VFD Line/Load

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C3PO

Senior Member
Location
Tennessee
The instructions in the drives that I use the most say not to run them in the same conduit. It is probably not a good idea, but if it was a really short run might not give you any problems????
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Can the line and load be in the same conduit for a variable-frequency drive?

the short answer is yes. But the real question is whether it is a good idea. The answer to that question is maybe, maybe not.

IMO, it is better to seperate them unless you are sure it won't make any difference.
 

Besoeker

Senior Member
Location
UK
I am inclined to agree that in general it is better not to do this.

However, there are cases where one can run into situations where routing two conduits to a VFD is difficult where one might want to take a closer look.
I would have thought that, as a rule, the supply and the motor would be in two different locations making it a bit impractical to use a common conduit for all the conductors.
It isn't something I've run into. We use separate steel wire armoured cables for input and output.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I took these from the AB Powerflex 40 manual.

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.

I don't know that I have ever had even two sets of VFD outputs routed in a single conduit, much less three.

I did not see any restrictions either in this manual or in the general AB drives instruction manual about not routing input power and VFD output in the same conduit. Maybe it is there, but I cannot recall ever seeing it, and I specifically went looking for such a caution just now.

In fact, I don't recall seeing it in any VFD manuals, but I mostly use AB drives.

I looked thru the manual for a Baldor drive I used last year and did not see anything there about this either.
 

Besoeker

Senior Member
Location
UK
I did not see any restrictions either in this manual or in the general AB drives instruction manual about not routing input power and VFD output in the same conduit. Maybe it is there, but I cannot recall ever seeing it, and I specifically went looking for such a caution just now.

In fact, I don't recall seeing it in any VFD manuals, but I mostly use AB drives.

I looked thru the manual for a Baldor drive I used last year and did not see anything there about this either.

As I said above, it isn't a problem we would encounter because we don't use conduit very much for anything.
For a VSD we commonly use, the installation section of a the manual lists three different types of cable for mains input, motor output, and control respectively.

This is a motor cable:
Symmetrical power cable equipped with compact low-impedance shield and intended for the specific mains voltage. 360? earthing of the shield with cable glands in both ends needed for EMC levels C and H.

The shield is to prevent the noise resulting from the fast switching edges interfering with other things.
If the motor and mains conductors are in the same conduit there is no shield between them and there would be would be some coupling between the conductors resulting in noise on the mains conductors. And thus the supply. Not good.
 

Besoeker

Senior Member
Location
UK
This is a tangent on the topic, but I tihnk the NEC eventually needs to address the issue of non-sinusodial and high frequency current rating of conductors along with derating for multiple conductors.
Possibly so.
In the context of VFDs, the input current is fairly non-sinusoidal but, if you use the RMS values for conductor current rating, you should be fine.
The output currents are usually fairly good sine waves. Although the inverter section of the VFD may have a switching frequency, the motor inductance attenuates the high frequency component.
Voltage stress is a different matter.
 
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