Calculating Panel FLA: Use VFD input current or motor table FLA?

If the motor has the UL mark on the nameplate and the plate has a value for FLA, you can use that instead of the value from the table. The complication arises from running a motor on a different voltage/phase count than the drive. I'm fairly sure you can just convert P=IV to a different voltage and use sqrt(3) to convert single and dual phase and still use the motor FLA.
 
CapinWinky said:
If the motor has the UL mark on the nameplate and the plate has a value for FLA, you can use that instead of the value from the table. The complication arises from running a motor on a different voltage/phase count than the drive. I'm fairly sure you can just convert P=IV to a different voltage and use sqrt(3) to convert single and dual phase and still use the motor FLA.
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What is your question?
 
CapinWinky said:
If the motor has the UL mark on the nameplate and the plate has a value for FLA, you can use that instead of the value from the table. The complication arises from running a motor on a different voltage/phase count than the drive. I'm fairly sure you can just convert P=IV to a different voltage and use sqrt(3) to convert single and dual phase and still use the motor FLA.
Click to expand...
This works for motor contactors where the motor IS the connected load. For a VFD, though, the VFD is the conversion device that supplies the motor. So, for example, a 480VAC, 3-Phase, 7.5 HP motor requires 11 FLA on a contactor. To run the same motor on a Vector VFD requires 12.4 A from the same mains. That doesn't seem like a big deal, until you have a panel that drives two of these motors and has a 150VA control transformer. The mill only needs a 30A breaker to serve the panel with simple contactors ahead of the motors, but two VFD driven motors will exceed the 30A breaker rating when derated to 80% for continuous use.
 
Tip DS said:
This works for motor contactors where the motor IS the connected load. For a VFD, though, the VFD is the conversion device that supplies the motor. So, for example, a 480VAC, 3-Phase, 7.5 HP motor requires 11 FLA on a contactor. To run the same motor on a Vector VFD requires 12.4 A from the same mains. That doesn't seem like a big deal, until you have a panel that drives two of these motors and has a 150VA control transformer. The mill only needs a 30A breaker to serve the panel with simple contactors ahead of the motors, but two VFD driven motors will exceed the 30A breaker rating when derated to 80% for continuous use.
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Put in a bigger breaker.

Why are you factoring for continuous use?
 
Tip DS said:
To run the same motor on a Vector VFD requires 12.4 A from the same mains.
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Where do you get that info from? I'm looking at a VFD data sheet for a 7.5HP VFD where the OUTOUT current is 12.1A, but the INPUT CURRENT is only 9.5A. Remember, the output current of the drive is INCLUSIVE of the reactive current due to the motor power factor. On the input side, i.e. "drawing from the mains", that is corrected to .95 by the drive, so the actual current drawn from the source is LOWER, not higher.
 
Jraef said:
Where do you get that info from? I'm looking at a VFD data sheet for a 7.5HP VFD where the OUTOUT current is 12.1A, but the INPUT CURRENT is only 9.5A. Remember, the output current of the drive is INCLUSIVE of the reactive current due to the motor power factor. On the input side, i.e. "drawing from the mains", that is corrected to .95 by the drive, so the actual current drawn from the source is LOWER, not higher.
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I don't always see that, so RTFM is important.
Depending on make and model, YMMV

Researching this a bit, I found a table:

1741691696014.png
None of these output currents relate (> or =) to the NEC HP table values.
The 480v drives 40-100 HP did.

Interesting.
 
The issue there is that those drives were DESIGNED for standard IEC motor kW sizes at 400V, then when MARKETED for NEMA standard motor HP sizes at 480V, end up over sized. The maximum input current RATING is still based on the IEC motor sizing, not the output current sizing.
 
Funny how these forums are full of responses that never address the questions asked.

OP: "What is the average weight of a Black Angus cow?"
Respondent 1: "You know, grass fed cows are healthier than corn fed cows."
Respondent 2: "What color is the cow?"
Respondent 3: "The color doesn't matter, idiot!"
Respondent 2: "Just say you don't know the answer, retard!"
Respondent 1: "The color has nothing to do with the health of the cow. Don't you know anything?!"
Respondent 4: "Chicken has less fat. Why do you even bother with beef?"
Respondent 5: "You're all stupid. If you knew anything, you'd be vegan."
 
Tip DS said:
Funny how these forums are full of responses that never address the questions asked.

OP: "What is the average weight of a Black Angus cow?"
Respondent 1: "You know, grass fed cows are healthier than corn fed cows."
Respondent 2: "What color is the cow?"
Respondent 3: "The color doesn't matter, idiot!"
Respondent 2: "Just say you don't know the answer, retard!"
Respondent 1: "The color has nothing to do with the health of the cow. Don't you know anything?!"
Respondent 4: "Chicken has less fat. Why do you even bother with beef?"
Respondent 5: "You're all stupid. If you knew anything, you'd be vegan."
Click to expand...
Why did you ignore respondents 6 thru 873?
 
This is what UL 508a says.

49.2 The full-load current rating of the panel shall, at a minimum, include the sum of the current ratings of all loads that are able to be operated simultaneously plus the primary current rating of all control transformers connected to the input voltage.
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So use the input rating of the VFD. Incidentally, you can increase the FLA from the minimum value the calculation gives you. Not sure why you would do that but it is specifically allowed.
 
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