dc motor vs. ac motor in power factor correction

[mwuniseal]

We are looking into correcting our power factor. It has been said that we could use more dc motors in place of ac motors to improve power factor. I do not believe this is correct due to both being a inductive load. All of our motors both ac and dc are on vfd.

 

[Jraef]

We are looking into correcting our power factor. It has been said that we could use more dc motors in place of ac motors to improve power factor. I do not believe this is correct due to both being a inductive load. All of our motors both ac and dc are on vfd.

Someone has some misconceptions here.

First off, you don't have power factor in a DC motor, inductive or not. Power factor is a relationship to voltage sine waves and current sine waves. There are no sine waves in DC, so power factor is a non-issue.

Assuming you are supplied with AC power, in order for you to use DC motors, the AC power would need to be converted. In that conversion process, you will have rectifiers (and probably transformers). The conversion process will have poor power factor itself, so using DC motors essentially solves nothing. Whomever suggested that doesn't know what they are talking about.

The simplest solution is to look at the power factor of each individual AC motor load and add power factor correction capacitors to them, connected as close to the motor as possible. This corrects each individual contributing load with just the amount it takes for that load when it is running. This method avoids over correcting, which makes your PF leading and that can be just as bad as lagging.

If you have VFDs on some of your AC motors, the power factor as seen by your system might be fine if it has harmonics mitigation, so don't add PFC caps to those. Also be careful if any of them have soft starters or Y-Delta starters, they take a little extra care in how they are connected.

If you have DC drives on some of your motors now, and the PF created by the drive is bad, you can fix that too with correction filters. The same is true of raw AC VFDs. There are lots of harmonics mitigation systems that can be used for drives and they usually take care of power factor distortion as well.

By the way, you apparently used the term VFD to describe DC drives, that would be incorrect because VFD is Variable FREQUENCY Drive, and DC has no frequency. VSD (Variable SPEED Drive) is better to use for DC drives, but just saying DC drives is best because even VSD can be applied to several technologies.

But scrap the DC motor idea as a way to improve power factor, that was worthless.

 

[Besoeker]

Someone has some misconceptions here.

First off, you don't have power factor in a DC motor, inductive or not. Power factor is a relationship to voltage sine waves and current sine waves. There are no sine waves in DC, so power factor is a non-issue.

Assuming you are supplied with AC power, in order for you to use DC motors, the AC power would need to be converted. In that conversion process, you will have rectifiers (and probably transformers). The conversion process will have poor power factor itself, so using DC motors essentially solves nothing. Whomever suggested that doesn't know what they are talking about.

The simplest solution is to look at the power factor of each individual AC motor load and add power factor correction capacitors to them, connected as close to the motor as possible. This corrects each individual contributing load with just the amount it takes for that load when it is running. This method avoids over correcting, which makes your PF leading and that can be just as bad as lagging.

If you have VFDs on some of your AC motors, the power factor as seen by your system might be fine if it has harmonics mitigation, so don't add PFC caps to those. Also be careful if any of them have soft starters or Y-Delta starters, they take a little extra care in how they are connected.

If you have DC drives on some of your motors now, and the PF created by the drive is bad, you can fix that too with correction filters. The same is true of raw AC VFDs. There are lots of harmonics mitigation systems that can be used for drives and they usually take care of power factor distortion as well.

By the way, you apparently used the term VFD to describe DC drives, that would be incorrect because VFD is Variable FREQUENCY Drive, and DC has no frequency. VSD (Variable SPEED Drive) is better to use for DC drives, but just saying DC drives is best because even VSD can be applied to several technologies.

But scrap the DC motor idea as a way to improve power factor, that was worthless.

I mostly agree with your points.
Except one. The power factor of a DC drive isn't always bad.
We have DC drive systems in operation from a common DC bus. Often it is either a 12 or 24-pulse system. Both displacement and distortion power factors are close to unity.