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Gas Motor vs. Electric Motor - Equal HP?

06/14/2010 7:40 PM

I am building a band sawmill and changing it from gas to electric. I have heard so much about the differences and need to know how to size the electric motor to take the place of a 9hp Honda. The electric will be 220v single phase.

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#1

Re: gas to electric hp equal?

06/14/2010 7:43 PM

Horsepower = Horsepower

It is torque that may be different.

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#46
In reply to #1

Re: gas to electric hp equal?

08/13/2010 3:46 PM

The gas motor efficiency is given by Carnot Cycle - practical value is 20%.

An electric motor has an efficiency of, say, 90%.

If the electrical energy is not provided by gas, but by wind, for instance, the INPUT energy to obtain the same OUTPUT power is 4 to 5 lower with electricity.

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#2

Re: gas to electric hp equal?

06/14/2010 7:48 PM

6.7 kW.

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#3

Re: gas to electric hp equal?

06/14/2010 7:55 PM

The common conversion is 3/5.

Three hp electric motor will do the same job as a five Hp gas engine. Electric motors are typically rated at continuous load capacity while gas engines are rated at peak capacity.

So for a 9 Hp Honda a 5 Hp electric with a service factor of 1.15 or greater would be sufficient. Service factor is the above and beyond extended overload capacity of an electric motor. Plus its not likely you will be running the electric motor for extended time periods in an heavily overloaded state anyway. If you are regularly overloading the saw then go with a 7.5 Hp electric motor and have extra reserve power and a bigger safety margin available for extended overload periods.

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#4
In reply to #3

Re: gas to electric hp equal?

06/14/2010 8:37 PM

GA

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#5

Re: Gas Motor vs. Electric Motor - Equal HP?

06/14/2010 9:03 PM

This is interesting, not sure it's applicable if you already have a motor. It actually compares brands: Energy-Efficient Electric Motor Selection Handbook

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#6
In reply to #5

Re: Gas Motor vs. Electric Motor - Equal HP?

06/14/2010 11:34 PM

Thanks Lyn, you've provided me many excellent bookmarks under "references."

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#7

Re: Gas Motor vs. Electric Motor - Equal HP?

06/15/2010 11:07 PM

RPM and Torque will be your guide. If you know what the torque and speed of the 9HP Honda Engine, all you need to do is find an electric motor with similar speed and torque output and Bob's your uncle. Most electric motors turn at different speeds than gas engines so you may need to use a gearbox to alter the torque/speed curve to match the original engine output.

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#8

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 12:07 AM

From a completely practical standpoint based on extensive research many years ago, when I was young and had the time to do such research, I found that a good equivalency is 1hp electric motor is equal to 3hp gasoline powered engine, output shaft rpm being the same.

At that time I was looking to replace the 1hp electric motor on a air compressor with a gas engine. Several scientific/engineering folks insisted that 1hp = 746 watts etc.. The 3hp gas worked very well. To prove the point I replaced the 3 hp gas with a 1 hp gas and the unit could only get to full speed with no load on it. Next put the 1hp electric back on and it worked fine. Finished off by putting the 3 hp gas engine back on and it ran just like the 1 hp electric.

After that I hit some catalogs (McMaster, Grainger, Surplus Center, etc) and found that most manufacturers will sell units with either an electric motor or a gas engine of 3 times the hp of the electric motor. Since that time I have converted many units from electric to gas or gas to electric based on 1 hp electric = 3 hp gasoline powered and have always been well within acceptable performance expectations.

Some of the unit converted have included water pumps, winches, material handiling equipment, welders, conveyor belts, hydraulic drives, cement mixers and a golf/utility cart.

I'm sure many contributors can find mathematical, scientific, engineering type reasons why this won't work but it is difficult to say it won't work when it does work! AND THIS IS FROM A GUY WHO IS A CHEM ENG!

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#13
In reply to #8

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 11:20 AM

Bunk.

1:3 in not a good, generalized equivalence. Your "extensive" research was not extensive enough, and you are too-broadly generalizing from a specific situation (or perhaps a few specific situations). Many industrial engines can produce 75% of rated HP continuously. There is no universal difference between continuous and peak ratings in electric motors -- in other words some peaks are for one minute, others are for 10 minutes, etc.

There is no substitute for looking at the spec sheets and the precise requirements of the job. A fan, for example, is likely to use near constant hp, and in that case a 4 hp engine (operating constantly at 3 hp) will do as much work, constantly as a 3hp (continuous rating) electric motor.

Many hobbyist and semi-professional electric car converters have rules of thumb somewhat like your own re hp equivalencies. But their conversions are good for 60-70 mph top speeds, about half the top speed of the original car.

There is no substitute for doing the math. For a very rough guide, TCMtech's guide is much closer to reality.

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#15
In reply to #13

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 2:03 PM

Dear MoronicBumble-

As with some of the other posters to this type of forum you have taken small parts of another's post and misrepresented it as unworkable, impossible, unlikely, etc. inorder to emphasize your viewpoint. Just because you might not agree or would have done things differently is blatent ignorace to "the process of research". Your statement that "Your "extensive" research was not extensive enough, and you are too-broadly generalizing from a specific situation (or perhaps a few specific situations)." shows more of a factor of ignorance than intellegence or creativity since how do you know what went into it? (ASSUME = ASS / U / ME). As first stated this research was done from a practical standpoint, not a highly theoretical approach. Spec sheets were the first sources consulted, then suplemented with others experience and more research! A practical standpoint is to cross the shallow stream by walking across the top of the stones or put some appropriate boots on. A highly theoretical approach is to design and build a dual suspension bridge to cross the stream. The stones afforded you the opportunity to reach the original goal, crossing the stream, vs. building the bridge, engineering out of control.

