Thanks for this response.
LetÂ’s clear up the terms there is no AC/DC common Buss. Common Buss by definition is DC.
In their manual on common bus applications, Rockwell calls it a "Shared AC/DC Bus", which is why I called it that.
I would not recommend powering both vfdÂ’s, there should only be 1 power source. The reason for this is they there in no way to ensure the charge rate to the buss will be the same on both vfdÂ’s even if they are the same size the components may vary a little and cause an over load or fault at any time.
It is possible to connect multiple vfdÂ’s on a common buss but there still should only have 1 power source.
Rockwell makes the point you're making here, but stops short of saying it shouldn't be done, and instead shows their recommended way to do it. For the 700 series power flex drives they recommend back-back diodes between drives (from Bonitron) in the DC bus to add an additional voltage drop, to overcome the issue of having different impedances at the VFD input causing overloading of the rectifier section of one of the drives. For the 500 series drives, they recommend line reactors on both drives instead of the diodes; I'm guessing to reduce the input impedence difference between drives, and/or reduce the reactive current at the drive input.
I would not recommend using a breaker anywhere on the buss connections, remember the buss voltage will be about 700VDC most breakers are rated for 500VAC or 600VAC max. While the breaker on an AC voltage will be able to break the fault current but DC will arc across and could destroy either or both vfdÂ’s. I have seen this to many times where it was tried and failed destroying things. You have to keep in mind that when the breaker needs to trip and open the breaker load it is already be above the load rating of the breaker. I have seen many breakers where the contacts weld closed.
Do not place a contractor at any point on the buss if you try to open it under load it could weld the contact closed and here again burn up components.
This is a good point; in this case though the VFDs are on 208V, making the DC Bus voltage around 300V. The braking chopper switches on at 395VDC.
A single breaker feeding both vfdÂ’s should work fine that way when one switch disconnects everything
This is what Rockwell recommends also (actually they just recommend a common disconnect), and works for shut down during maintenance, but it doesn't exactly solve the problem where the mains fuses on one drive blow and the other doesn't, unless this is somehow communicated to the breaker (via a shunt trip). Is that what you meant here? Or how did you envision this working if one of the drive fuses blow? This is sorta the problem I'm trying to figure out, how to shut down the other VFD if fuses on one blow.
But you should still consider installing a Buss loader resister (DB Brake Resistor) on each vfd to prevent buss overvoltage trips.
I think you're probably right on this.
I would recommend a Yaskawa vfd for this project they have an advantage here the power to controls board comes from the buss. The ac line powers the buss the buss powers the vfd controls so as long as there is power on the buss the control will have power and control the vfd
Although I haven't tested it yet (I will), I would imagine this is the case for powerflex 52X drives as well, as Rockwell does give examples of how to set up multiple drives with a single rectifier source. If the common bus didn't power the other drive, then controlling the additional motors wouldn't be possible. Additionally, we already have the drives, reactors, etc., so would prefer not to switch models at this point. But will keep that in mind for next time.
Depending on your application the micro 850 plc may not be needed I would need more details on that.
Most likely this isn't needed for VFD control, but I wanted to interface a PanelView with it to read out DC bus trends, and there will be other features of this system that will eventually require PLC control, assuming everything works correctly. Also the drives will be enclosure mounted, so easier to control everything from a single panel.
So how would you recommend handling the situation where fuses blow to one of the drives, and it's desired to shut down the other?