 at 17:27
  • \$\begingroup\$ (1). Yes, if its lightly loaded, then having low power factor is understandable because the power being drawn is less. (2) Ok. this explains it better. If I remove the capacitors from the DC Link and directly feed the DC output to the switches, will that be a problem? (3) If the harmonics do not travel backward, how does adding Inductance in series with the AC line help improve distortion factor? Need more explanation here. \$\endgroup\$
    – Vishal
    Commented Jun 23, 2019 at 19:06
  • \$\begingroup\$ Look at electronics.stackexchange.com/questions/435880/… Note that that question was closed as too broad. Adding questions by comment or revision is also a problem. I am not very familiar with DC drive design, but I suspect that the capacitance is less than what is typically used for VFDs. The designer of the DC drive will decide that. At 1 MW, & 750 V, it is not a project for someone without substantial experience. \$\endgroup\$
    – user80875
    Commented Jun 23, 2019 at 19:26

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