Of the several dozen projects converted all of them worked, none failed to meet expectations. Whenever my technical consultant (the original 746 watts = 1 hp guy) wanted to investigate any of the conversions he had full access to all materials and research. (He was a Magna Cum Laude Elect Eng graduate from MIT with extensive experience in other than his areas of expertiece).

Also, I am now retired with 40 years plus in engineering, the original research was done when I was a high school/college student. That was before the development of personal computers, the internet and when calculations were accurate to 3 places since that was the accuracy of a slide rule. My personal rule of 1 hp elect = 3 hp gasoline hasn't been disproven in that time. I have learned many things in that time, most important to put some trust in others research when applicable. No, I haven't sailed the world but I trust that Columbus was correct in that the world is round. Once met Albert Einstein but never insisted that he explain E=mc2 to me. The Wright Brothers never explained to me how an airplane can fly like a bird but not flap it's wings, but I still fly in planes.

Lastly, if you don't like my posting don't blatently denounce it as "BUNK". Repeat the research, inquire about the research and development, ask some questions, learn more! Once they started to listen to Edison he wasn't such a bumbling idiot!

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#16
In reply to #15

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 2:42 PM

This motor will drive a band saw re-sawing timbers. With the right blade for the hardness of the wood the head can be pushed through the timber easily by hand. No mechanical feed is intended. There are more links and video about home made mills out there than I can count. The three to one rule seems prevalent enough to have some basis, and if it proves to not work out, like the guy said "electric motors are cheap." I'll just bump it up a notch. Thanks to everyone for the help. I'll post back when I;m done with the build.

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#45
In reply to #16

Re: Gas Motor vs. Electric Motor - Equal HP?

06/22/2010 9:44 AM

if you look up honda's page on your 9hp motor and match the top of the output torque to an equivalent electric motor, then this will match the expected performance providing the rpm's are matched

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#17
In reply to #15

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 8:54 PM

Hi Old Salt,

You might have noticed that I did not write "utter and total BS" but only "bunk" ... meaning only that you are both over-simplifying and over-generalizing. There is no difference between electric hp and gasoline hp: a gas engine rated for 7.5 hp continuous duty produces the same continuous power as an electric motor rated for 7.5 hp continuous duty: if both are running a conveyor all day long, continuously lifting grain up an inclined plane, and if 7.5 hp is required for the operation, then both motors will have done the same amount of work at the end of the day, and neither will have been overloaded. For such duty, an electric motor of 2.5 hp would be too small, and would overheat. If there are large variations in load over the course of the day, then one needs to understand the nature of those variations to say more than 1 continuous hp = 1 continuous hp.

The Tesla roadster, with its 150 kW (200 hp) electric motor has a top speed 30 mph slower than the 190 hp (gasoline) Elise upon which the Tesla is based. If, from that small sample, I were to overgeneralize, I might say that an electric motor must be of substantially higher hp to match a gasoline engine. I could hardly be offended it you wrote "Bunk," because I would be grossly oversimplifying and overgeneralizing.

It is tough to successfully argue that your rule-of-thumb is better than TCMtech's. His results in a more conservatively loaded electric motor and its associated components (including bearings, etc) all under less relative load and more likely to last longer. His rule of thumb results in about twice the electric hp of yours. You cannot both be "right" for all situations. His solution will work in many applications; yours will work in fewer.

Compare these compressors produced by the same manufacturer and both rated for 100% continuous duty:

http://www.northerntool.com/shop/tools/product_531387_531387

http://www.northerntool.com/shop/tools/product_211720_211720

One is 5 hp electric: 18.1 cfm (at 90 psi). One is 5.5 hp gasoline: 11.8 cfm (at 90 psi). For the gasoline-powered compressor to equal the electric one, its engine would need to be uprated to 5.5 x 18.1/11.8 = 8.4 hp. Your rule of thumb suggests that 15 gasoline hp is required. TCMtech's is obviously much closer to the real world. (5hp x 5/3 = 8.33)

So when I wrote "bunk, I only meant 1. your figure does not work for real, engineered products like the Ingersoll Rand compressors, 2. that it does not work for continuously rated hp in continuous load applications (in which case, post number one is correct: 1hp = 1hp); 3. it does not work for electric vehicles, 4. as an opinion to be taken on faith, it is no better than the opinions expressed by others, who have, in this thread said 1:1, 5:3, 2:1, etc.

You mention many things that you take on faith, without understanding how they work. That is certainly your prerogative. It may be unreasonable, however, to expect that engineers share your apparent disdain for doing the math or your enthusiasm for oversimplification.

I have learned many things in that time, most important to put some trust in others research when applicable.

You seem to be suggesting that I should trust your "research" but not that of the others whose opinions differ from yours. Ignore Ingersoll Rand; believe in you? Ignore TCMtech, common engineering practices, and my own experience. Is that the idea?

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#20
In reply to #17

Re: Gas Motor vs. Electric Motor - Equal HP?

06/17/2010 4:03 PM

MoronicBumble:

"meaning only that you are both over-simplifying and over-generalizing" is very clearly a relative thing. What is over-simplifying or over-generalizing in your viewpoint? I think, based on my career and the needs of the projects I have worked on for many years, that my accuracy has an excellent track record. No, I don't usually use pi to the 20th place, almost every time 3.14 or 3.1415 was good enough but I do know where to get pi to the 100th place should it be necessary. I don't waste time or money over engineering things (1st thing learned from my engineering mentor) but 1st I figure out how accuarate it needs to be.

