ND vs HD VFD cable sizing

[PlasticPanda]

NEC 430.122(A) has a little informational note that says “Power conversion equipment can have multiple power ratings and corresponding input currents.” but provides no guidance on how to handle those. I have a 250hp positive displacement pump driven by a 300hp motor. The VFD is configured for 250hp (HD) but is also rated for 300hp (ND). Now there are two engineers fighting over whether the feeders need to be sized for 250hp or 300hp. Who is right and why?

 

[MTW]

The feeder and overcurrent device needs to be sized by what the VFD calls for on it's input, not by the motor size. Read the manual.

 

[don_resqcapt19]

You have to look at the manual. Many VFDs provide for alternate motor sizes less than the maximum for that drive, and provide a lower input amp rating for the smaller motor. If your drive manual does that, then there will be a lower input amp rating in the instructions for the 250 hp motor.

 

[Deltaforce]

NEC 430.122(A) has a little informational note that says “Power conversion equipment can have multiple power ratings and corresponding input currents.” but provides no guidance on how to handle those. I have a 250hp positive displacement pump driven by a 300hp motor. The VFD is configured for 250hp (HD) but is also rated for 300hp (ND). Now there are two engineers fighting over whether the feeders need to be sized for 250hp or 300hp. Who is right and why?

Your vfd dual rated 250hp(hd) and 300hp(nd)
Normal 300hp(nd), later it may configured variable torque pump application replacement positive displacement pump, worst case scenario for 430.122(A) use 125% manual input current for 300hp continuous duty, size feeder

 

[Jraef]

NEC 430.122(A) has a little informational note that says “Power conversion equipment can have multiple power ratings and corresponding input currents.” but provides no guidance on how to handle those. I have a 250hp positive displacement pump driven by a 300hp motor. The VFD is configured for 250hp (HD) but is also rated for 300hp (ND). Now there are two engineers fighting over whether the feeders need to be sized for 250hp or 300hp. Who is right and why?

There is no world in which a PD pump is a Normal Duty load for a VFD. So the problem here is not the wire size, it is that you have the wrong VFD. You need a 300HP HD rated drive, next size up. anyone telling you differently does not know what they are talking about.

Then in the drive tech data, you can find the HD rating Input Amps, that is what you use to size the cables.

 

[Deltaforce]

There is no world in which a PD pump is a Normal Duty load for a VFD. So the problem here is not the wire size, it is that you have the wrong VFD. You need a 300HP HD rated drive, next size up. anyone telling you differently does not know what they are talking about.

Then in the drive tech data, you can find the HD rating Input Amps, that is what you use to size the cables.

But vfd HD mode only 250hp pd pump op case
If pd pump size increase 300hp, still vfd ok if hd input current equal or greater than motor fla and internal change settings required

 

[winnie]

There is no world in which a PD pump is a Normal Duty load for a VFD. So the problem here is not the wire size, it is that you have the wrong VFD. You need a 300HP HD rated drive, next size up. anyone telling you differently does not know what they are talking about.

Then in the drive tech data, you can find the HD rating Input Amps, that is what you use to size the cables.

If a 300 Hp motor is used to drive a 250 Hp mechanical load, is that a 300 Hp HD electrical load or a 250 Hp HD electrical load?

Thanks

 

[ptonsparky]

If a 300 Hp motor is used to drive a 250 Hp mechanical load, is that a 300 Hp HD electrical load or a 250 Hp HD electrical load?

Thanks

I wondered that myself.

We know for sure no one would use a larger motor on equipment that was possibly undersized and just needed a little extra humph on occasion.
(Please note a degree or two of sarcasm)

 

[Jraef]

If a 300 Hp motor is used to drive a 250 Hp mechanical load, is that a 300 Hp HD electrical load or a 250 Hp HD electrical load?

Thanks

Yes. The issue with HD vs ND sizing is about the overload capability of the transistors, and a key aspect of that is the instantaneous over current capability, as encountered by a rapid increase in the torque requirement. Essentially the load suddenly demands Break Down Torque from the motor, typically 180-200% of the motor FLT. The associated current it will require is based on the MOTOR rating, not the load. So what will typically happen is that something causes the load speed to rapidly decrease to almost stall speed, the motor slip rapidly increases so it attempts to compensate by drawing more current and if the drive is not sized right, the current spike triggers an IOC (Instantaneous Over Current) fault on the VFD hardware protective system to keep from destroying the transistors.

Been there, done that, hit the T-sort and the hat… it’s a messy conversation when you have to tell someone that the only fix is to replace the VFD with another one after the fact.

 

[winnie]

Oh. I can see how I'd get tripped up by that.

The larger motor makes the _transients_ worse, and that requires greater capability from the VFD. The steady state operation probably improves (lower current density, less heating, etc.) and for steady state operation the drive is probably sufficient.

What would happen if the base voltage of the motor were reduced in order to decrease break down torque? My guess is that would reduce the intensity of the current transient, but beyond that I don't know if it would help enough for the OP.