upgrading to vfd's

Status
Not open for further replies.

eadd9000

Member
Our mcc panel at our facility had degraded to the point that it needed to be replaced.

we are running 5 motors of the starters in the mcc panel i intend to replace the panel with vfd's that being said i was going to run a 2 inch conduit from the main breaker box to a wireway ( total run of pipe 60' ) pull in my 24 # 12 awg conductors
three hots and a ground for each vfd then i would like to pull another 24 #12 awg conductors through the same conduit out of the vfd's to run the motors.

two questions do i need to pull a ground for each vfd and is pulling the incoming and outgoing power through the same conduit a bad idea.

i called ac tech and they said i might get some noise on the line but it should be fine but i just wanted to double check with you guys.

p.s. i'm a newbiesorry if i insult anyones intelligence and thank you mike holt your books got me to the point where i can proudly tell my dad i'm a master electrician.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
How are you handling derating with that many hots in one pipe?

The second thing is, I personally don't like stuffed conduits just because if one wire blows apart a few years down the road there's a good chance the other wires will take collateral damage. Plus the derating issue...
 

cadpoint

Senior Member
Location
Durham, NC
...three hots and a ground for each vfd then i would like to pull another 24 #12 awg conductors through the same conduit out of the vfd's to run the motors.
...
two questions do i need to pull a ground for each vfd and is pulling the incoming and outgoing power through the same conduit a bad idea.

I think there's a lot more to consider than your two questions and hopefully I've covered enough of them! :)

First off a granda conduit size doesn't help with required derating of putting more wires into a larger conduit, Chap. 9, Table 1. I didn't do the math to qualify the two inch conduit. Since you said wireway, I'd also re-read all of Article 376.

I figure that you have 3 wires and a EGC per motor, so that's (if bundled) 16 wires -per, see Article 250.122(C) read all in respects to your EGC. Since your present design at 45% with 30 wires that leaves you at 13.5 amp without consideration for an added percentage required for a motor and any * heat derating, like but not limited to Article 430.7(C) Did you make it?
Opps I didn't throw in any consideration of this as 3 ph power!

As earlier mentioned * Table 310.15(B)(2)(a), I'd also look at Table 310.15(B)(3)(a) mandated by Article 310.15(B)(3).

I believe I'd review in the Index, Motors; Adjustable Speed Drives, Branch circuits, and Conductors, Disconnecting Means OK frankly Go A to Z in Motors because you didn't qualifiy what the motors are or doing!.

I called ac tech and they said i might get some noise on the line but it should be fine but i just wanted to double check with you guys.

I'd ask for a complete book/pamplet or installation guide of the VFD. Most new books on VFD's installations that I've read (and read here) state one conduit per Line and one conduit per Load. Now this is not set in stone(one per), but we have to follow the instructions of the manufacture... :)

Are the VFD's going to be rated as a means of disconnect, and still be in line of sight sixty foot away?

Requirements are just different and more strict when dealing with motors.

Welcome to the Forum!
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Code issues aside, my experience is that if you just run a lot of VFD output cables in the same raceway / conduit, you are asking for all kinds of trouble. Putting them nto the same raceway as the incoming cables is a suicide wish.

If you were not going to run the motors at different speeds, you would not need separate VFDs. If you are therefore running at different frequencies and currents, a lot of noise and induced voltages get expressed on the various VFD leads because the inductive coupling no longer is in sync with the adjacent conductors and sooner than later, you start damaging the motor windings and/or the VFD output transistors. Usually if you read the VFD manual they will tell you this, but in looking at a couple of AC Tech manuals I have, they fail to mention it. Tech support guys at VFD mfrs are not electricians, they are usually propeller heads. I would not trust their advice on bigger issues like this. One of the bigger VFD installation disasters I have had to go in and fix was one where an end user rant the output cables for 8 x 1HP AC Tech VFDs in one 3" conduit out to a chain of conveyor motors where the first one was about 75' from the cabinet and the last one was 800' away. They were losing their VFDs at a rate of about 1 per month, all due to transistor failures. After we separated the conductors in individual conduits, that problem went away.

The newer caveat is the advent of "Inverter Cable" which is a triplexed shielded conductor set. Example. But these have an overall jacket for each set and pulling 5 sets in one conduit might be a daunting task at the very least. Still, better than running separate conduits though.
 
Code issues aside, my experience is that if you just run a lot of VFD output cables in the same raceway / conduit, you are asking for all kinds of trouble. Putting them nto the same raceway as the incoming cables is a suicide wish.

If you were not going to run the motors at different speeds, you would not need separate VFDs. If you are therefore running at different frequencies and currents, a lot of noise and induced voltages get expressed on the various VFD leads because the inductive coupling no longer is in sync with the adjacent conductors and sooner than later, you start damaging the motor windings and/or the VFD output transistors. Usually if you read the VFD manual they will tell you this, but in looking at a couple of AC Tech manuals I have, they fail to mention it. Tech support guys at VFD mfrs are not electricians, they are usually propeller heads. I would not trust their advice on bigger issues like this. One of the bigger VFD installation disasters I have had to go in and fix was one where an end user rant the output cables for 8 x 1HP AC Tech VFDs in one 3" conduit out to a chain of conveyor motors where the first one was about 75' from the cabinet and the last one was 800' away. They were losing their VFDs at a rate of about 1 per month, all due to transistor failures. After we separated the conductors in individual conduits, that problem went away.

