VFD Caculation

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Gustav Eriksson

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Hi

If you conect 7 VFD 1HP 3 Phase in a cable where each VFD supplying a 0,5HP Motor and the fuse is 16A. This is a common way of bilding Conveyor Power supplu in EU."Look at the JPG picture". What i understand is that in US you must calculate with the input current of each VFD. That would give us half the motors on each fuse compared of what we are using in EU. Is this realy true or have i missed something ?
 
Gustav Eriksson said:
Hi

If you conect 7 VFD 1HP 3 Phase in a cable where each VFD supplying a 0,5HP Motor and the fuse is 16A. This is a common way of bilding Conveyor Power supplu in EU."Look at the JPG picture". What i understand is that in US you must calculate with the input current of each VFD. That would give us half the motors on each fuse compared of what we are using in EU. Is this realy true or have i missed something ?
Click to expand...

There may be a language thing going on, but I am having a hard time understanding your question.

You can connect as many motors to the load side of a VFD as you want. You just have to provide an external overload for each motor that will shut off the drive.

NEC article 430 part X discusses the specific requirements for VFDs.

Generally the line side conductors are sized for 125% of the rated input of the VFD, without respect to the line side fuse size. Normally the manufacturer will put this information in the manual.
 
VFD Calc.

VFD Calc.

We are using 1 VFD for each motor and they are connected to the same conductor with a 16A fuse for example. Here in EU we calculate the current for each motor not depending on the VFD rated iput current that gives us 7-8drives on a 16A fuse group. If we must calculate with the input current of VFD that will give us only 3-4 VFD on the same conductor and fuse. Hope this will make my question clearer.
 
I agree with Bob.
You seem to be approaching it from the standpoint of "How many VFDs can I squeeze in behind this fuse?" rather than "What is the proper way to fuse a set of VFDs?"

The conductors feeding each of the VFDs must be at least 125% of the VFD maximum input current rating. Then the fuses must be sized to protect the conductors. So if you have 1 set of conductors feeding all of the VFDs, you would add up the max. amps of all of them and add 25%, then select the cable to supply that much current and then fuses to protect that cable. So assuming these are 480V drives and motors, 1HP drives are probably rated for 2.5A input(?), so 7 drives would be 17.5A, x 1.25 = roughly 23 amps. You would need to size the conductors for 23A (#10 AWG) and fuse accordingly. But in some of the 1HP VFD manuals I can see, they say the maximum fuse size in front of them is 10A anyway, so you would need to further break down this grouping no matter what if that is the case.

As was said by Bob, read the installation manual for your drive because that's where they will lay this out for you.
 
Random thoughts on this topic:

Jraef: do you use the _maximum_ or the _rated_ input amps of the VFD? My understanding is that most VFDs have a continuous rating and in addition have some higher value specified for 30 seconds or 1 minute. I'd thought that you were supposed to use the continuous rating for sizing with respect to the NEC.

Second thought: Would it be reasonable for the VFD manufacturer to apply _different_ ratings based upon the load connected to the VFD?

Third thought: Are we discussing a field installation covered by the NEC, or the internal wiring of a piece of equipment, in which case the applicable rules are governed by the testing labs, and there is quite a bit more room for 'engineering supervision'.

-Jon
 
Gustav Eriksson said:
We are using 1 VFD for each motor and they are connected to the same conductor with a 16A fuse for example. Here in EU we calculate the current for each motor not depending on the VFD rated iput current that gives us 7-8drives on a 16A fuse group. If we must calculate with the input current of VFD that will give us only 3-4 VFD on the same conductor and fuse. Hope this will make my question clearer.
Click to expand...
Gustav

That may be the way you have seen it done.
I'm also in the EU (UK).
We (as a company) always fit individual isolation on each drive. Usually, that is a fuse-switch and usually an ABB OS or OESA type.
If the drive feeds a number of motors, each has it's own overload protection.
Others may do it differently.
 
winnie said:
Second thought: Would it be reasonable for the VFD manufacturer to apply _different_ ratings based upon the load connected to the VFD?
Click to expand...
Most VFD manufacturers give high overload and low overload ratings.
Typically, the high overload rating is used for "constant torque" loads and the low overload for variable torque applications (centrifugal pumps and the like).
 
Excuse my ignorance but, you guys are saying that you have installed 1 VFDs for 2 or more motors in a conveyor system? I guess that if you do that then over current protection would be for both motors on the line side of the VFD and the thermals for each individual motor at the load side? I have never encounter this, I have done a lot of conveyor systems, everything was speck out for individual motors. If you needed to daisy chain the motors we would use a speed pot for the control or the plc`s.
 
codewarrior321 said:
Excuse my ignorance but, you guys are saying that you have installed 1 VFDs for 2 or more motors in a conveyor system? I guess that if you do that then over current protection would be for both motors on the line side of the VFD and the thermals for each individual motor at the load side?
Click to expand...
For a multi-motor installation from one VSD we'd put all individual motor protection on the load side of the VSD.
Line side protection would be for the VSD.
 
codewarrior321 said:
Where is the Branch Short Circuit And Ground Fault protection for the Individual Motors as required by NEC 430.52.
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Where does it say you have to have one for each motor in a group installation?

The VFD or its input fuses will trip if you have a ground fault or short circuit on any of the motors attached to the VFD. In clearing the fault on one motor it will stop the other ones as well.
 
winnie said:
Random thoughts on this topic:

Jraef: do you use the _maximum_ or the _rated_ input amps of the VFD? My understanding is that most VFDs have a continuous rating and in addition have some higher value specified for 30 seconds or 1 minute. I'd thought that you were supposed to use the continuous rating for sizing with respect to the NEC.
Click to expand...
You would use the continuous input current, not the short time OL rating (which is a numbers game anyway). The issue is, you cannot use the output or connected current rating because just like in your case, what if the motor is significantly smaller? If you sized everything for the connected motor only, then someone comes along later and sees the higher rating on the VFD nameplate and changes the motors to get "more power", they may end up creating a hazardous situation. The rule is there to prevent that. So if the VFD is rated for 15A input current but the motor only has a 5A FLC, if someone some day connects a 12A FLC motor to it, they are covered.

Second thought: Would it be reasonable for the VFD manufacturer to apply _different_ ratings based upon the load connected to the VFD?
Click to expand...
Happens all the time, but again, the issue is for that future situation where someone is reading the current rating of the VFD. So whatever the VFD says on the nameplate is what the rule is based on.

Third thought: Are we discussing a field installation covered by the NEC, or the internal wiring of a piece of equipment, in which case the applicable rules are governed by the testing labs, and there is quite a bit more room for 'engineering supervision'.

-Jon
Click to expand...
The NRTL would be the standard to build a panel by, but it must still be installed in the field. If the AHJ doesn't like what he sees, he can still red tag it. You are welcome to argue the issues and you may very well win on a technicality, but the person who's job is held up in the process is not going to like you very much...;)
 
petersonra said:
Where does it say you have to have one for each motor in a group installation?

The VFD or its input fuses will trip if you have a ground fault or short circuit on any of the motors attached to the VFD. In clearing the fault on one motor it will stop the other ones as well.
Click to expand...

NEC Article 430.53 has a lot of restrictions, in making the suggested installation. Not over 1 HP, Must be listed, ECt.....
I stand to be corrected
 
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