amps vs. hp

[LeeB]

manufacturers pretty much put whatever hp rating on a motor they think they can get away with these days. Can anyone tell me the relationship between amps and horsepower? LeeB

[Tom]

I think the numbers are here but I'm too tired to put them together right now. :D

The story goes that Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. He found that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. He then increased that number by 50 percent and pegged the measurement of horsepower at 33,000 foot-pounds of work in one minute.

In the strictest sense  watt=volts X amps


1 horsepower is equivalent to 746 watts. So if you took a 1-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts.

1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).

One BTU is equal to 1,055 joules, or 252 gram-calories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.

watt=joules/seconds

[Don P]

Amps per motor HP at 115 volts single phase, for 230 volt 1 ph divide amps by 2
This is from Graingers, I remember Fine WW doing an article on it some years back but my issues are in storage.
1/4 hp 3450 rpm 3.5-4.1 full load amps
1/4 hp 1725rpm 3.0-6.3

1/2-3450-6.6-11.8
1/2-1725-6.3-9.6

3/4-3450-9.2-14.8
3/4-1725-9.8-13.3

1-3450-11.4-19.2
1-1725-12.6-15.0

2-3450-15.0-23.0
2-1725-19.0-24.6

A rule of thumb I ran across is...
at 115v a single ph motor draws 14 amps per hp at rated output
at 230v a single ph motor draws 7 amps per hp at rated output
at 230v a 3 phase motor draws 2.5 amps per hp at rated output

[J Beyer]

It seems like it is a benefit to run three phase to keep the amps down.  What benefit is ti to run three phase and lower amps?

[Minnesota_boy]

3-phase is a smoother power because it's always pulling.  With single phase there is a build up, a power time, a release and a coast before it starts the cycle over the other way.

[J Beyer]

So basically another way of putting it is it would be like switching from a  two-cylinder to a six-cylinder engine with the exact same power output at the same rpm.  And with cycles in between helps to keep things smooth pulling without a "jerking" motion when the next power cycle occurs.

Thanks for the input.

[dan-l-b]

Then why don't we have more available three phase electric?  Around here it is really expensive to hook up, If you can get it at all.  Otherwise, converters are the way we go.  Great lessons Tom and DonP.

[Minnesota_boy]

Three phase power requires 4 wires while single phase only takes 2.  For most installations, there is little difference in the power delivered so the power companies opt for the lesser expense.  Unless you really need 3 phase, you won't get it.

[LeeB]

much good info here. knowledge base stuff me thinks. LeeB ;)

[WV_hillbilly]

 Here are some the formulas to find HP and Amps for single and three phase motors.

 Single phase :

    HP =        volts X amps X % efficiency X power factor
                   ____________________________________
                                
                                                                        746



  Amps =                        HP  X 746
                   ______________________________

                    volts X % efficiency  X power factor



Three phase :


     HP =   volts  X amps X % efficiency X power factor X 1.73
               __________________________________________

                                                                       746



   Amps =                      HP X 746
               __________________________________

               volts X % efficiency X power factor X 1.73


As you can see three phase is a more efficient at transferring power cause it divides the load with the square root of 3   (1.73)  in all the calculations. :


  If you guys would like a handy little book try to find a book titled    : Ugly's Electrical References  by  George V. Hart. There is all kinds of information about motors ,capacitors and electric in general in this little pocket reference book . It should be availiable at your nearby electrical supply house. And I ain't talkin about the big box stores. They are only about $ 6.00 .  If you can't find them and want one let me know as our supply house sells them.



Hillbilly

[Don P]

I was playing around with the math and a Graingers catalog. Seems efficiency is a BIG variable. I'm getting a range of efficiencies from 40%-95%! looks like the small motors are the lowest eff. Are there any rules of thumb for motor efficiency?

I put Hillbilly's math on a calc to make it faster to play with.
https://forestryforum.com/members/donp/HPcalc.htm

[Mark M]

Hi Don

Say how do you do that fancy Java stuff? do you just hand code it or is there a program that helps write the code? I might have a use for that someday.

Mark

[Jeff]

He hand codes it. It's what you call Javascript. Very different from Java. Two completely different things and differnet languages. Javascripting is bacically a set of instructions to a web browser. Its the power of the browser that is the real program. On the other hand Java does not need the webbrowser to function.

[Don P]

Thread turning left :D, I still want more motor stuff if its out there.

I'm just larnin this coding myself, so take this all as the blind leading the blind :D
First I right click on the field of anything I want to see the "code" of, in this case just put your pointer in the middle of the calculator and right click, then when the box comes up, click "view source" (you can do this to see a webpage's code too). Then start figuring out the script. I wrote it on an html editor, I downloaded a free one from here
 http://www.chami.com/html-kit/download/
One neat thing, once you get your tree, is the free webspace Jeff is kind enough to let us use. If we learn how to write these simple programs, instead of just posting a formula we can post a link to a calculator that does the math.
Something I saw Mitch do and I should go back and do is to write the math formula on the calculator so you can easily see what its doing. I've been talking to him, his calculators work on a different language, and I ain't got a clue on them...yet  ;)

Below is the address to as stripped down a calculator as you're likely to find, its the kind of stuff I've been looking at to learn how they work. Then I just keep piling more stuff together. Trial and error and error ???
Ron, Jeff and the pro's are probably rolling on the floor by now...I don't really understand this very well yet I'm just copying and modifying, so don't copy my code its probably just hanging together.  

https://forestryforum.com/members/donp/multicalc.htm

[Mark M]

Thanks Don

Thats what I needed to know.

