Multiple PowerFlex 755’s Sharing One Braking Resistor?
- Thread starter sanger
- Start date
No #1 - what if 2 of them want to brake at the same time, the resistor would have to be rated for double, then triple for 3, etc.
No #2 - That could, potentially, cause a path for current between the 2 buss's of the VFD's or even the power outputs to the motors.
No #2 - That could, potentially, cause a path for current between the 2 buss's of the VFD's or even the power outputs to the motors.
GaryS
Member
Check out common buss configuration
I don't know if the PF755 can be set up for common buss or not but it's worth checking out
Some drives can and some can't
But generally its a bad idea on most drive the built in brake transistor is only good for about 10% power
you can add an external braking unit that can handle what ever power you need
I don't know if the PF755 can be set up for common buss or not but it's worth checking out
Some drives can and some can't
But generally its a bad idea on most drive the built in brake transistor is only good for about 10% power
you can add an external braking unit that can handle what ever power you need
janner_10
Lifetime Supporting Member
Why? just do it properly.
jraef
Member
Yes, if you use the Common DC Bus setup, you can have multiple drives share the same braking resistor. But this will be based on the idea that in a Common DC Bus arrangement, one drive is braking while others are motoring, consuming some of the excess energy, so the resistor is only there to dissipate any excess. If you are sharing loads like that, you will need what are called “sharing diodes” that prevent circulating currents from happening between drives. A-B has a manual specifically for setting up and using Common DC Bus arrangements with PowerFlex drives, it describes how to do it and what to use.
https://literature.rockwellautomation.com/idc/groups/literature/documents/at/drives-at002_-en-p.pdf
If all motors will be braking at once, the braking transistor of one VFD will not be sufficiently sized to dissipate the energy of multiple motors; the transistor will be sized only for that one VFD. So if that is the case, you must use an external braking transistor module sized for the total connected load that will be braking at the same time. You can buy those from third party vendors like Bonitron. They will be able to guide you through the selection process. But be aware, you will need a lot of specific details about the load and motion profile. Guessing is not something you want to try with this. The A-B manual includes information on Bonitron units they recommend for these type of applications.
https://literature.rockwellautomation.com/idc/groups/literature/documents/at/drives-at002_-en-p.pdf
If all motors will be braking at once, the braking transistor of one VFD will not be sufficiently sized to dissipate the energy of multiple motors; the transistor will be sized only for that one VFD. So if that is the case, you must use an external braking transistor module sized for the total connected load that will be braking at the same time. You can buy those from third party vendors like Bonitron. They will be able to guide you through the selection process. But be aware, you will need a lot of specific details about the load and motion profile. Guessing is not something you want to try with this. The A-B manual includes information on Bonitron units they recommend for these type of applications.
Thanks for the great info. This is not a configuration that we would typically design but it is a drive upgrade project where the original drives were configured with one large common braking resistor.
If that's the case, I'd be inclined to take the opportunity to "do it properly" and give each drive their own braking resistor. No doubt - if it's so old that you're now upgrading it - it was originally done that way for cost reasons, but technology has advanced a long way and the cost of components has come down. You may find that the cost involved in properly re-engineering and building such a complex system now outweighs the costs of individual braking resistors for each drive.
If you do choose to stick with a single resistor for reasons like lack of panel space or management who insist that you do it this way because they think it will be cheaper, then make sure you get the right people involved. Jraef has probably forgotten more about VSD's than I'll ever know, and his advice is very good - you can't do this by guesswork and estimates, you need someone who knows how to do it properly to advise you.
Gene Bond
Member
Not necessarily!
It could have a common bus to share energy from one load to the others, like a web line with payout (braking) and takeup (motoring)... So more information may be required before deciding whether or not to duplicate the older system.
Assuming the old system worked well, the resistance and wattage of the original brake resistor compared to the total HP (and recommended values from AB) will be an important bit.
True - I should probably have put a few more qualifiers in that statement rather than imply it's a universal truth. If it were me, I'd still be looking very hard at whether I could do it in a more "standard" way without significantly deviating from the existing design or re-inventing the wheel, but you're right - there are definitely applications where it may well still be a very good option.
So it is someone else's design ?
You absolutely have to investigate the application. Like others have mentioned, it could be that one drive sends regenerative energy to the other drive, and the brake resistor is only to take up excess energy.
And/or if the above scenario is during normal production, the regenerative energy is much greater (because it is constantly on) than the energy used for braking (the dissipated power may be greater, but only for a short time).
The drive manufacturers usually state the recommended size of brake resistors based on stopping, not taking up regenerative energy for a long period of time - so the drive manufacturers recommendations are too small for constant regenerative energy.
Depending on the application, connecting the DC-buses together may be the only way to solve the problem with regenerative energy.
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