Selecting a VFD

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faresos

Senior Member
Hello Everyone,

I have a general question, can I use 7.5HP VFD (rated 11 Amps @ 480V) for 5HP pump? In general, Is there any issue for using oversized VFD on motors? The situation is, I'm working on a project that we are replacing one 3HP with 5HP pump but we have an unused 7.5HP VFD that I wanted to use. I know the VFD's are rated for the output Amps which technically should work but not sure if there will be an issue. Thanks,
 
faresos said:
Hello Everyone,

I have a general question, can I use 7.5HP VFD (rated 11 Amps @ 480V) for 5HP pump? In general, Is there any issue for using oversized VFD on motors? The situation is, I'm working on a project that we are replacing one 3HP with 5HP pump but we have an unused 7.5HP VFD that I wanted to use. I know the VFD's are rated for the output Amps which technically should work but not sure if there will be an issue. Thanks,
Click to expand...
Not as far as I am aware. You need a different overload.
 
Most VFDs have a lower limit on the size of motor you can use, usually around 50% of the rating, just because the accuracy of the current sensors gets to be out of tolerance when the current is so low. But 5HP on a 7.5HP drive is fine.

One word of caution when using an existing drive; find the parameter that does a “Reset to factory defaults” and do that before doing any other programming. Otherwise you can end up with weird stuff happening that you can’t understand.

Any UL listed VFDs sold since 2005 have the motor overload functionality built in, so when you enter the motor nameplate data, the VFD sets up the OL protection for that motor.
 
And remember, the NEC requires the circuit feeding the VFD to be sized for the input FLA, not that of the motor.

This is probably not an issue with this install, but I thought it should be pointed out.
 
faresos said:
Hello Everyone,

I have a general question, can I use 7.5HP VFD (rated 11 Amps @ 480V) for 5HP pump? In general, Is there any issue for using oversized VFD on motors? The situation is, I'm working on a project that we are replacing one 3HP with 5HP pump but we have an unused 7.5HP VFD that I wanted to use. I know the VFD's are rated for the output Amps which technically should work but not sure if there will be an issue. Thanks,
Click to expand...
To add to what has already been mentioned, if you have a motor that is stopped, started, reversed, or accelerated/decelerated frequently enough you very well might need to "de-rate" the VFD so it can handle the excess heating that those frequent activities will cause.
 
kwired said:
To add to what has already been mentioned, if you have a motor that is stopped, started, reversed, or accelerated/decelerated frequently enough you very well might need to "de-rate" the VFD so it can handle the excess heating that those frequent activities will cause.
Click to expand...
Not so likely with a pump.
 
jim dungar said:
And remember, the NEC requires the circuit feeding the VFD to be sized for the input FLA, not that of the motor.

This is probably not an issue with this install, but I thought it should be pointed out.
Click to expand...

True but 11 A x 125% is 14 A, still under what the minimum motor feeder (#14) will carry.
 
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