Mark

[Bro. Noble]

Mark,

I DonT understand the electric stuff nor the Java stuff.  I DonT understand what you're saying about not understanding that you put at the end of your posts.  I'm getting about as confused as that notorious Canadian Mountie.  I do like your picture-----you look like you have a plan in mind.  There is one thing that bothers me a little though.  If you are a man now, but changed; what would you change into? :

Noble

[C_Miller]

Neighbor asked me to stop by when the local power company rep stopped by to see what we could work out for a three phase drop to our properties.

   The engineer showed us a schematic of their 3(O with slash through it) system they would put in. It was just two transformers hooked up open delta. don't see why a feller couldn't have single phase brought into his place of business then set up a similar system for his own use instead of paying the power company to do the same thing outside.

But I never did much with engineering side of the house.

C

[Bibbyman]

I think this can be done with TWO single-phase sources with the correct phase relationship.   It's called duplexing. I asked my power company about this and they not only said NO! they said "Hell NO!!"  

Appears they would have to manage to keep both lines in the correct phase relationship for it to work. Out in my area it would limit their ability to change out service to balance loads or for outages.  If they swap out a line,  my motors could run backwards or not at all, etc.  

Now if you have a three-phase line they can tap into, then it should work fine.  The only thing they are saving is a three-phase transformer and using two single-phase that are more common and cheaper.

In the end,  we went roto-phase converter.

[Larry]

Don P your doing a great job on this motor stuff and the calculators.  Keep it up so I can learn a little more.

Got a question that maybe somebody can help me with.  I am building a sawdust blower and right now it has a Robbins & Myers 5 hp motor on it just to test it out.  I know I will be way over powered but was thinking once I get the fan rpm set I can put an ammeter on the motor and see what it is pulling under full load to determine the correct hp motor I will need.  I am guessing I will need something around 2 or 3 hp to run the size fan I have.  Don't want to leave the 5 hp motor on it because it is slow starting plus I have to run a new circuit for the blower.  Any thoughts?

[Mark M]

Hi Don

Thanks for the info on Java Script, I might be able to figger out how that works.

Noble - have you ever seen the Red Green show on PBS Tv? That's his picture, not me. We look similar only I am over-inflated by about 100 lbs. If you haven't seen the show check it out, it's amazing what he can do with duct tape. I once saw him hook two Gremlin cars together to make a Hum-Vee. Pretty amazing

Sorry about getting off topic, I'll shutup now.

Mark

[Bro. Noble]

Mark,

No I havn't seen that guy on TV.  We only get channel 3 and I DonT watch it but on rare occasion.

So that's Red Green in the picture with your mill and cat?  That was some fancy duct taping he did on it's tail.

I sure do appreciate the opportunity to hang around with you and DonP and these other brainy guys-----learnin about electric and java.  Well electric anyway,  I still don't have airy an idee whats java.

Been pondering something and maybe you guys can help-----If electricity is made by electrons,  is morality made by morons?

Noble

[Mark M]

Hi Noble

I think you are right about those morons, but I don't know if they actually make morality or if they just try to control it  ???

I'm not sure bout this Java stuff neither, I think it sumptin you drink, maybe it causes you to lucinate and think in starange symbols

Mark

[Don P]

Only if you put milk from those Irish cows in it :D

Larry, you threw a curve ball :D, I got a book that tells how to do it another way but I'll be going through more java than I got in the house trying to make a calc for it. So here's the long form.

This is from "How to Design and Build Centrigugal Fans for the Home Shop" by Dave Gingery

HP= horsepower required
C= volume of air in cubic feet per second
h= pressure in inches water column
E= efficiency (.40-.60 average)

HP= ( C * h * 5.2 ) / ( 550 * E )

If we assume .50 for efficiency it can be further simplified

HP=( C * h * 5.2 ) / 275

...................................................

To get volume of air, C, in the above equation.

First you need velocity,
Assume a 10 inch diameter wheel 3 inches wide spinning at 1725 rpm

10 * 3.1416=31.416" wheel circumference
31.416 / 12 = 2.618 feet per revolution
2.618 * 1725 rpm = 4516.05 feet per minute

..........................................................
Next we need velocity pressure in inches of water column (h)

velocity is in ft per second here
4516ft/min /  60 seconds/minute = 75.27 feet per second

h= (velocity / 66) * (velocity / 66)
h= (75.27 / 66)  *  (75.27 / 66)  
h= 1.14 * 1.14
h= 1.3 inches water column pressure

.........................................................
Output

A=blast area in sq inches
D= wheel diameter in inches
W= wheel width in inches

A=(D * W)/3
A=(10 * 3)/3
A=(30)/3
A=10 square inches blast area......

10 square in/ 144 sq inches per sq foot= .069 square feet blast area

4516 ft per minute * .069 sq ft blast area * .80 efficiency = 249.28 cubic feet per minute

250 cfm / 60 seconds per minute = 4.17 cu ft / sec
....................................................
Back to where we started, you know HP required

HP= ( C * h * 5.2 ) / 275
HP= ( 4.17cfs * 1.3" WC * 5.2 ) / 275
HP= 28.19 / 275
HP= .103 ....1/8 horse would pull this blower


If the women don't find you handsome, they should at least find you handy ;D

[Bro. Noble]

MARK,
 I know what you mean about lucinating------that's howcome I'm online at 2 or 3 AM sometimes.  Got up to lucinate. :

Noble

[Larry]

Don P,
Thanks for the info.  I better wait for a rainy day try before I work my way through those calculations.

I really like reading that Gingery guy.  Wouldn't surprise me if he comes out with a book on how to build a nuclear submarine in the backyard someday.  I didn't know he had a book out on fans.