# Help me understand this amperage thing.



## OlCatBob (Mar 20, 2012)

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


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## Hawkeye (Mar 20, 2012)

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.


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## Tony Wells (Mar 20, 2012)

1 hp motor pulling 1.28 amps? I want one. :biggrin:


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## OlCatBob (Mar 21, 2012)

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


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## randyjaco (Mar 21, 2012)

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


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## Hawkeye (Mar 21, 2012)

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.


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## OlCatBob (Mar 22, 2012)

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


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## mitsue (Mar 22, 2012)

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.


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## 75Plus (Mar 22, 2012)

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


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## mitsue (Mar 23, 2012)

75Plus said:


> 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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## OlCatBob (Mar 23, 2012)

Well... I did find this picture on PM. It's another Nichols motor, 1 hp, 3 phase, but dual voltage. I'll post a pic hopefully, or at the least include a link to the page.
http://www.practicalmachinist.com/vb/antique-machinery-history/nichols-mill-222085/index3.html

This is interesting, if it IS 460V how the heck am I running this thing on 230V???
Bob


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## R.G. (Mar 23, 2012)

OlCatBob said:


> I was just trying to understand why it appears that the listed amperage is only for one leg of the 220 volts.


It is the practice in the motors and multiphase electricity world to list only the current per phase, as (1) they try really hard to make all the currents balanced and (2) the current per phase tells you how big your wires need to be, while the sum of all three phase currents is just confusing if you're trying to pick wire size. 



OlCatBob said:


> This is interesting, if it IS 460V how the heck am I running this thing on 230V???


Many electrical motors will run on a lower voltage of the right frequency if the moon is in the right phase and the wind is from the right direction. 

Or maybe it's internally jumpered for 230V.


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## OlCatBob (Mar 23, 2012)

Hey RG, it sounds as if you know something about this, how could one determine if "it is internally jumpered"? Since I am running it off a freq drive, could that maybe "make the wind blow properly"? I'm obviously not an EE, just trying hard to understand. Thank you for your input, all of you that have chimed in.
Gratefully, Bob


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## R.G. (Mar 23, 2012)

Actually, not many EEs know this stuff any more. Power specialists try to get out of AC power line stuff all they can, and no one who can do the math wants to be stuck in power engineering. No money there. But I did have some motors classes and did design transformers for a while. 

The only way to tell if it's internally jumpered, etc. is to open it up and look for any possible jumpering. Then the problem is that it may have been rewound to some particular voltage. 

The only non-laboratory way I know to see what voltage it's supposed to be running at is to slowly increase the voltage to it a nominal power line frequency, and watch the current it draws with no load on the motor, just spinning itself. The current will increase more-or-less linearly until at some point it will start increasing more quickly than it has before. That's the onset of magnetic saturation in the iron, and the voltage-divided-by-frequency there should not be exceeded. This is where energy starts going into the iron, not moving the motor. The current there is much less than full load current, because it's not turning a load, so the magnetizing current is the majority of the power in. Gotta have good, easy spinning bearings to find this point. A mechanical load will cover up the place where magnetizing current increases. It's not an easy test, even though it only needs a variable voltage source and a current meter. 

The volts-per-Hertz is a maximum for the motor. Using a variable frequency drive allows you to run the motor at less than the design frequency, and that means that your VFD has to be smart enough to reduce volts with frequency. Most are, but it's something to be aware of. 

You know, the more I think about it, maybe the best thing if you want to know about it is to find a motor rewinding place and find the old, grouchy guy in the back room and ply him with donuts, or cigars or something to tell you all he can about the motor. These guys usually feel underappreciated, so it often works. He should at least be able to tell you what not to run it over.


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## OlCatBob (Mar 23, 2012)

RG, I laughed out loud on reading your last paragraph, There's a motor shop in town, maybe I'll just take 'er down there and see what they can determine. I really do appreciate your input. The thing starts up nicely as it is, and though I haven't really put a lot of load on her, it seems to run fine.
Thanks again, and if I can get some good info, I'll post back.
Respectfully thankful.
Bob


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## R.G. (Mar 24, 2012)

Let me know what you find out. My advice may not help much, but it doesn't even cost a donut.


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