Perhaps your viewpoint may be skewed by your lack of experience of knowledge on the subject. To make a statement such as "There is no difference between electric hp and gasoline hp: a gas engine rated for 7.5 hp continuous duty produces the same continuous power as an electric motor rated for 7.5 hp continuous duty" shows a lack of understanding of the subject. Electric motors based soley on how they operate vs. a gasoline recip engine are much more efficient. An electric motor rotates with very efficient bearings while it goes directly from electrical energy to mechanical rotary energy. A gasoline engine suffers from several inefficiencies including the need to have comparitively vast quantities of lubricant, going from fuel, to heat, to expansion, to recipricating linear motion, to rotorary motion, through less efficient bearings all the while supporting the necessary ancillary equipment such as the ignition system, battery recharge alternator if so equipted. Lastly energy is lost in the use of a muffler. All this is then compounded by the wasted other "3 cycles" of a four cycle engine.

Go out and get your hands dirty doing some "development work" instead of doing "pure research" with a sharp pencil and a PC. It would be an eye opener! How can you state: "It is tough to successfully argue that your rule-of-thumb is better than TCMtech's. His results in a more conservatively loaded electric motor and its associated components (including bearings, etc) all under less relative load and more likely to last longer. His rule of thumb results in about twice the electric hp of yours. You cannot both be "right" for all situations. His solution will work in many applications; yours will work in fewer." TCMtech's and I are not arguing as to whose rule of thumb is better, we are simply offering the results of 2 different investigations into the subject. How do you know "yours will work in fewer" if you don't have an open mind to the subject and have investigated the 2 different rules of thumb. How do you know what is "obviusly much closer to the real world"?

Your example of air compressors is very light weight as evidence. What about the more than two dozen projects that have been converts? Also they clearly disspell your 1hp = 1hp theory.

Your statement-- "You mention many things that you take on faith, without understanding how they work. That is certainly your prerogative. It may be unreasonable, however, to expect that engineers share your apparent disdain for doing the math or your enthusiasm for oversimplification." You know nothing about my interests in doing math (I like math and sometimes do calculus and diff equations in my head when I am bored) and only seem to be interested in what you only perceive as the correct path to take. One of the biggest problems with some engineers is their pursuit of what they perceive as "accurate". What many forget is that time is money and things can be wastefully over-engineered. I wish I had a dollar for every chem eng who designed a system with no regard for what materials were available and what were really required. Industrial and residential plumbing measurements only require a tape measure or a folding rule not a caliper or a micrometer! More engineers make household plumbing more complicated than it needs to be than accountants do.

If I had what you state as apparent disdain for doing math or an enthusiasm for oversimplification I certainly would not have lasted over 40 years in industry as a Chemical Engineer. Most of those years I was responsible for costly operations or functions and was the "go to guy" when others couldn't resolve a problem or get things done. I was successful because I kept an open mind and tried to put the appropriate priority on each thing. I and my peers think I was good at it, we didn't over-engineer. If you are an engineer as you seem to proport to be, you should have found out by now that there are alot of bright and intelligent people out there who can run rings around you in their own little specialty. Don't try to bully them with what you perceive to be your intelligence. It will only result with alot of "doggy do" on your face and credibility. Remember- "It is very difficult to fly with the eagles when you work with turkeys!" and "It's hard to remember your goal is to drain the swamp when your up to your ass in alligators!"

TRUE ENGINEERS ARE DRIVEN BY GETTING THE JOB DONE NOT BY HAVING A HAPPY TIME ALONG THE WAY.

One final thought for you-- In your statement "It may be unreasonable, however, to expect that engineers share your apparent disdain for doing the math or your enthusiasm for oversimplification." you make some pretty strong assertions about someone who you know nothing about. This is commonly known as the "A-- Hole Process" not the "engineering process" that you proport to be a strong supporter of! Kid, come back when you have something constructive to say, not just looking for an argument! You are a shame to most other engineers.

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#21
In reply to #20

Re: Gas Motor vs. Electric Motor - Equal HP?

06/18/2010 12:58 PM

This is quite a diatribe.

All this because I said your rule of thumb is "bunk". Although you have suggested several times now that I am an ass, I have done nothing of the sort regarding you. This is a forum of ideas, and ad hominem attacks are ineffective here. Self-declared experience is rarely effective here, too -- we have had hundreds of posts from all sorts of nut cases promoting one scheme or another, and they are often self-declared experts in over unity machines, etc. In other words, claiming some limited experience in a particular area does not gain you any credibility here. A couple dozen conversions that you claim to have worked well is a drop in the bucket compared to the hundreds of thousands of compressors sold by companies like Ingersoll Rand. I used to work for Ingersoll Rand, and can tell you that they are very cost-conscious and thoroughly engineer their products to provide high value. They would be foolish to oversize engines, or to oversize motors -- either case erodes the value proposition.

Whether or not I think you are an ass is immaterial, and it would be rude of me to suggest that you are one, or to imply that you are one. My opinions of you as a person are best kept to myself.

But my opinions of your ideas, particularly when they fall outside common engineering practice, are best not kept to myself. I make no money from my posts here. My only motive is to help people avoid making the engineering mistakes that I and others have made. This thread has many recommendations re motor sizing. To the extent that they are poorly-written and over-generalized, they reduce the utility of this forum as a database of sound engineering practices. In contrast, if they are well-written and supported, they increase the utility of the forum. If one believes in the value of making a contribution rather than simply being belligerent, then one should point out errors of thinking, and highlight well-accepted principals and practices instead. TCMtech's principal has support in actual engineered products, as I have shown. Your idea does not, and you have provided no support at all for your idea, other than your personal belief that your factor works well.

It is your idea and the way that it is expressed that is wrong. Your conversion is not broadly applicable as a rule of thumb. In real engineering, rules of thumb sometimes work, but often do not. The Tesla reference shows that in vehicles, your rule of thumb does not come even remotely close, and TCMtechs rule of thumb is better but still not applicable. Many applications required real engineering. If we want to provide an engineered solution to the OPs question, then we need to know the continuous and peak load on the saw. Who knows who sized the original gas engine? If the sawmill is home-built, then the original engine could be much too large. Until we know the continuous load on the original 9 hp motor, we can say no more than to provided an accepted rule of thumb, as TCMtech did.

