VFD Disconnects

[infinity]

Wouldn't the drive burning out fall under "or introduces additional or increased hazards to persons or property"?

 

[mkgrady]

All I will say is lockouts are not acceptable unless the installation meets one of these conditions from 430.102(B).




Keep in mind 'impracticable' basically means impossible, it does not mean difficult.

I think B. Would apply. The roof hatch is kept locked. It's a big university in Boston that has strict maintenance and access rules. I can't even get in an electric closet without calling facility management.

 

[mkgrady]

Wouldn't the drive burning out fall under "or introduces additional or increased hazards to persons or property"?

Are you saying the disconnect is or is not needed?

 

[iwire]

Wouldn't the drive burning out fall under "or introduces additional or increased hazards to persons or property"?

I think B. Would apply. The roof hatch is kept locked. It's a big university in Boston that has strict maintenance and access rules.

I think both of these questions can only be answered by the AHJ.


It will be up to the AHJ to decide if the safety of personal is less important than the safety of the drive that could be protected from damage with the use of the AUX switch.

 

[Jraef]

After sending the rfi, the hvac contractor purchasing the drives and motors has contacted the drive manufacturer Allen-Bradley to try to resolve the auxiliary contacts issue. They say the contacts in the switch are not necessary due to new designs and the disconnects are not needed because the controllers can be locked out. Really? More on this below.

Which series of AB drive? Some of the older ones, I would be concerned. the newer ones (PowerFlex 70, 700, 750, 520) have the added protection, but again, that will NOT save you from potential damage if someone closes the disconnect with the VFD in the Run state already.

As a seperate issue I have been struggling to find a route for the four runs of conduit that run from the controllers to the disconnects and motors. They are on the roof and the drives are about 24 feet below. We are replacing four old existing motors that are straight 480 three phase line voltage. ALL twelve of the existing conductors are in one conduit from the electric room to the roof. I'm told the manufacturer said that we could run all of the conductors from the drives to the motors in the same conduit. I'm no vfd expert but this surprises me. Does this sound right?

Absolutely NOT! You must NEVER run the OUTPUTS of different drives in the same conduit, it's a recipe for drive failure, motor failure or both. Whomever told you that was unqualified to respond, or they misinterpreted the issue (or I have). Inputs To the drive in the same conduit is fine, but NEVER the outputs. Normally, as in no VFD involved, the phase conductors will not induce voltages on one another because everything is operating from the same source, so all magnetic fields are are in sync and one phase induction is cancelled out by the others. But when you have multiple VFDs, the output conductors are all operating at their own DIFFERENT sources, (even if the output frequencies are the same), so the natural cancellation of magnetic fields no longer happens and the cables induce voltages on one another, damaging the transistors in the drives and / or exacerbating the Reflected Wave phenomenon in the circuits to where the motor insulation breaks down. I recently saw one where the user had a good quality Inverter Duty motor, meaning it had the "Spike Resistant" magnet wire, but they ran the input and output conductors in the same tray for only about 25ft. Same issue, the input phases are out of sync with the output phases and the motor was seeing 3000V spikes, which punched right through that spike resistant insulation in about 6 months.

If we go by the manufactures suggestions we get rid of the motor disconnects, the auxiliary contacts (not in contract anyway), the four runs of conduit to the roof and maybe even the wire if it's the right size. I'm meeting the hvac contractor and the owner tomorrow. Any other comments are welcome and thanks to all for responding.

Whether or not you need or don't need the disconnect switches is a matter subject to the installation, facility etc., and I agree, it will come down to the AHJ. But if you are retrofitting into a situation where there never have been disconnects, that's a good argument for not needing them again. Just adding the VFDs doesn't really change anything in that context in my opinion.

 

[mkgrady]

I met with the owner, designer and the hvac contractor today. Turns out the engineer is insisting on seperate drive output conduits. Based on responses here that seems important and maybe they were just wrong about putting them all in one. It also turns out the fan motors are being supplied with disconnects included. Now I'll just have to make sure these disconnects have auxiliary contacts or I can add them in the field. Thanks for all the responses.

 

[Aleman]

Disclaimer: I don't advocate or suggest running multiple VFD motor circuits in the same conduit. That said....where I work we have a lot of drives and various controls with 100's of motors, mostly pumps. Our older controls were built by a real hack. I am in the middle of a project to clean these older systems up which is a lot of fun. Examples; multiple VFD outputs in the same pipe, mixed with misc low voltage controls, minimal or no circuit protection. My only point here is that even with these bad installs, I haven't seen any failures I can attribute to the wiring methods over a 4 year period. These drives are all Hitachi's. While we use AB drives now because my co workers hate Hitachi and the AB's have Ethernet IP, in my opinion the Hitachi's are fairly bullet proof. I have also seen multiple instances of production workers or maint guys shutting down a motor with a local disconnect and the drive seemed to be unconcerned. No failures I have seen caused by this. So while I know and have read and I believe these practices are no good, I have yet to see any negative consequences.

 

[Jraef]

Yes, but still, luck is not a strategy...

 

[Besoeker]

Which series of AB drive? Some of the older ones, I would be concerned. the newer ones (PowerFlex 70, 700, 750, 520) have the added protection, but again, that will NOT save you from potential damage if someone closes the disconnect with the VFD in the Run state already.

Which should not be possible if the disconnect has auxilliary contacts in the Run enable circuit. I wouldn't consider having a disconnect, manual or otherwise, without them.

