VFD DC bus extension
VFD DC bus extension
(OP)
We plan to use a common bus VFD system (charging unit and 4 motor inverters). Usually, this would be a line-up with DC bus bars going along the line-up and dropping down connecting bars to respective inverters. In this specific case, the only possible location is a charging unit with radial connections to stand alone inverter cabinets. My question: is the difference between using bus bars or cables for these DC connections? Is there a limitation on the length of such a run? (For reference: each inverter has rated input current about 1000 ADC at 1100 VDC). To our surprise, the drive supplier can not provide a clear answer to these questions.





RE: VFD DC bus extension
At 1kA, those will be some mighty thick cables/bus bars. As it's DC, you'll have no skin effect to deal with, so that's good. How much voltage drop can the inverters tolerate? How long are your cable runs? It's a simple application of Ohm's law to find out the resistance - and thus, cross section - of the needed conductors.
Other thoughts:
Ensure that your cables' insulation is rated for the voltage it will see.
At 1kA, methinks fusing is a good idea; I wouldn't want to see what an unprotected bolted fault would do to that bus. Remember that DC faults are harder for overcurrent protection to clear, and thus fuses/breakers need to be spec'ed and rated properly.
SceneryDriver
RE: VFD DC bus extension
Realize that your 1100vdc bus will have 2200V+ noise on it that will try to get into EVERYTHING else on your machine (and probably even your neighbors!).
We are working on some mining applications for 100kw 1500vdc bus drives, and leads here are also only 2-3 foot max. I would not recommend longer if possible.
We have some 7axis machines with 20-85 amp each drives on common dc link; during design phase drive designers decided we could not mount these in a way to make the dc link wires longer than about 2 feet max....
We recently did a multiaxis 460vac robot and customer tied the dc bus between 2 drives with 2 foot long untwisted wire that they pushed into the wire raceway with other wires on machine to look pretty: result was I had a 4 hour drive to help 'fix' the machine that would not run - computers and sensors all over the machine had so much noise they were not working right. removing this single set of wires solved the issues.
I suspect that if you search drive manuals from the likes of Bosh Rexroth and Siemens you will find the official guidelines you search.
So moral of my story is these link wires have probably the most horrendous high voltage and frequency noise on them in a machine so need real RFI thought and shielding.
RE: VFD DC bus extension
RE: VFD DC bus extension
This usually will create the most severe limitation in such systems.
RE: VFD DC bus extension
You might want to give some thought to that potential sticky point.
old field guy
RE: VFD DC bus extension
Could the design essentially treat the DC bus as a type of transmission line, and be designed accordingly? Since you would know the switching frequency of the IGBT's and thus the harmonics that would be created, could the DC bus be designed to not be resonant at those frequencies?
Machines that large are something I've only seen in pictures. Managing that much energy safely is impressive to say the least.
SceneryDriver