In the medium scale generation world (megawatt level), engines are rated for continuous hp, just as they are in the ships. In these cases hp is hp -- if you need 1000 hp continuous to move a ship, it does not matter whether an electric motor or a diesel provides the 1000 hp. Even in designing compressors and generators like the Ingersoll Rand units I mentioned, the engines real continuous hp is used -- this is the reason IR claims that both compressors are rated for 100% continuous duty. In these cases, the real continuous hp of and electric motor and an engine are identical.

You make many assumptions about my age, experience and mode of working, all of which are wrong. Garrth has observed that many CR4 members are old farts, and I fit that mold. Like you, I started my engineering career with a slide rule. (Unlike you, however, I would correctly round 3.14159 to 3.1416, however. If you are going to take the time to work to greater precision than is usually necessary, you might as well do the math right.)

This is revealing:

TRUE ENGINEERS ARE DRIVEN BY GETTING THE JOB DONE NOT BY HAVING A HAPPY TIME ALONG THE WAY.

I could hardly agree less -- if you're not having fun, you should be in a different line of work .

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#22
In reply to #21

Re: Gas Motor vs. Electric Motor - Equal HP?

06/18/2010 2:32 PM

MoronicBumble:

Obviously you and I come from different background and philosophies in engineering. That doesn't bother me in the least. Mine is based on my personal experiences, those of my peers, past and current research and development, etc. This is the great USA! Everyone has opportunity and choice!

When I made my first post where I expressed "my rule of thumb" of 1 hp elect for 3 hp gasoline I prefaced it with--

From a completely practical standpoint based on extensive research many years ago, when I was young and had the time to do such research, I found that a good equivalency is 1hp electric motor is equal to 3hp gasoline powered engine, output shaft rpm being the same. This meant good, not perfect in all situations, not the solution to all the worlds problems, not the Holy Grail. GOOD is a relatively suitable place to start if someone has no idea where to start. It's not perfect but better than poor!

I also concluded with--

I'm sure many contributors can find mathematical, scientific, engineering type reasons why this won't work but it is difficult to say it won't work when it does work! AND THIS IS FROM A GUY WHO IS A CHEM ENG! I still believe this and your contributions have clearly verified it. My engineering consultant was my father, yes the guy with all "glory" on his degree as he termed it and the original 1 hp = 746 watts guy. Several things he first taught me were: don't make it any more complicated than you have to i.e. KISS; always get a 360deg size-up first; always do the best you can; don't strive to be perfect because perfectionists end up being utterly depressed from not reaching their goal; have fun in doing what ever you do; and lastly make it work so good that you are proud of it. The best hand shake he ever gave me was when he mathematically proved himself wrong and me right with my "rule of thumb". He was extremely proud that I had taken practicality into consideration over his pure theory.

In the medium scale generation world (megawatt level), engines are rated for continuous hp, just as they are in the ships. In these cases hp is hp -- if you need 1000 hp continuous to move a ship, it does not matter whether an electric motor or a diesel provides the 1000 hp This is not small sawmills, small air compressors or golf carts. Use applicable criteria for the matter at hand.

The difference in pi be it 3.1415, 3.14159 or 3.14 is very insignificant in real world use and very seldom needed. When it is needed good engineers know what to apply. The difference in pi between 3.1415 and 3.14159 at the world's equator is approx. 440 yards. Tough to measure with inexpensive devices and almost always not significant. As I have advocated all along- DON'T OVER ENGINEER THINGS!

As for your inference that if engineering isn't fun you should get out of it I am still driven by TRUE ENGINEERS ARE DRIVEN BY GETTING THE JOB DONE NOT BY HAVING A HAPPY TIME ALONG THE WAY. Life isn't always a bunch of roses, put the same effort into getting those less desirable tasks done as you do with those tasks you like. It all pays off in the end!

Finally, you have shown a strong preference for IR air compressors. I too like IR compressors, have a 5hp one at home along with a 5hp Quincy. While in industry, with IR screw compressors of 100hp+ I found Quincy to be better, not because they were a better machine but because they had less down time due to waiting for parts, waiting for a service rep, etc. The Quincy was better because it ran more often. Time is money and money is what pays us and the stockholders.

If in the future you again feel the need to selectively discect someone's posting please pass over mine. My posting are not made as statements of irrefutable theory, just statements based on classical engineering development, known facts and practical applications. Discecting what is posted here will kill this forum faster than anything.

"A practical guy"

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#23
In reply to #22

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 12:27 AM

Don't worry

someone else will be happy to dissect your posts should MB decline

when you express your opinion in such a way to lead the reader it is fact & then omit any real data

what do you expect?

the rest of us to bow down & praise your superior wisdom

Your preference for Quincy is special, but has no relevance

unless of course you can provide some links to information from Quincy supporting your "rule of thumb"

If the op provides some more detail about the requirements it would be much easier to make much more accurate recommendations.

TCMtech's scheme has other attributes beyond the sizing of the motor. Using a freq drive is also going to act as a protection device, soft start & be able to display the load being asked of the motor, which would provide data to actually answer the question.

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#24
In reply to #23

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 2:49 PM

Garthh:

Thank your for your civilized, although perhaps slightly slanted, reply. After having reviewed all of subject communications I am of the viewpoint that perhaps all the communications are due to different people's definitions and application of one phrase "rule of thumb" and a few words "bunk" and ""accurate".

