Help me understand this amperage thing.

OlCatBob

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Hi guys, I have a Sheldon shaper and a Nichols mill, both will run on my VFD. I do have to change settings on the Freq Drive, because the shaper runs a 1-1/2 horse motor, while the mill has a 1 hp motor. Part of the info plate on the mill has been scratched, but the only part really affected is the voltage listing. Theamps for the 1 hp motor clearly shows it being 1.28 amps, and clearly shows 1 hp. If I calculate 1.28 X 220, / 746, it equals .37 hp. With 3 phase, do I realy need to multiply the amperage reading by 3 to get actual amperage for the VFD setting. The 1-1/2 hp motor has a much higher amperage of 4.8 for 220 volts equaling 1.41 hp.
I hope my question is clear enough, it just seems as though one motor is rated one way the other another way. #.84 seems a lot closer to me than 1.28 for setting the ratings.

Thanks for looking, and especially thanks for replying.
Bob
 
First, you are correct in using 746 watts per HP, but that would be at 100% efficiency and unity Power Factor. No motor has achieved that just yet. At rated load, the efficiency may be as high as 85 - 90%. Power Factor will improve as efficiency does, but there are differences. Too complicated for this discussion.

If the motor windings are Y-connected, the total power is a product of all three windings. At 220 volts, 3 x 1.28 x 220 = 844.8 W. Divide that by 746 and we get 88.3% efficiency. As always, most of the inefficiency comes out as heat.
 
1 hp motor pulling 1.28 amps? I want one. :biggrin:
 
Hey Hawkeye, I would have to assume that the VFD "realizes" that nothing is 100%; with that said, the book states that I should put the numbers in that appear on the motor plate. The VFD has never really "liked" the 1.28 amps that are listed on that plate, so when I initially fired up my mill, I actually input into it the "learn" mode. It's never really shut itself off since I did that, but I now have another motor that has been entered into the mix and I was just trying to understand why it appears that the listed amperage is only for one leg of the 220 volts.

It's a Master gearmotor that came original with the Nichols mill.
Bob
 
I don't know how you have wired up those machines, but you may have some problems if you have on/off switches on each of those machines.

Randy
 
I didn't realize that you were talking about data to enter into the VFD, since my Teco VFD doesn't need that input. If the 1.28 A is an input, the VFD would 'know' that it is on three legs. That brings back the possibility that the voltage rating that is missing is something other than 220.
 
Hey Guys, thanks for getting back to me on this issue. Randy, I have both of these machines running basically straight off the VFD. One at a time of course.
Hawkeye, you stated that your TECO doesn't require you to enter that kind of info into the controller? How does it know what it's supposed to be doing? My VFD is a Cutler-Hammer, (a somewhat respected and esteemed name) and I have to enter the data into it so that it has a baseline to work with. In all, not a problem, and makes sense to me. I bought the VFD to use with the mill, and only recently bought the shaper, but I have 20 amp 3 ph plugs placed on both pieces, and have a socket coming off the VFD, and just unplug one and plug in the other.
I am going to input 3X the 1.28 A reading for a 3.84A total next time I fire up the mill and see how that works for me.
Bob
 
The vfd just needs to know what the max current rating is so it can tell when there is an overcurrent condition is my guess. The plate rating probably is for each leg it seems. Tell the drive whichever motor amperage rating is higher.
 
That 1.28 amp rating is much closer to what you would find at 440 volts. Is there a possibility that the motor is dual voltage and the 220 volt amperage was lost when the name plate was damaged? If so the amperage would be doubled for 220 volt operation.

Here is a full load amps (FLA) table that may be helpful.

http://www.engineeringtoolbox.com/elctrical-motor-full-load-current-d_1499.html

"As a "rules of thumb" amps horsepower rating can be estimated as

  • 115 Volts motor - single-phase : 14 amps/hp
  • 230 Volts motor - single-phase : 7 amps/hp
  • 230 Volts motor - 3-phase : 2.5 amps/hp
  • 460 Volts motor - 3-phase : 1.25 amps/hp"

From the link it looks like you nailed it.
 
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