DC Motors and PFC 1

[pwcwatt]

Greetings,

I am trying to figure out the correct steps in applying PFC caps to some large DC motors. I would prefer to do this on the line side at the DC load as opposed to putting a large bank at the service entrance but was wondering at what level of harmonic distortion (both voltage and current) I would be required to use a filter in addition to the capacitor.

Also I would like to know at what degree harmonic a filter is absolutely required regardless of the amount of distortion.

Any input would be greatly appreciated.

Thanks

 

[davidbeach]

Capacitors on the DC bus will do nothing for your AC power factor; the caps would have to go ahead of the rectifiers. I would approach with caution unless you know the configuration of the rectifier front end as you may get undesired interactions with any filters on the rectifier.

 

[rbulsara]

To furhter clarify what davidbeach said, DC does not and can not flow through a capacitor. Capacitor is an open circuit to a DC circuit.

 

[pwcwatt]

My intention was to apply PFC Caps to the AC side of the DC motor. Since the caps will draw a possible overload of current depending on the amount of harmonics present in the circuit I was looking for some input on what the threshhold might be.

Thanks

 

[rbulsara]

Have you measured the PF at on the "AC" side of the DC drive? What percentage of the total load is the DC motors.

I would suspect, your low pf at the service is not due to DC drives..What is the average pf at the service?

 

[pwcwatt]

Power factor at the entrance is in the .75 range which is pretty close to the power factor on the AC side of the DC motors. Anything below .85 incurs a penalty around here. The AC motors in the facility are too small to account for the poor overall power factor.

If I have to treat at the main service entrance I will but like I said I would like to treat at the load to mitigate the line losses as well.

 

[jraef]

Only older DC drives that used SCR converter sections had poor enough pf to worry about. Newer PWM DC drives are just like AC drives in that they present a .95 pf thoughout the speed range. Before you go any further, find out which type of DC drive you have.

If you do have an older DC drive, the tricky thing is that the pf will vary with speed. Determine the lowest speed you will run the system most of the time, then measure the pf at that speed and correct for it to about .95 by looking at correction charts from the PFC mfgrs. But an engineered system that is also a harmonic filter is really the best solution, because as davidbeach said, you can run into resonnance problems putting caps in front of drives by themselves if you don't know what you are doing.

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