Having verified my understanding and use of these I offer the following information:

"RULE OF THUMB"-

http://www.phrases.org.uk/meanings/rule-of-thumb.html

http://en.wikipedia.org/wiki/Rule_of_thumb

http://rulesofthumb.org/

http://rulesofthumb.org/about.php

http://en.wiktionary.org/wiki/rule_of_thumb

http://desktoppub.about.com/library/glossary/bldef-ruleofthumb.htm

http://www.businessdictionary.com/definition/rule-of-thumb.html

"BUNK"

http://www.thefreedictionary.com/bunk Def. #2

http://www.thefreedictionary.com/bunkum

"ACCURATE"

http://www.businessdictionary.com/definition/accurate.html

My information, as previously stated, was presented as a "rule of thumb". Specific wording was "From a completely practical standpoint", a statement of less than irrefutable research and conclusion. As you stated it is an opinion and if I may refer to that infamous and practical philosopher DIRTY HARRY- "Opinions are like ass holes, everyone has one!" There was no attempt to present supporting data because it is a rule of thumb based on research many years ago and the original poster didn't seem to need volumes of supporting data, just a general rule of thumb as to where to start with his development project. Any presumption of it being irrefuable fact is probably due to the lack of common sense on the part of the reader.

My wisdom, or lack thereof, is superior to no one. Only my wife knows the difference between my IQ and Albert Einstein and she tries to forget it. It is irrevelant to almost everything of concern in this world and expecially at my stage of life. I am not so convinced that only my reasoning and engineering skills are correct as some other posters are inclined. If I am wrong help me, not damn me. I am a human being which doesn't like when a person would so indiscrimately skew a "rule of thumb" statement as to be completely untrue. As I originally stated " I'm sure many contributors can find mathematical, scientific, engineering type reasons why this won't work but it is difficult to say it won't work when it does work! AND THIS IS FROM A GUY WHO IS A CHEM ENG!" I personally find it hard to say something can't work if it does work. As I stated to MB, "The Wright Brothers never explained to me how an airplane can fly like a bird but not flap it's wings, but I still fly in planes".

What would I expect? Someone to ask a question before using a statement such as "BUNK"! (Please see definitions for "bunk" and "bunkum") This is certainly a very poor choice of words, but is completely consistant with the tone of MB's postings, statements and conclusions.

My statement concerning IR vs. Quincy was a statement of the experience of several plant sites and is very relevant to the subject. MB made several references to IR air compressors and their quality, I too like them. What we experienced was that they were good operating units but overall, when all things were considered, were not as good as the Quincy's because they were not operationally available as much of the time. Quincy's were slightly more expensive to operate overall but could be depended on to be running when they were supposed to be. TIME IS MONEY. This data was an internal accounting record, not available for internet posting.

TCMtech's scheme has many attibutes to it that are extremely good. I have no argument with his rule of thumb or as he stated "common conversion", I think he put alot of thought and work into it. I support his statement fully and enthusiastically. It deviates somewhat from my experience but I never stated that my "practical approach" was more than a "Rule of Thumb". Just remember though that my reply was to the original poster who stated "I am building a band sawmill and changing it from gas to electric. I have heard so much about the differences and need to know how to size the electric motor to take the place of a 9hp Honda. The electric will be 220v single phase." My post answered his question as I best saw it (no pun intended). As well as TCMtech in his initial reply, I didn't feel it necessary to provide supporting data.

What does bother me is when someone makes a post to this forum and obviously knows nothing about the question they are asking and provides irrevelant or very little informations concerning the question, i.e. I am building a 1,000mw hydroelectric generating station, what size wire do I need? Equally as bothersome is someone who provides an answer with blatant presumptions and errors who feels that their answer is the "only one". We are humans, let's act like we are good ones.

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#25
In reply to #24

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 4:05 PM

My comments. are certainly somewhat biased, having been acquainted with Mr. Bumble here on this site for 5 years, I'm not at liberty to discuss the circumstances regarding his present situation...

I won't tell you how to react, to divergent opinions & my favorite Moron has been known to express his opinions in no uncertain terms.

you certainly fit right in our little band of people who know & have done shyt

I don't think the actual IR vs Quincy argument, is relevant, other than IR had some easily obtainable data directly comparing gas & electric. I personally had good experiences with both brands, though my satisfaction had more to do with the dealer, than the quality or reliability of the equipment.

We are certainly in agreement, about the relative quality of questions. I have a certain bias against guests & generally steer away for discussions started by guests.

Equally as bothersome is someone who provides an answer with blatant presumptions and errors who feels that their answer is the "only one"

you seem to be one of the "someone's" who piis you off

then again the joint is lousy with Someone's [including me from time to time]

the give & take [or lack] of this many smart people in this small a place is part of the beauty

Welcome to CR4

Enjoy

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#26
In reply to #25

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 10:29 PM

Easy goes it all. I'll provide more info for clarification if it helps. There are 2 band wheels 18" dia running a 144" x 1-1/4" x .042 blades at 800-1000 fpm typ. The reason for two speeds is a friend of mine set his up for a slower speed for green wood and he gets less loading. The cured wood would be resaw work or sawing old cut or downed wood. This is not a high production set-up, but more to get use of some unique timber for rustic furniture. The shop is an old city mule barn with single phase power. The drive motor will be speed reduced via twin belt pulley to get the right band speed. This machine is fed by pushing the cutter head by hand through the stock. Load will be controlled by ear. If you look at any of the available video you'll see it's a slow process at best so there are no-load running times equal to or greater than load times. Don't know if this helps any, but thanks again.

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#28
In reply to #26

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 11:21 PM

Hi DGCI:

I've done a fair bit of sawing and have rarely wished for less power on a saw. An underpowered saw can be more of a safety hazard than a powerful one, and can mean that you spend more time working your way through a cut when you could be doing something else.

A simple answer to your question is here. If you read about Mister Sawmill Model 22, you can see that it is powered by a 9 hp gasoline engine. An option is a 5 hp electric motor. They say:

  • Electric motors last longer, cost less to operate and are pleasantly quiet, and a 5 h.p. commercial electric motor produces just about as much cutting power as the 9 h.p. gasoline engines, so you lose no time or capacity going that route if it meets your needs.

