VFD Circuit design

[infinity]

For all of you who design these circuits what would you use: (OCPD, input conductor size, output conductor size) for a 480v, 60 HP motor {77 amps T430.250} and a 90 amp VFD input?


OCPD=
Input Conductor=
Output Conductor=

 

[Jraef]

For all of you who design these circuits what would you use: (OCPD, input conductor size, output conductor size) for a 480v, 60 HP motor {77 amps T430.250} and a 90 amp VFD input?


OCPD=
Input Conductor=
Output Conductor=

OCPD= No less than 100A, no more than whatever the VFD manual says is appropriate. In some cases, you have to pay close attention to what it is LISTED to be used with, and there is little room for deviation, and many VFDs are ONLY listed with fuses. So you CAN have a CB, but you MUST also have the fuse, sized per the manual. So for example on A-B drives it is 225A max. CB or 100-170A TD fuses. But on ABB drives it MUST be 125A fuses, the breaker is optional in addition to them. You have to pay attention to the details of YOUR specific VFD.

Input Conductor= 90A x 1.25 = 112.5A minimum per 430.122, adjusted for distance / voltage drop, fill, ambient etc. But certainly no less than #1

Output Conductor= 77A x 1.25 = 96.25, adjusted as above, but no less than #3

 

[romex jockey]

We just installed some VFD cable due to power quality woes from a freq drive to a 4803ph 7.5hp

In the instructions the shield is to be married to the ECG at both ends of the run

I don't understand this, i guess i had thought a 'drain' (that which picks up stray anything) would be only run back to source voltage

~RJ~

 

[infinity]

OCPD= No less than 100A, no more than whatever the VFD manual says is appropriate. In some cases, you have to pay close attention to what it is LISTED to be used with, and there is little room for deviation, and many VFDs are ONLY listed with fuses. So you CAN have a CB, but you MUST also have the fuse, sized per the manual. So for example on A-B drives it is 225A max. CB or 100-170A TD fuses. But on ABB drives it MUST be 125A fuses, the breaker is optional in addition to them. You have to pay attention to the details of YOUR specific VFD.

Input Conductor= 90A x 1.25 = 112.5A minimum per 430.122, adjusted for distance / voltage drop, fill, ambient etc. But certainly no less than #1

Output Conductor= 77A x 1.25 = 96.25, adjusted as above, but no less than #3

So the OCPD is determined more by the drive manufacturer than the designing engineer. Is there much leeway in that design? I'm reviewing some drive branch circuits and seeing small variations from about 150%-175% of the drive input current with time delay fuses. I guess that fits within the parameters that you've outlined.

 

[petersonra]

for drives you follow what it says in the manual for the input OCPD. Most times I use a 250% circuit breaker, as most modern drives are listed to allow it.

the output side is just a standard motor circuit, however, if you are using VFD cable, I am not convinced that the size of the EGC in them is always compatible with a 250% input CB.

 

[Jraef]

We just installed some VFD cable due to power quality woes from a freq drive to a 4803ph 7.5hp

In the instructions the shield is to be married to the ECG at both ends of the run

I don't understand this, i guess i had thought a 'drain' (that which picks up stray anything) would be only run back to source voltage

~RJ~

In shielded SIGNAL wires, the shield and drain carry EXTERNAL sources of noise away from your signal wires to ground without adding to it by making a ground loop.

In shielded POWER cables, the shield and drain are containing INTERNALLY created EMI/RFI interference from getting OUT of the cable and affecting other things. So what you are doing is creating a faraday cage around the cables.

 

[romex jockey]

Thx Jraef



~RJ~