SC study for single-phase system feeding VFD to run 3-phase motor

Status
Not open for further replies.
M

morgan3520

Member
Location
Dallas, TX
Occupation
Electrical Engineer
Has any body ran or did a short circuit study and an arc flash analysis of a 240V single-phase system feeding a VFD that produces a “third leg” to run a 3-phase motor. The VFD is rated for 5hp and the actual motor is 2hp, 240V, 3 phase.Screenshot 2023-03-27 111503.png
 
You shouldn't put contactors downstream of VFDs, it's a good way to kill the VFD. What's the purpose of them? The VFD should be controlling the On-Off of the motor.

As far as an "arc flash analysis", there is not difference in how you would determine the power going in the box. The fact that the VFD changes the feed to the motors to 3 phase is irrelevant.
 
Jraef said:
You shouldn't put contactors downstream of VFDs, it's a good way to kill the VFD. What's the purpose of them? The VFD should be controlling the On-Off of the motor.

As far as an "arc flash analysis", there is not difference in how you would determine the power going in the box. The fact that the VFD changes the feed to the motors to 3 phase is irrelevant.
Click to expand...
But depending on the design of the VFD you may or may not have to consider motor contribution to the current upstream of the VFD. yes?
 
I've not had reason to run such an analysis. My thoughts:

If the VFD has line regeneration capability (very unlikely with these small units), then it might make a slight contribution to available short circuit current. Most VFDs just have a simple input rectifier and cannot regenerate back to the supply side.

If the fault is in the input rectifier itself, then the DC rail capacitor could source energy into the fault in combination with the supply fault current. When this happens the drive will enable its rapid self disassembly feature.

Between the VFD and the motor you pretty much have a local system with its own fault capability; the drive and the motor can both feed a fault on that single circuit.

The available short circuit current at the utility transformer secondary is huge. But after the 250 feet of #4, not so much.
-Jon
 
GoldDigger said:
But depending on the design of the VFD you may or may not have to consider motor contribution to the current upstream of the VFD. yes?
Click to expand...
Only if it is an Active Front End Line Regenerative VFD used for constant braking applications. For the most part nobody makes them smaller than 10HP, but one could possibly use a 10HP AFE LR VFD for a 2HP motor if one had a penchant for wasting mass quantities of money.

On second thought, I don’t believe that any of the AFE LR VFDs would work on a single phase input, because they would detect the “lost” phase and shut down.
 
Last edited:
That non-fused main looks like a violation of 230.91. There is no upstream protective device to clear an arcing fault on the incoming lugs of the 100A MCB in the control panel. You'll get a red sticker instead of an orange one. IMHO, you don't need a sticker anywhere downstream of the panel, so why try to crunch the numbers?
 
JoeStillman said:
That non-fused main looks like a violation of 230.91. There is no upstream protective device to clear an arcing fault on the incoming lugs of the 100A MCB in the control panel. You'll get a red sticker instead of an orange one. IMHO, you don't need a sticker anywhere downstream of the panel, so why try to crunch the numbers?
Click to expand...
Some utilities require a disconnect at the meter, it’s usually non-fused.
 
Yes, but there are circumstances wherein it does not. I’ve seen it discussed before, if not here then in another forum. I think it has something to do with being a “site isolation device”, for example like at a large farm where where there is one main utility service drop, then customer overhead distribution lines to feed different metered sites after the utility drop, something like that.
 
Status
Not open for further replies.
Top