VFD question

[Davebones]

Have a VFD rated for 8 amp out put at 208 volt . Can we install a motor on it that is rated for 14 amps if we only run it it at half speed or less than the rated 8 amp output ?

 

[Besoeker]

Have a VFD rated for 8 amp out put at 208 volt . Can we install a motor on it that is rated for 14 amps if we only run it it at half speed or less than the rated 8 amp output ?

Half speed does not mean it will take half the current.

 

[Jraef]

Have a VFD rated for 8 amp out put at 208 volt . Can we install a motor on it that is rated for 14 amps if we only run it it at half speed or less than the rated 8 amp output ?

Temporarily, sure, but I remommend programming a Current Limit of 8A into the VFD if available (many cheap ones don't offer that). Current Limit in a VFD acts by overriding the speed command once you hit that current level. That way if some bozo in the future tries to speed it up, he doesn't trip the drive off line, it just ignores him.

In order to do this though, you are technically violating 110.3, the so called "suitable for the intended use" clause. I've seen AHJs red tag this arrangement on that basis.

 

[Besoeker]

Temporarily, sure, but I remommend programming a Current Limit of 8A into the VFD if available (many cheap ones don't offer that). Current Limit in a VFD acts by overriding the speed command once you hit that current level. That way if some bozo in the future tries to speed it up, he doesn't trip the drive off line, it just ignores him.

In order to do this though, you are technically violating 110.3, the so called "suitable for the intended use" clause. I've seen AHJs red tag this arrangement on that basis.

I agree. I think it's not a good idea.

 

[Davebones]

It's just for a test on a R/D test stand . Just going to be temporary installation . Thanks for your help on this

 

[Jraef]

Half speed does not mean it will take half the current.

True, but in more ways than one. If it's a centrifugal load, 1/2 speed could mean even less than 1/2 current, in a constant torque load, 1/2 speed may be more than 1/2 current. That's another good reason to use Current Limit instead of relying on a speed setting.

 

[Jraef]

It's just for a test on a R/D test stand . Just going to be temporary installation . Thanks for your help on this

Ok, yes, done all the time on test stands. If your drive lacks a Current Limit feature, use an analog output to drive a current meter so you can watch it as you test.

If your drive doesn't offer a current output or current limit, get a better drive...

 

[Besoeker]

True, but in more ways than one. If it's a centrifugal load, 1/2 speed could mean even less than 1/2 current, in a constant torque load, 1/2 speed may be more than 1/2 current. That's another good reason to use Current Limit instead of relying on a speed setting.

Or reducing the v/f ratio might work.

 

[Z3R0]

Temporarily, sure, but I remommend programming a Current Limit of 8A into the VFD if available (many cheap ones don't offer that). Current Limit in a VFD acts by overriding the speed command once you hit that current level. That way if some bozo in the future tries to speed it up, he doesn't trip the drive off line, it just ignores him.

In order to do this though, you are technically violating 110.3, the so called "suitable for the intended use" clause. I've seen AHJs red tag this arrangement on that basis.