variable frequency drive motors

[Cold Fusion]

Laszlo -

Request you read post 9 and 10. My example is a current source drive.

[1]What would be the value of a capacitor on the output side of an ASD?
3 Wye connected banks of 7200V - 500kvar. So 1500kvar reflected to 6500V or 1500 X (6500^2)/(7200^2). Although why the actual number of kvars/farads would matter I don't know. It's what Rockwell included when they sold us the drives.

[2]What is it intend to accomplish either from the technological or the finanacial side?
Well, I didn't discuss this with Rockwell. My assumption is the motor side caps reduce the lagging power factor (lower the motor magnetizing current) seen by the inverter.

I also suspect CS drives use caps on the output to smooth the current output delivered by the inverter. The inverter delivers power in a 400hz or 800hz fairly narrow pulse. When the inverter semiconductors turn off, the motor, being inductive, continues to draw current. As you are aware, most semiconductors have trouble in this quadrant - high voltage, high current.

Designing CS drives is not my area of expertise - I'm only guessing. There are others here that are more knowledgable than I. Perhaps one of them will jump in.

Couldn't tell you anything about the financial side - probably made Rockwell some money selling us drives that worked and met spec.

[3]Since the capacitor is not variable a resonant circuit at some frequencies, especially with varying carrier frequency is practically unavoidable.
No question here - I'll assume that you wish to know my views on how the drive avoids the "unavoidable".

Maybe the engineers avoided carrier frequencies that could resonate. That would be my guess.

[4]How would one surmount the resultant problems?
What problems? The drives work fine.

[5]What about the increased discharge voltages due to the harmonics?
I don't have a definition for "increased discharge voltages due to the harmonics". I don't know what that means in this context. If you are asking on how Rockwell keeps voltage spikes developing from harmonics, from damaging the inverter semiconductors - well, I don't know there are any spikes, harmonic or otherwise. Looking at the inverter output with a 200mhz scope, I don't see anything that causes me heartburn.

There are snubbers across each semiconductor to keep the voltage even across each semiconductor in each series string while that string is turned off.

OK, I'll bite.
Just for starters.........

Just for finishers .......

I'm not in the drive design business. Nor am I particular knowledgable about the intricacies of drive design. I don't want to be. I'm in the drive application business. The contract was for Rockwell to supply drives and motors to meet a spec. We did not tell them how to do it. Rockwell did not include the output caps without needing them to meet spec.

I answered your questions. However, let's not get side-tracked. You made statements without supporting evidence. Are you going to support your statements? I'm not going to do your homework.

cf

 

[Cold Fusion]

petersonra -
I'm thinking you did not read post 9.

What possible benefit would there be to putting capacitors on the load side of a VFD?

I included post 9 and highlited the section.

It does not improve the PF the utility sees at all,
I pretty sure I knew and understood that. Please see post 9

and thats really the only reason for PF correction.
I'm thinking current source drive designers may have different thoughts than that.

Originally Posted by CorElec Is it correct to connect capacitors to variable frequency drive motors for power factor and voltage improvements?

Probably not - unless they came with the drive, or maybe specified in the installation manual.

Yes I have seen drives with capacitors on the output - current source, MV. (example is AB PowerFlexf7000).

If you are trying to correct the low power factor the line side of the drive is showing to your system, yes I have also seen drives with caps on the line side. In the cases I have been involved with, the caps were selected to control the harmonics, raise the pf, and not cause too much disturbance when they were switched on with no load on the VFD.

If you are thinking putting caps on the motor side will fix the low pf on the line side - no that won't work. The DC bus between the line side and the load side stops that. You can't reflect vars across the DC bus. The output inverter has to handle whatever pf the motor (including installed caps) puts up.

I have seen a drive that automatically adjusts the output voltage to minimize the vars the motor draws, That one also had caps on the drive output.

Read the installation instructions that came with the drive.

really good advice

cf

 

[weressl]

Laszlo -

Request you read post 9 and 10. My example is a current source drive.

[1]What would be the value of a capacitor on the output side of an ASD?
3 Wye connected banks of 7200V - 500kvar. So 1500kvar reflected to 6500V or 1500 X (6500^2)/(7200^2). Although why the actual number of kvars/farads would matter I don't know. It's what Rockwell included when they sold us the drives.

[2]What is it intend to accomplish either from the technological or the finanacial side?
Well, I didn't discuss this with Rockwell. My assumption is the motor side caps reduce the lagging power factor (lower the motor magnetizing current) seen by the inverter.

I also suspect CS drives use caps on the output to smooth the current output delivered by the inverter. The inverter delivers power in a 400hz or 800hz fairly narrow pulse. When the inverter semiconductors turn off, the motor, being inductive, continues to draw current. As you are aware, most semiconductors have trouble in this quadrant - high voltage, high current.

Designing CS drives is not my area of expertise - I'm only guessing. There are others here that are more knowledgable than I. Perhaps one of them will jump in.

Couldn't tell you anything about the financial side - probably made Rockwell some money selling us drives that worked and met spec.

[3]Since the capacitor is not variable a resonant circuit at some frequencies, especially with varying carrier frequency is practically unavoidable.
No question here - I'll assume that you wish to know my views on how the drive avoids the "unavoidable".

Maybe the engineers avoided carrier frequencies that could resonate. That would be my guess.

[4]How would one surmount the resultant problems?
What problems? The drives work fine.

[5]What about the increased discharge voltages due to the harmonics?
I don't have a definition for "increased discharge voltages due to the harmonics". I don't know what that means in this context. If you are asking on how Rockwell keeps voltage spikes developing from harmonics, from damaging the inverter semiconductors - well, I don't know there are any spikes, harmonic or otherwise. Looking at the inverter output with a 200mhz scope, I don't see anything that causes me heartburn.

There are snubbers across each semiconductor to keep the voltage even across each semiconductor in each series string while that string is turned off.


Just for finishers .......

I'm not in the drive design business. Nor am I particular knowledgable about the intricacies of drive design. I don't want to be. I'm in the drive application business. The contract was for Rockwell to supply drives and motors to meet a spec. We did not tell them how to do it. Rockwell did not include the output caps without needing them to meet spec.

I answered your questions. However, let's not get side-tracked. You made statements without supporting evidence. Are you going to support your statements? I'm not going to do your homework.

cf

I don't have a homework. You are making a bunch of statements above that do not have any supportive, independent backup. My statements reflected commonly known principles of physics and I no longer have my college textbooks.

You are switching the arguments.

The OP discussed LV applications.

Carrier frequencies are 2, 4, 6, 8, 12 and 16kHz, not 400 and 800. Any time when you lower the voltage, the capacitor will discharge. The faster the voltage drop is - and it is nearly instantenous from FV to 0 in case of semiconductors - the faster the capacitor will discharge its stored quantity. Harmonics will fiuther agravate this.

Carrier frequency remains constant, so there is no way to 'avoid' resonant frequencies.

Why didn't Rockwell include the capacitors in the unit itself if it is to serve the motor and not the drive? Overcome a design defficiency before they modified their drive design? They did NOT offer it on my application.