VFD & Motor compatibility

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There are several things that differentiate a motor to be inverter duty;
1. increased insulation value, typically 1600V versus 1000V
2. additional phase paper between the windings
3. larger rotors, accounting for the larger size of an inverter versus non-inverter duty

The issue with the reflective wave causing a motor failure is more common with motors running at 460V. On a 460V VFD, the VFD DC bus is around 660-680V. I have measured line to line almost 1200Vp-p at the motor on 225ft +/- lead length. I have seen consultants specify 230V on applications were the lead lengths will be long with non-inverter duty motors. Even with reflective wave the motor usually never sees a spike that exceeds the 1000V insulation rating. Fractional motors are more at risk because a NEMA standard does not exist for them. IMO you get what you pay for, use a cheap VFD with a cheap motor and you get an expensive installation to maintain. In my experience, the most important recommendation to follow on a VFD installation is the grounding requirements.

Tony
 
tony_psuee said:
There are several things that differentiate a motor to be inverter duty;
1. increased insulation value, typically 1600V versus 1000V
2. additional phase paper between the windings
3. larger rotors, accounting for the larger size of an inverter versus non-inverter duty

The issue with the reflective wave causing a motor failure is more common with motors running at 460V. On a 460V VFD, the VFD DC bus is around 660-680V. I have measured line to line almost 1200Vp-p at the motor on 225ft +/- lead length. I have seen consultants specify 230V on applications were the lead lengths will be long with non-inverter duty motors. Even with reflective wave the motor usually never sees a spike that exceeds the 1000V insulation rating. Fractional motors are more at risk because a NEMA standard does not exist for them. IMO you get what you pay for, use a cheap VFD with a cheap motor and you get an expensive installation to maintain. In my experience, the most important recommendation to follow on a VFD installation is the grounding requirements.

Tony
Click to expand...

Best post so far.
I will only add that the better brands of Inverter Duty motors are also now coming with shaft grounding bushings as well so that the bearing races don't get fluted by ground current discharge (as mentioned in an earlier post). This is a very real problem not to be taken lightly.

BTW, one reason why people get the impression that inverter duty motors are just over sized is that some motor mfrs actually did that when the issue of inverter applications first came up about 15 years ago. I know for a fact that Lincoln Electric Motors did that, their salesmen even went around badmouthing other inverter duty motors as being a snow job!
 
Jraef said:
I will only add that the better brands of Inverter Duty motors are also now coming with shaft grounding bushings as well so that the bearing races don't get fluted by ground current discharge (as mentioned in an earlier post). This is a very real problem not to be taken lightly.
Click to expand...

How will the brushes stop current from flowing through the bearings unless they are also installing non-conductive bearings at the same time?
 
Jraef said:
Not really an issue, the bushing just provides a lower resistance path to ground than the bearings.

http://www.est-aegis.com/about_aegis.htm
Click to expand...

Yes.

So are you saying that electricity only takes the path of least Resistance?

Unless the bearings are non-conductive the current will likely split close to 50-50 between the brushes and the bearings.

It's smoke and mirrors, it also does not address the cause of the current it only puts a band aid on the problem.
 
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