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PF Correction of DC Motor (thyristor control)


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We have a 90KW DC motor drive a plastic extruder. The power supply is limited to 150A (415V 3 Phase). This is a new machine and only once connected, did we realise how inefficient DC variable drives are!


On low viscosity resins at full speed (430 rpm) the PF=0.89 and current ~ 130 Amps, at 200 rpm the current is 100A at PF=0.5 and at 50 rpm currrent is 110A at PF=0.35.


In all cases there seem to be a linear relationship between AC and DC amps ie. 100amp DC = 100amps AC (per phase).


With high viscosity material at 50 rpm the current rises to 190A and the PF =0.2 - Due to supply current limitations we have only maintained this for 1-2 minutes. It is impossible to increase speed due to current limitations.


As a trial we connected 6 * 30 KVA capacitors in Delta (3 banks of 2 caps in series). This gave us an ~ 20amp reduction in AC current with low viscosity material (down to 110-115amps) and a 40Aps reduction in AC current with the high viscosity material (down from 190A to 150A).Current drawn by the caps varied between 72A-76A. This correction was insufficent we need to reduce the current from 190A to 90A (guess) to run high viscosity resin at a commercial rate.



1. What amount of PF correction would be needed to improve the P.F. from 0.2 to ~ 0.9 (to enable the processing of high viscosity resin). Is this amount of correction possible?

2. We know harmonics are being generated and from the literate on this site - inductive choke are stated as essential. Any suggestions of what size chokes would be necessary?

3. Due to cost, our preferred out come would be to connect a fixed bank of capacitors and chokes at the motor. Is this practical or is it necessary to have controlled (microproccessor)PF unit to cope with the varing speeds under load?

4. Would a 90KW AC variable speed motor be a more efficient option in this application considering the low speed torque reqirements?


Any comments would be greatly appreciated.



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Hello Sandy


Welcome to the forum


I would not expect that you would achieve anything by adding power factor capacitance to the input side of the drive.

Basically, power factor correction capacitors should only be added to a load that draws a sinusoidal current lagging behind the voltage. i.e. and inductive load.

The DC motor and drive does not fit into this category at all. The DC drive comprises a controlled rectifier system that rectifies the incoming supply, converting it ti a variable voltage DC supply. This supply is presented across the DC motor. The current would normally be in phase with the supply, but would be distorted due to the rectifier action. This waveform distortion is what causes the poor power factor rather than the angle between voltage and current.

I expect that the poor power factor is due to the harmonic currents rather than the current lagging the voltage. This can only be corrected by some form of harmonic filtering. (active or passive)


Your questions:

1. If it is a standard DC drive system, then I dont believe that you can "correct" the power factor with capacitors.

2. Because you have a lot of harmonics present, you should add reactors in series with any capacitors connected to the supply to reduce the current through them. The inductors should be selected such that they form a seres resonant circuit with the capacitors at a suitable frequency such as 180 Hz. The capacitor suppliers can recommend the detuning reactors.

3. As in 1 above, I do not believe that capacitors will solve your problem.

4. DC drives are much better than AC for high torque at low speed. A.C. drives are best suited to applications where the torque falls with speed.


Best regards,

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  • 1 month later...

The only way I have seen a DC drive power factor compensated for was by isolating the drive and the motor from the system. Then the isolated branch was then corrected for poor fower factor.



1. The question on increasing the power factor is one that requires a few steps. The first is that a isolation transformer feed the DC drive. Power Factor correction caps will then need to be installed on the primary side of the transformer, remember that these correction caps will have to be switchable because the power factor will change as the drive and motor change speed and load.

2. The above mentioned caps can be "tuned" to help filer out some of the harmonics.

3. The best place would be at power feed side not the motor side.

4. Now, the AC Vector drive is suppose to be able to replace most DC drives. If you want to look at this option you may want to talk with your local AC drive supplier.

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