can you feed 2 vfd's from 1 disconnect?

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cskinney86

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can you feed 2 vfd's from 1 disconnect? Here are the specs. 230v 3phase power. I have A 60amp breaker with #6g wire going into a 60A non-fusible disconnect.
Then was wanting to feed 2 vfd's with #10g wire out of the disconnect to 30a fuses for each vfd. The motors after the vfd's are rated for 26a fla. So 60a breaker(6g)>60a disconnect>(10g)30a fuses>vfd's(10g)>26fla motors. I hope I explained that right. Thanks for the help!
 
cskinney86 said:
so I need to run 6g wire from the disconnect to the 30a fuses before the vfd's?
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I did not say that. If you ran #6 wires to the 30 A fuses they would not be tap conductors.

The ampacity of the wires going to a VFD is not determined by the FLC of the motors but by the rated input current of the drives.

The arrangement you are describing seems kind of sketchy to me, code wise.

430.122 covers sizing of conductors to VFDs.

430.130 covers the CB or fuse requirements. USUALLY, 430.130(A)(2) is the most appropriate.

Potentially, 430.131 would apply in your case.
 
With VFD inputs the final concern is input short circuit ratings. Many VFDs have atrocious input ratings like 5 kA, especially small ones. As a rule of thumb if the kVA of the transformer exceeds 10 times the kVA of the VFD, in all likelihood you have an issue. The fix is simple. Install a 3% line reactor. You can use smaller line reactors or even substitute a transformer if you also need voltage conversion but the 3% line reactor rule works for most cases. Current limiting fuses don’t give you enough current limiting but if you insist on going down that path then you need properly sized semiconductor fuses as an alternative to a line reactor. The reactor also helps with surge protection as well. Don’t count on it curing harmonics though. It’s pretty useless for that.

Also not mentioned...do NOT use input contactors unless they are rarely used. This can cause premature failures with the precharge circuit.
 
paulengr said:
With VFD inputs the final concern is input short circuit ratings. Many VFDs have atrocious input ratings like 5 kA, especially small ones. As a rule of thumb if the kVA of the transformer exceeds 10 times the kVA of the VFD, in all likelihood you have an issue. The fix is simple. Install a 3% line reactor. You can use smaller line reactors or even substitute a transformer if you also need voltage conversion but the 3% line reactor rule works for most cases. Current limiting fuses don’t give you enough current limiting but if you insist on going down that path then you need properly sized semiconductor fuses as an alternative to a line reactor. The reactor also helps with surge protection as well. Don’t count on it curing harmonics though. It’s pretty useless for that.
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This was sort of true with old VFDs but the new ones that are listed to the latest UL standards are a lot better. Even the small ones often take on the short circuit current rating of the AIC rating of the overcurrent protection device feeding it. Not universal but a lot better than it used to once be. I don't think you can even get the older style VFDs anymore. They can still be built as long as they are building the exact same thing they've been building for 20 plus years but once they change to a different model UL won't list them anymore unless they go to the new standards.
 
petersonra said:
This was sort of true with old VFDs but the new ones that are listed to the latest UL standards are a lot better. Even the small ones often take on the short circuit current rating of the AIC rating of the overcurrent protection device feeding it. Not universal but a lot better than it used to once be. I don't think you can even get the older style VFDs anymore. They can still be built as long as they are building the exact same thing they've been building for 20 plus years but once they change to a different model UL won't list them anymore unless they go to the new standards.
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It’s not all VFDs by a long distance. Some are very, very good. Others are total crap. You just can’t make that assumption. And it’s not UL...they didn’t increase their standards. It’s two things. The word about this is getting out, and have you bothered to try to read and interpret the input specs? And the spec sheets themselves are either trivially easy to use or a mine field of garbage that tries to upsell their equipment (AB is particularly bad). In the old days almost every VFD manual stated that it should be driven from an isolation transformer. That’s the change.
 
paulengr said:
It’s not all VFDs by a long distance. Some are very, very good. Others are total crap. You just can’t make that assumption. And it’s not UL...they didn’t increase their standards. It’s two things. The word about this is getting out, and have you bothered to try to read and interpret the input specs? And the spec sheets themselves are either trivially easy to use or a mine field of garbage that tries to upsell their equipment (AB is particularly bad). In the old days almost every VFD manual stated that it should be driven from an isolation transformer. That’s the change.
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I have not used any junk drives in a long while. AB , Siemens, Schneider is about it. They are easy to find info on.
 
Ok those are brand names. Schneider assembles some but the components come from elsewhere. AB doesn’t even build them. Siemens makes some, especially medium voltage (Robicon) but not all. The drive modules themselves are decidedly not Siemens and 5 minutes taking one apart reveals the name Semikron.

So pray tell what is the difference between a Toshiba, Schneider, Mitsubishi, and say an older GE? Are any of these “junk drives”? Hint: they all come off the exact same production line.

And if you’ve spent as much tine trying to clean up the output of an AB as I have you’d think differently of them. Here is a common mode filter for a 900 HP, 480 V Powerflex. it’s still tearing the crap out of a motor with 90 A of common mode current (!!!) with insulated bearings and two shaft brushes. Need 4 more cores. Cable is 500 MCM so impractical to wind multiple turns. Of course all AB had to say for themselves is it needed VFD cable which predictably did nothing at all. All this giant filter because the drive output is total crap.

da1537f0b73f6e9d2350f06a559aee4a.jpg


It’s far more beneficial to find out where your drives come from.
 
No one makes much of anything anymore.

What version of power flex is it? The 753 and 52x line are pretty solid.

The 4, 40,400, 70, and 700 are pretty iffy imo.
 
petersonra said:
No one makes much of anything anymore.

What version of power flex is it? The 753 and 52x line are pretty solid.

The 4, 40,400, 70, and 700 are pretty iffy imo.
Click to expand...

A 753 “put together”. Two drives with one controller firing both to get the current up.
 
cskinney86 said:
can you feed 2 vfd's from 1 disconnect? Here are the specs. 230v 3phase power. I have A 60amp breaker with #6g wire going into a 60A non-fusible disconnect.
Then was wanting to feed 2 vfd's with #10g wire out of the disconnect to 30a fuses for each vfd. The motors after the vfd's are rated for 26a fla. So 60a breaker(6g)>60a disconnect>(10g)30a fuses>vfd's(10g)>26fla motors. I hope I explained that right. Thanks for the help!
Click to expand...
Where are the fuses going to be, in the VFD cabinets with no additional disconnect? Not a good plan. If one fuse blows and the only disconnect is the 60A one feeding both drives, you lose both drives when you have to shut down, troubleshoot and replace a fuse (or drive). I prefer having a disconnect for each individual drive. It’s simpler, cleaner and safer. The exception would be if the two drives are feeding a machine in which, if one goes down, the entire machine must be shut down anyway. In that case you can do what you planned, but as Bob said, you must then follow the Feeder Tap rules.

What size fuse and type of fuse you need for each drive is dictated by what the drive is listed with. You cannot just pick one out on your own, you must follow the drive mfr instructions. Bob is correct on that too, the UL listing standard for VFDs HAS changed relatively recently and fusing has become much more strict.

Side note: SCCR for non-fused disconnects has become problematic too. Although a few have been series listed with specific breakers of the same mfr as the switch, most are not so they typically only carry a 10kA SCCR at best, some are as low as 5kA.
 
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