Paralleling load side of a VFD

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hbeery10

hbeery10

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Sardis, Ohio
We are wiring 3 VFD's. They are all 480 VAC, 3 phase. Sizes are 350 HP, 150 HP, and 125 HP. Is there any reason we can't run the conductors out of the load side of the VFD's in parallel? Does that cause any issues with harmonics or anything? Thanks!

Garth
 
hbeery10 said:
We are wiring 3 VFD's. They are all 480 VAC, 3 phase. Sizes are 350 HP, 150 HP, and 125 HP. Is there any reason we can't run the conductors out of the load side of the VFD's in parallel? Does that cause any issues with harmonics or anything? Thanks!

Garth
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Garth -
I'm guessing you are meaning to run the conductors side by side, as in a cable tray; as opposed to connecting the outputs together?

ice
 
I hope you mean next to each other rather than electrically in parallel.
If the conductors are not individually shielded you might get some unwanted interactions that might cause problems. I would at least bundle/cable each set of wires seperately.
 
First, the HP ratings I posted are the size of the VFD's, not the motors they are running. The motors are smaller, but I don't know for sure what the motor sizes are. Not that it makes a big difference for our purposes, but just wanted to clarify that.

The dwgs speced 500 kcmils from the biggest drive and I am just trying to avoid running wire that big (as well as save money), so I was going to run 3/0 in parallel. We weren't concerned about feeding the VFD with parallel 3/0, but we just weren't sure if that would cause any problems out of the load side.
 
GoldDigger said:
I hope you mean next to each other rather than electrically in parallel.
If the conductors are not individually shielded you might get some unwanted interactions that might cause problems. I would at least bundle/cable each set of wires seperately.
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So each set of 3/0 bundled conductors would not need shielded as long as they are bundled, correct? Is zip tying the conductors together in the cable tray typically how they would be bundled? We have thrown the idea around of buying premade tray cable, but we may end up running individual TC conductors and then bundling them in the cable tray since the lead time is so long for the prebundled tray cable.
 
hbeery10 said:
First, the HP ratings I posted are the size of the VFD's, not the motors they are running. The motors are smaller, but I don't know for sure what the motor sizes are. Not that it makes a big difference for our purposes, but just wanted to clarify that.

The dwgs speced 500 kcmils from the biggest drive and I am just trying to avoid running wire that big (as well as save money), so I was going to run 3/0 in parallel. We weren't concerned about feeding the VFD with parallel 3/0, but we just weren't sure if that would cause any problems out of the load side.
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What makes you think you can just change something that is spec'd on a drawing?

It might not make any real difference, but if you bid to a set of drawings it is potentially criminal fraud not to install it that way.

OTOH, if the drawings are just vague suggestions that you are free to comply with only if you want to, then it does not matter any.
 
petersonra said:
What makes you think you can just change something that is spec'd on a drawing?

It might not make any real difference, but if you bid to a set of drawings it is potentially criminal fraud not to install it that way.
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We are going to run it by the owner before doing it, but just wanted to make sure it was a recommendable change. Also, this is an industrial setting (not a taxpayer funded project) and as far as I know it was not publicly bid, we just gave a quote for the work.
 
hbeery10 said:
We are going to run it by the owner before doing it, but just wanted to make sure it was a recommendable change. Also, this is an industrial setting (not a taxpayer funded project) and as far as I know it was not publicly bid, we just gave a quote for the work.
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Do these drawings have an engineer's seal on them?

Did they go through any kind of plan review by the local inspection agency?
 
If you run those in cable tray without any shielding, you are asking for trouble. You are going to create a very powerful local FM radio transmitter. Think about what FM radio ntands for; Frequency Modulation, exactly what a VFD does.

Parallel runs is fine, if inside of grounded steel conduit. In open cable tray or PVC conduit , all VFD output conductors must be shielded. Aside from the RF radiation, cross induction from the different frequencies alone will give you no end of problems. Remember, when we run conductors next to each other in tray, they are all at the same frequencies, so any urial induction gets cancelled out. If the conductors are carrying different frequencies, as they would with VFD outputs, they do not. This makes reflected wave spikes extremely more likely and dangerous, often exceeding the insulation resistance of "inverter duty" motors. I recently witnessed a 250HP Baldor inverter duty motor get killed by this exact thing, and Baldor motors are not crap, their insulation is rated 2200V.
 
Our local wastewater plant just had four new 40HP Siemens blower motors installed that are being ran on AB drives. They have now all shorted out phase to phase, the motor rewind shop says it's because European motors are wound to a different standard with a lower insulation rating. I don't believe the motors nameplate even made it known what insulation class they were, F, etc.
 
Cow said:
Our local wastewater plant just had four new 40HP Siemens blower motors installed that are being ran on AB drives. They have now all shorted out phase to phase, the motor rewind shop says it's because European motors are wound to a different standard with a lower insulation rating. I don't believe the motors nameplate even made it known what insulation class they were, F, etc.
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I think it unlikely. Siemens motors sold in the US generally have dual NEMA and IEC ratings. If they have a nameplate that mentions HP, it is a good bet they have a NEMA rating as well as their IEC rating.

In any case, VFDs are probably more common in Europe then in the US, so it is doubtful a company like Siemens would make a general duty motor not suitable for service there with VFDs.

No doubt there are some specialty motors made that are not suitable for use with VFDs, but they are special.
 
petersonra said:
I think it unlikely. Siemens motors sold in the US generally have dual NEMA and IEC ratings. If they have a nameplate that mentions HP, it is a good bet they have a NEMA rating as well as their IEC rating.

In any case, VFDs are probably more common in Europe then in the US, so it is doubtful a company like Siemens would make a general duty motor not suitable for service there with VFDs.

No doubt there are some specialty motors made that are not suitable for use with VFDs, but they are special.
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I agree. If it says "40 HP" on the plate, it's not an IEC motor. IEC motors will show a kW rating, and 40HP comes out to 53.6kW, but a standard size IEC motor would be 55kW, then because it is rated at 50Hz, if they re-rated it in HP because of 60Hz operation, it would be closer to 50HP. Also, IEC motor design specs for insulation are the same as NEMA in most cases, so the nameplate should have F or H or whatever it is.

But the insulation temperature rating has little to do with being capable of surviving the potential voltage spikes from VFDs. Basic magnet wire in days gone by was typically rated 1200V because with sine wave power, that was more than enough. But VFDs and the cables used in connecting them can cause very high voltage spikes to be created, easily over 1200V, often over 1600V and under the wrong conditions, such as in open tray and next to each other over distances, over 2000V. Motors that just say "inverter rated" do not necessarily have higher VOLTAGE rated magnet wire, you must read the fine print, and/or use a sine wave filter on the output of the drive.
 
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