As you can see, this squares quite well with TCMtech's rule of thumb. "Just about as much cutting power" hints that the 5 hp might not be quite as powerful, but it's very close. The Lumberjack, further down the Mister Sawmill page, is ordinarily 5hp electric, with 7 hp gas being an option.

Any electric motor draws power at a rate heavily dependent on load (and dependent to a lesser extent on bearing drag and some other losses). So a large motor will not cost noticeably more to operate, (given the same idle times and cutting speeds) but will be less likely to overheat under difficult conditions, will have lower relative stresses on its bearings, etc.) A larger motor also gives you the future option to run a higher higher blade speed while maintaining high torque. As others have said here, 5hp should be fine. 3hp would be too small.

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#29
In reply to #28

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 11:43 PM

I was looking at the chainsaw version last year

while exploring the purchase of 40 wooded acres,

the thought being to sell redwood to off set some of the purchase price

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#40
In reply to #26

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 2:49 PM

DGC1-

Don't have much to add to your fairly complete list of parameters. Since you are hand feeding the stock your apparent only slow speed limitation is to feed the stock fast enough as to not burn or char the wood and not gum up the blade. High speed rate as you stated is best determined by ear and visually. Since you have a wide range of feed rates, combined with choice of blade speeds, choice of blade thicknesses (kerf), choice of blade tooth configurations, etc. the hp required is a very wide range, limited only by the speed rate you want to feed at.

If you think you might be cutting denser wood in the future, it probably would be better to oversize it a little bit now if not a quantum step in cost.

I use an 50+yr old Do-all 20" throat, 156" blade with a 2hp motor. Blade speed is around 50fpm to 60mph. Never had any problems cutting from 1-1/4" 316 St Stl to 12" oak, pine and balsa. Have done some resawing and never with any surprise problems. Just try to remember to put a small wedge in the cut at an appropriate place after the blade to keep the wood from binding on the blade (similar to what is done with a table saw for long rip cuts. As you have stated, I find the ear to be the best judge of optimum operation.

Good Luck, interesting project.

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#41
In reply to #40

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 3:17 PM

DGC1-

As an after thought-

I realize the motor that Do-All equipted my saw with many years ago is not within the acceptable power range of some contributors but it is the one Do-All chose to install. Some of my uses have been similar to what you are setting up to do, cutting thick wood, resawing, etc. so please accept this information as experiences or lab research conducted by me and others using the unit. It worked for us on many projects. It is yours to consider and utilize for whatever you choose to.

Again, good luck. You seem to have a good grasp on what you are doing. Try to enjoy the project as you progress through it!

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#30
In reply to #24

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 12:08 AM

As well as TCMtech in his initial reply, I didn't feel it necessary to provide supporting data.

The difference, which perhaps continues to elude you, is that TCMtech's response is common practice -- a good rule of thumb. Your theory is not common practice, and results in what would ordinarily be considered an undersized motor. Therefore, because you are proposing a new and unacccepted theory you need to supply some backup data.

I gather you didn't like my compressor example. Let's try a power washer. Power washers convert shaft hp into hydraulic hp. The shaft can be powered by (for example) an electric motor or by a gasoline engine. Northern Equipment provides examples of each. (And no, Northern Equipment is not my favorite brand.)

They sell a 3000 psi 2.5 gpm (3000*2.5/1714 = 4.38 hp hydraulic output) unit. It uses a 4.8 hp Honda 160 cc engine. They also sell an electric model with the same (i.e., identical) hydraulic output. Its electric motor is 5hp. Obviously, your 3:1 theory does not work at all in this case, unless you are yet another over-unity believer, who is convinced that a 1.66 hp electric motor can cause a hydraulic output of 4.38 hp.

Having verified my understanding and use of these...

I just realized that English must be a second language for you, and want to compliment you on your use of it. You do pretty well. However, I think all the other readers of this thread are completely familiar with the terms you took so much space to define. (To me these definitions come across as just so much drivel.) When I used the term "bunk" I meant "claptrap" or "nonsense" -- and I believe those words appear in your definitions.

You presented your theory as having broad applicability, but it has very narrow applicability -- as you can see, in the common example above, it would produce a conversion that is off by a factor of about three. By saying "bunk" I was simply saying that your statement makes little sense. (Broad ≠ Narrow) Perhaps, being unfamiliar with English, you thought bunk was an insulting term (perhaps slang for some sexual deviance), and therefore that caused you to become insulting in retaliation. Rest assured that the only meaning I intended was that what you were proposing made little sense from a practical engineering perspective.

I look forward to your contribution in matters of chemical engineering.

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#31
In reply to #30

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 12:38 AM

Geez Bumble,

your gonna have a couple o 100 good answers before you know it

I'm pretty sure he can show us how to make Hydrogen with aluminum....

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#27
In reply to #22

Re: Gas Motor vs. Electric Motor - Equal HP?

06/19/2010 10:38 PM

Hi Old Salt,

Very little in your post is on topic, and little of it is of interest to me (although I can see how it could be interesting to you).

However, this caught my eye: "my posting are not made as statements of irrefutable theory, just statements based on classical engineering development, known facts and practical applications." I can understand that you may actually believe this.

The issue that seems to be causing you some stress is that nothing you have written here supports your contention that your "statements are based on classical engineering development, fact, or practical application." You have supplied no documented application in which your theory holds. By way of example, I have supplied data on IR compressors (which you incorrectly interpreted as my having a particular fondness for the brand... I was using it only as an example of standard practice and ordinary competent engineering.) I could supply similar data from other manufacturers, but there is no need: the IR data clearly shows that your numbers are wrong -- there is no need to hammer home the point.

