sharing DC bus between VFDs

[fab1961]

Dear all,

while with drives applications with permanent magnet motors it is a common practice to share
the DCbus between drives, I've never come across a similar connection with VFDs (I refer to
manuals, case histories etc. - we have been using in time Japanese and European VFDs of different makes).
When there are for instance two identical async motors driven by identical VFDs why not connecting the DC
buses together to increase the current braking capacity ? Or one can save money using only one braking
resistor for two VFDs (of course if proper conditions are met for sizing the resistor, contemporaneity factor etc).
Or even do without any braking resistor due to the increased energy storing.
We work with low voltage motors in the range 0,75 kW to 4 kW with no special functions, simple
V/f applications and I am interested in your point of view.

Thank you

 

[controlsdude]

Multple motors on 1 vfd is common. Just look at google pdfs from different manufacturers.

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/140m-at003_-en-p.pdf

https://www.toshiba.com/tic/datafiles/10.2_S11_Multiple_motors_on_one_VFD_6509.pdf

https://library.e.abb.com/public/fd...ing_multiple_motors_in_parallel_500-600_V.pdf

 

[waross]

Energizing may be a problem. When only one drive is started, it will be charging both capacitor banks and may not be up to the challenge.
Running only one drive may lead to unintended consequences.
That said, it is possible to charge a large capacitor bank (DC bus) and run multiple inverters from it.
In most cases the modifications and rewiring are not cost effective.
If you are building 1000 machines, each with multiple drives off of one DC bus, it may become cost effective.
You would have a captive audience for replacement inverters.
Off the shelf models may not work.
Consider the controls and protections that depend on an AC input. These may or may not work properly with a modified DC bus.
Bite the bullet and buy a larger resistor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

Vacon (now Danfoss, I believe) is one such manufacturer that has options for a shared DC bus across multiple drives. I don't recall as to whether it was an option for such small motors though.

EDMS Australia

 

[LionelHutz]

We have 3 VFD's and a brake chopper connected common DC bus for testing. It's a good solution when it makes sense, but it has drawbacks. A single VFD failure could short the bus of the other VFDs and damage the other VFDs. All VFD's are connected together with the DC bus so you can't service a single VFD unless you install a means to isolate the DC bus to the VFD requiring service.

There are manufacturers that make common bus systems for the motor size range you mention. They use a single rectifier unit coupled to multiple inverter output units. The upside is that it's cheaper and more compact since it eliminates repeating common components. It can also allow a single inverter to regenerate into the other units instead of dumping the energy as heat. The downside is that a failure of 1 unit can take the whole system down.

 

[waross]

Lionel; Do you have any problems or special procedure when energizing the VFDs?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fab1961]

LionelHutz, Waross,

thank you for advice. What I was thinking of is using "normal" VFDs and wiring the DC bus using the available terminal blocks.
I can confirm that a failure on one drive burns the other ones connected, it happened to us a couple of times using servodrives.
In our applications the servodrives are used for drills or axes on machine tools and they are much more "stressed" than the async
motors we use to move material on rollerways, we have never had failures on VFDs due to the smooth use we make of them.
LionelHutz can you tell me which manufacturers sell the single rectifier system?

Have a nice evening everybody

 

[itsmoked]

Disregarding possible cost reduction schemes as noted above. It only makes sense when you have a blatant opportunity to re-use energy. For instance you have a bunch of conveyors and some are trying to slow downward conveyors while others are raising material. Or, perhaps elevators where someone is frequently going down while someone is going up. Good stuff that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Lionel; Do you have any problems or special procedure when energizing the VFDs?

Just turn the bigger ones on first so the little one isn't charging the bus of the big regen unit. There are DC bus fuses for each VFD and there are diodes in the DC bus for each unit so they can't back feed each other but only the regen unit.

 

[wde0665]

FWIW, had a Rockwell Powerflex700 system on a bridge crane with this type of common bus system. Believe the drive model numbers started with 20PD...., not sure about the inverter model. Three drives - 50hp bridge, 20hp trolley, 100hp hoist - on a common DC bus. Perhaps a unique experience with our Rockwell-engineered application, but it was a nightmare - herky-jerky operations, couldn't get things to settle down even after two field engineer visits. Ended up replacing the system with three separate integrated drives - the problems and the related late-night calls and complaints went away.