The newer caveat is the advent of "Inverter Cable" which is a triplexed shielded conductor set. Example. But these have an overall jacket for each set and pulling 5 sets in one conduit might be a daunting task at the very least. Still, better than running separate conduits though.
Exactly. To restate it:

Not only the input/output are should not be run in the same conduit, but there is a clear warning that EACH output should be run separately from each other.
ASD's themselves are also to be mounted their indiviual shielded enclosure with a maintained distance from each other as some manufacturers point out. (Experience has shown that the radiated noise CAN interefere with adjacent ASD's operation, resulting not only mis-operation but eventual failure.)The shielded enclosures can be mounted in common housing as long as you take care of the heat removal from it.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Exactly. To restate it:

...
ASD's themselves are also to be mounted their indiviual shielded enclosure with a maintained distance from each other as some manufacturers point out. (Experience has shown that the radiated noise CAN interefere with adjacent ASD's operation, resulting not only mis-operation but eventual failure.) The shielded enclosures can be mounted in common housing as long as you take care of the heat removal from it.

Most small drives (i.e. 15A and under) now are internally shielded and can be gang mounted in the same box, in fact several mfrs specifically advertise what they call "bookshelf" mounting and vertical cooling efficiencies that allow low clearances in between drives. A while ago you would have had to leave 4" or so between drives, some of the newer ones are now 1" or maybe even less side-to-side. But for sure this changes as the drive power increases because the RFI/EMI energy level increases as well, so this statement still holds true at that level.

It's always best to read that manual thoroughly. Before reading the AC Tech manuals earlier today I would have thought it a given that they addressed this, but AC Tech unfortunately does not seem to think these issues are important enough to mention. Maybe that explains their high field failure rate a few years ago?
 
Most small drives (i.e. 15A and under) now are internally shielded and can be gang mounted in the same box, in fact several mfrs specifically advertise what they call "bookshelf" mounting and vertical cooling efficiencies that allow low clearances in between drives. A while ago you would have had to leave 4" or so between drives, some of the newer ones are now 1" or maybe even less side-to-side. But for sure this changes as the drive power increases because the RFI/EMI energy level increases as well, so this statement still holds true at that level.

It's always best to read that manual thoroughly. Before reading the AC Tech manuals earlier today I would have thought it a given that they addressed this, but AC Tech unfortunately does not seem to think these issues are important enough to mention. Maybe that explains their high field failure rate a few years ago?

In my previous response I left out the highlighted world "....ASD's themselves are also to be mounted in their individual shielded enclosure with...". I was trying to call the attention to the dangerous practice where some panel builders would install chassis mounted drives adjacent to each other without shielding.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
In my previous response I left out the highlighted world "....ASD's themselves are also to be mounted in their individual shielded enclosure with...". I was trying to call the attention to the dangerous practice where some panel builders would install chassis mounted drives adjacent to each other without shielding.
Oh yeah, bad idea for sure.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It is probably best to route them separately if you can.

Having said that, there are many, many thousands of drives installed the way you describe that work fine. But, you might just get lucky and have an issue.

Just what is degraded about your MCC? If the drives are small enough, you might be able to just remove the existing starters and put the drives in the old buckets. No need for new conduit at all.

We sometimes just bolt new sections onto the end of existing MCCs as well. You can get MCC sections from every manufacturer that will accommodate VFDs.
 
Last edited:

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It is probably best to route them separately if you can.

Having said that, there are many, many thousands of drives installed the way you describe that work fine. But, you might just get lucky and have an issue.

Just what is degraded about your MCC? If the drives are small enough, you might be able to just remove the existing starters and put the drives in the old buckets. No need for new conduit at all.

We sometimes just bolt new sections onto the end of existing MCCs as well. You can get MCC sections from every manufacturer that will accommodate VFDs.

Just one minor caveat to that. If they are MCP starters, you would need to remove the starters and the MCP breakers, then replace them with regular Thermal Mag breakers for the VFDs.

But I too wonder what "degraded" means in an MCC. I regularly work on MCCs that are 40+ years old. The original parts may not be available but if they work, they work. I just did a project with some old 9600 Series Goulds MCCs from the 1960s. Goulds is long gone and although by legacy they are now supported by Siemens, the old Rowan contactors inside are still working fine.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Just one minor caveat to that. If they are MCP starters, you would need to remove the starters and the MCP breakers, then replace them with regular Thermal Mag breakers for the VFDs.

But I too wonder what "degraded" means in an MCC. I regularly work on MCCs that are 40+ years old. The original parts may not be available but if they work, they work. I just did a project with some old 9600 Series Goulds MCCs from the 1960s. Goulds is long gone and although by legacy they are now supported by Siemens, the old Rowan contactors inside are still working fine.

Since we are doing some field engineering with an old MCC anyway, wouldn't installing a set of fuseholders on load side of the MCP and prior to the VFD work as well and be a bit more economical?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Since we are doing some field engineering with an old MCC anyway, wouldn't installing a set of fuseholders on load side of the MCP and prior to the VFD work as well and be a bit more economical?
Hmmm... not a bad idea, assuming of course the fuse holders fit, something like a Class J maybe? Technically in that scenario the MCP can be thought of as a "Molded Case Switch" then. I suppose if an AJH agrees, it would be technically fine.

Of course one could argue that this combination of devices would not have been in the original UL845 file of the MCC mfr., but chances are nothing else you do would be anyway, so an AHJ would be making the determination in the field no matter what.
 
Status
Not open for further replies.
Top