There are many aspects of real day-to-day practical engineering for which being wrong by a large margin (as in your case, where your theoretical number produces a value of about half of ordinary accepted practice) is effectively a disaster. Consider a system that has five processes chained together, with each operating at 80% efficiency vs one in which the processes are optimized, with each at 95%. That seemingly small difference becomes a very large difference in overall efficiency: 32.8% in one case, 77.4% in the other. In some processes such differences can represent millions of dollars in energy cost. So there are many times when "close enough for government work" is not really close enough, in a practical day-to-day engineering sense.

If this site were "House and Garden" on line, or "Modern Maturity" on line, then your incorrect theory would not be worth commenting upon. But engineering, (real, practical engineering) tends to be based on reasonably precise numbers for many good reasons -- there are loads of cases in which the wrong numbers get people killed. You cannot pass a typical PE exam with slipshod math and rules of thumb that miss the mark by huge percentages.

Discecting what is posted here will kill this forum faster than anything.

I'd call this bunk, but will not because that word seems to have some special meaning to you. Instead, I'll call it nonsense. Highlighting BS as BS is a strength of this forum. You will find, as you poke around CR4 that some recommendations are both flat wrong and dangerous. Those of us who give a hoot should and do point out such BS. We've been cranky old men bickering about all sorts of things for years. I gather you have not read through any of the challenge question threads. The thing that draws people to this site is not poorly thought out "Oh this might work" answers as much as the discussion and hair splitting. As a crowd, engineers are very apt to say "prove it." For many people, most of engineering can be considered hair splitting: they'd say "Just put some friggin motor in it. If it breaks, put a bigger one in."

Clearly you've made some useful contributions elsewhere on CR4. I hope you keep up the good work.

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#32
In reply to #27

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 1:09 AM

MoronicBumble:

Against my better engineering and human judgement, I am sure I will accumulate several "off topic" votes, I am making my last reply to your postings. Any others will be address to others and from a more technical standpoint.........

I feel very sorry for you that you are unable to accept the concept that engineering might be a relative thing and not an exact science. Some things/concepts/solutions might be better than others but it is always a team effort to some degree. Very few things in engineering are "exact", most are a compilation of "the best" from several facets of the overall problem or goal.

I also feel very sorry for you in that you apparently think that only you have the correct or "exact" answer. Based on my education, experience, and my life's experience I feel that there is most often more than one correct answer. Mine isn't automatically the only right one or the best one solely based on the fact that I did the work. Your contention that "The issue that seems to be causing you some stress is that nothing you have written here supports your contention that your "statements are based on classical engineering development, fact, or practical application." You have supplied no documented application in which your theory holds." is yours only, no one else said it was required especially the original poster who asked the original question. Also, others have have made other postings of "rule of thumb" answers such as the "The common conversion is 3/5." made by TCMtech. What about them?

One of my first bosses instilled in each of his underlings that it might be fun to spend as much time on each project as we as individuals wanted to but the job needed to get done within the timeline. There were more aspects to the operation than we as individuals felt was best. We can be engineers but we have to be team players in the rough game of life and commercial competition. Time is money and "GIT 'R DONE".

What you profess to be engineering standards are really only your opinion, nothing that you have added to these discussions has suggested that they are any more accurate than anyone else's opinion including mine. As previously stated I am a great believer in the philosophy of DIRTY HARRY. No, I don't have a PE as part of the "glory" after my name. There are several others though which I am even more personally proud of than a PE, no offense to any PE's intended. Don't make any presumptions about people you don't know anything about!

When you come to the realization that this is an open forum, available to all, open to all philosophies of engineering perhaps you will enjoy it more. I enjoy it. Just because I might disagree with someone doesn't mean that I have to force my viewpoint on him/her. (among many options, making a right turn can be done by either turning right 90deg or left 270deg, sometimes it's more practical to turn left 270deg). If you don't like my "incorrect theory" I don't mind, its no insult to me. That's why I went into engineering (chem, elect, mech) instead of pure research, I wanted to solve problems not theorize possibilities. My career provided my with the opportunity to solve problems which were successfully and profitably solved.

As a human being I feel sorry for you, you do the same things you accuse others of doing but in a damning way. Where is your support, other than your opinion, that my "rule of thumb" did not meet the criteria for a "rule of thumb". Please check previously posted definitions.

When you become open minded enough to accept that no one on this earth has all the right answers and we are all entitled to our own conclusions perhaps you will understand engineering and problem solving better. Maybe you might enjoy life a little more. I love life and always take every opportunity to "stop and smell the roses". Maybe the fact that I have had 2 fatal heart attacks influences that (each time they were lucky enough to be able to bring me back after several minutes) but what the heck, it is too wonderful a world to waste time arguing with someone who takes there opinion as the only answer. I'm always open to other's answers, just don't insist that yours is the only correct one. I may be "old and worn at heart" but my mind and body are still relatively young!

FLASH!!!

While composing this final reply I received notice that you had posted another senseless dissertation of irrelevance.

To keep the matter straight, as you seem to make all sorts of irrelevant inferences,

1. I an USA born and raised. English is my primary language and the one most used. Profanity is my second language and you ain't seen nothin' yet!