Absolutely NOT! You must NEVER run the OUTPUTS of different drives in the same conduit,

Totally agree. It isn't likely to be an issue here. I can't say for certain that it hasn't/doesn't happen here. But for the drives I've dealt with over the years have been in industrial environments and SWA cable is used for both power and signal. Different cables for each you understand........

 

[Badams]

Combining drive output wiring

Combining drive output wiring

Which series of AB drive? Some of the older ones, I would be concerned. the newer ones (PowerFlex 70, 700, 750, 520) have the added protection, but again, that will NOT save you from potential damage if someone closes the disconnect with the VFD in the Run state already.



Absolutely NOT! You must NEVER run the OUTPUTS of different drives in the same conduit, it's a recipe for drive failure, motor failure or both. Whomever told you that was unqualified to respond, or they misinterpreted the issue (or I have). Inputs To the drive in the same conduit is fine, but NEVER the outputs. Normally, as in no VFD involved, the phase conductors will not induce voltages on one another because everything is operating from the same source, so all magnetic fields are are in sync and one phase induction is cancelled out by the others. But when you have multiple VFDs, the output conductors are all operating at their own DIFFERENT sources, (even if the output frequencies are the same), so the natural cancellation of magnetic fields no longer happens and the cables induce voltages on one another, damaging the transistors in the drives and / or exacerbating the Reflected Wave phenomenon in the circuits to where the motor insulation breaks down. I recently saw one where the user had a good quality Inverter Duty motor, meaning it had the "Spike Resistant" magnet wire, but they ran the input and output conductors in the same tray for only about 25ft. Same issue, the input phases are out of sync with the output phases and the motor was seeing 3000V spikes, which punched right through that spike resistant insulation in about 6 months.


Whether or not you need or don't need the disconnect switches is a matter subject to the installation, facility etc., and I agree, it will come down to the AHJ. But if you are retrofitting into a situation where there never have been disconnects, that's a good argument for not needing them again. Just adding the VFDs doesn't really change anything in that context in my opinion.

I recently designed a project with Allen Bradley PowerFlex 400 drives. Their literature says in part "Do not route more than three sets of motor leads in a single conduit to minimize “cross talk”. If more than three drive/motor connections per conduit are required,shielded cable must be used."

Here's a link to the entire manual. http://literature.rockwellautomation.com/idc/groups/literature/documents/um/22c-um001_-en-p.pdf

Why would Allen Bradley allow for up to 3 sets of motor leads in a single conduit and only require shielded cable if more than 3 motors are in the same conduit?

 

[Jraef]

I recently designed a project with Allen Bradley PowerFlex 400 drives. Their literature says in part "Do not route more than three sets of motor leads in a single conduit to minimize “cross talk”. If more than three drive/motor connections per conduit are required,shielded cable must be used."

Here's a link to the entire manual. http://literature.rockwellautomation.com/idc/groups/literature/documents/um/22c-um001_-en-p.pdf

Why would Allen Bradley allow for up to 3 sets of motor leads in a single conduit and only require shielded cable if more than 3 motors are in the same conduit?

Context, context, context. Read the rest of that sentence;
"... Shielded cable must be used."
This is what Besoeker was saying, and I should have said. You can do this IF you are using the shielded "VFD cable" where each set of 3 cables + ground is in a shielded jacketed bundle. Even then, this statement is warning not to put more than 3 of these shielded cable sets per conduit.

 

[Aleman]

Which should not be possible if the disconnect has auxilliary contacts in the Run enable circuit. I wouldn't consider having a disconnect, manual or otherwise, without them.

Well, the 'hack' didn't believe in local disconnects either despite any code requirements. So I have been cleaning things up, protection on the drives, local disconnects with interlocks, getting rid of the multiple runs in conduits etc.

I know a guy who recently interviewed with the person who originally built our older systems. During the interview the interviewer explained that he didn't believe in local disconnects. At least he is now putting drives on individual circuit protection. Seriously, some of these panels have 3 wires right off the main disconnect and daisy chained along multiple drives, protected by only what feeds the panel. Or should say had because I have fixed most of them.

 

[qcroanoke]

Yes, but still, luck is not a strategy...

+1

 

[kwired]

Disclaimer: I don't advocate or suggest running multiple VFD motor circuits in the same conduit. That said....where I work we have a lot of drives and various controls with 100's of motors, mostly pumps. Our older controls were built by a real hack. I am in the middle of a project to clean these older systems up which is a lot of fun. Examples; multiple VFD outputs in the same pipe, mixed with misc low voltage controls, minimal or no circuit protection. My only point here is that even with these bad installs, I haven't seen any failures I can attribute to the wiring methods over a 4 year period. These drives are all Hitachi's. While we use AB drives now because my co workers hate Hitachi and the AB's have Ethernet IP, in my opinion the Hitachi's are fairly bullet proof. I have also seen multiple instances of production workers or maint guys shutting down a motor with a local disconnect and the drive seemed to be unconcerned. No failures I have seen caused by this. So while I know and have read and I believe these practices are no good, I have yet to see any negative consequences.

I've had fairly good luck myself on multiple VFD outputs in same raceways in situations where at the time I didn't know any better. The one's I had the most trouble with were long runs and we ended up installing line reactors on the drive output and that lessened their frequency of failed motors, but they may have needed that even if run as single circuits in raceways.

Most if not all those were under 3 HP, don't know if that is also a factor or not. Most any larger then that I can recall running were always single drive output in the raceway.