Vfd

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Timboe

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Is there any disadvantage other than cost to using a VFD rated at 5hp on a 2hp motor? 480volts 3 phase. I have a 5hp and don't want to spend the money on a replacment for the 2hp. Do you think the 5hp VFD will last longer on the 2hp motor? I dont think it will make a difference but want someone eles opinion.:D
 
Won't make a difference in terms of "making it last longer". Most of the time the things that damage drives are voltage related.

The only thing that might possibly be an issue is if the VFD programming will allow you to set the motor FLA down low enough. Some drives (although very few) only allow a minimum setting of 1/2 of the maximum current rating. That setting is what the overload protection is based on, so if it doesn't go low enough you will need to use an external OL relay downstream of the VFD. But most likely it will let you if it is a newer digital drive. In a pinch one time, I used a 40HP drive on a 5HP motor; no problem at all.
 
It won't mean much with this small of a drive but, the NEC requires the branch circuit to be sized based on the maximum input rating of the VFD not its connected load
 
It's a fairly common practice in an industrial environment to do what you are suggesting, mainly for sparing parts. It makes more sense to have 1 each 5HP spare to cover numerous 5HP, 3HP, 1HP installations vs. stocking 1 of each frame.
 
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Jraef said:
The only thing that might possibly be an issue is if the VFD programming will allow you to set the motor FLA down low enough. Some drives (although very few) only allow a minimum setting of 1/2 of the maximum current rating. That setting is what the overload protection is based on, so if it doesn't go low enough you will need to use an external OL relay downstream of the VFD. But most likely it will let you if it is a newer digital drive. In a pinch one time, I used a 40HP drive on a 5HP motor; no problem at all.
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yeah. what he said.
 
Timboe said:
Is there any disadvantage other than cost to using a VFD rated at 5hp on a 2hp motor? 480volts 3 phase. I have a 5hp and don't want to spend the money on a replacment for the 2hp. Do you think the 5hp VFD will last longer on the 2hp motor? I dont think it will make a difference but want someone eles opinion.:D
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The 5hp VFD will run the 2hp motor just fine.
But, to make best use of the protection afforded by the VFD, you may need to set a few parameters differently:
  • Motor rated current
  • Current limit
  • Motor cosphi
  • Motor thermal protection
  • Stall current
for example
 
I have noticed that the vfd does seem to last longer if over sized if your not using bypass contractor. That is one reason Ab will sell you a drive rated for example from 40 to 50 hp for normal duty and if it severe duty maybe a 50 to 65 hp drive.
 
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