2. My use of definitions was for your benefit since you show strong tendencies to disregard published definitions (perhaps my understanding and the usage the English language is greater than yours, I don't care)

3. The definition for bunk is:

Empty talk; nonsense. [Short for bunkum.] The thesaurus list for Bunkum is:

Noun

1.

bunkum - unacceptable behavior (especially ludicrously false statements) buncombe, guff, hogwash, rot, bunk dogshit, horseshit, Irish bull, bullshit, crap, shit, bull - obscene words for unacceptable behavior; "I put up with a lot of bullshit from that jerk"; "what he said was mostly bull" drivel, garbage - a worthless message

4. You are a disgrace to the engineering profession! The things you expound as being incorrect applications of engineering are exactly what you are doing! If you were to see yourself as others see you and your babble you would be ashamed. On second thought, you probably wouldn't with your almightier than thou attitude.
According to your posting of 06/20/2010 12:08 AM Your statement that: "The difference, which perhaps continues to elude you, is that TCMtech's response is common practice -- a good rule of thumb. Your theory is not common practice, and results in what would ordinarily be considered an undersized motor. Therefore, because you are proposing a new and unaccepted theory you need to supply some backup data." is pure personal opinion on your part and to me it is unsupported in every aspect" Where is your proof? You prove it's wrong! It's worked for me as a "rule of thumb" for many years, I have the completed conversions to prove it. Get your friggin' hands dirty and prove your theory is as big as your mouth/touch typing finger tips are! Again, I feel sorry for you. I appears that engineering has perhaps overtaken your life and you are missing much of it. Your incessent insistence that you and only your conception of what is correct is the only solution/answer is very harmful to the engineering profession and yourself. As for my future contributions to CR4, please don't pay any attention to them. They are intended as being helpfull, not the subject for disection. Also, you will find that most of my significant contributions are in other subjects than Chem E. I have several other areas of knowledge other than only engineering and Chem E. It helps in life to have varied interests. Life is more enjoyable and you are able to contribute more to the world.
Adios!

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#33
In reply to #32

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 1:27 AM

http://www.mistersawmill.com/

Since when is providing a link to the exact application the Original Poster is asking about "Theory"?

Somehow you equate providing a link to other direct comparisons is also "Theory"?

This is an open forum, you are welcome to look as foolish as you want.

I'd like to congratulate you on a job well done

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#35
In reply to #33

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 11:01 AM

Garthh:

I was unaware of your posting with the link when it was posted since I was in the middle of my last and final post to MoronicBumble. Your statement that "Since when is providing a link to the exact application the Original Poster is asking about "Theory"? Somehow you equate providing a link to other direct comparisons is also "Theory"?" is your statement and your statement alone. I am not the source. I have no alternative but to attribute it to the distortion and askewing of postings for one's own personal glorification or lack of credible substance to substantiate. I can assure you that I take full responsibility if I am wrong or have made a mistake, in this instance you have distorted!

Before you make any additional statements concerning other posters such as myself, those that you apparently disagree with, this forum would certainly appreciate it you would limit yourself correct references. As for- "This is an open forum, you are welcome to look as foolish as you want. I'd like to congratulate you on a job well done" it you, not me who has made a fool of himself.

Adios!

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#36
In reply to #35

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 12:03 PM

You continue to assert the superiority of your theory, but have yet to provide any actual data to support that claim & then get all butt hurt when Mr. Bumble calls Bullshine

I certainly take exception with your notion that you in any way represent "the forum".

You speak for yourself & no one else until otherwise noted by those representatives of the forum or members who would [for reasons that are unclear to me] wish to affiliate with your utterances.

Your previous posting is actually in violation of site policy, containing several instances of profanity...

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#34
In reply to #32

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 10:58 AM

Old Salt/MoronicBumble, I'm sure you both believe strongly in your POV. Might I suggest this would be a good time for each of you to drop this little flame war?

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#37
In reply to #34

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 2:01 PM

Rorschach:

THANK YOU, THANK YOU, THANK YOU! It was never my intention to become involved with or to maintain an argument on this forum expounding either one theory or another concerning a "rule of thumb", "engineering method", "exact", etc. I simple stated what was my personal "rule of thumb" that I developed many years ago relevant to the original posters question. I made no statement as to it's accruracy, be it +-0.000001% or +-50%, just that to some people it might not be mathematically reproducable but that it had worked within it's expectations through the years for me. Just a suggestion to the original poster, not a declaration of war on anyone in particular. (As a matter of principal I usually use the "DEL" key if I disagree, unless it is a matter of definite life safety).

Again, THANK YOU! Now for my next project, perhaps you can make a contribution. I won't yell, shout, stomp or tamtrum if I disagree with you. Probably say "thank you for your contribution" and go on my way. See the problem is: 1) I'm am the Fire Chief for the local volunteer fire co. I'm usually pretty good at it, I like what I do and my peers say I'm very good at it. 2) I am an avid sailor and have been for over 50 years. I'm told I'm also pretty good at it but I sometimes question some of the complimentors' motivation. I'm good enough to really enjoy it. I enjoy it even more than anything else except my wife, of course! (she's smart enough to never make me choose between her or the boat). The boat is a 50 yr old sloop that my dad and I built and have sailed ever since. He're's the real problem- think it would feasible to make a sail powered fire boat? Where I sail is 60miles away and there are only small rivers within our response area. How could a river capable fireboat be constructed that is sail powered and enjoyable enough for my wife to want to go on it so she will give me "home time" credits when I'm actually doing "fire company time". I'm stumpted so far but perhaps this forum could make suggestions.

Rules are: no strict adherence to established conventional engineering thought processes; "rules of thumb", no matter how fat/skinny the thumb is, are acceptable; prefer an auxillary power of either electric (battery preferred) power or gasoline power (83 octane max); and finally all suggestions are not final.

Again, Thanks for your timely contribution!

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#42
In reply to #37

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 5:10 PM

Did you ever consider starting your own thread?

Hijacking this one is rude

sounds like a fun discussion

the comment I'm replying to is off topic {as is this one}

note the 5 off topic votes I've given my self

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#39
In reply to #34

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 2:47 PM

I think your suggestion is a good one. I've marked those of my own posts that contribute nothing of a technical nature off topic recently. The recent ones only provide advice to Old Salt re the benefits of being less belligerent and the value of supplying verifiable data.

However, the posts that I have not marked off topic provide technical detail that I think you will agree is of some value. In my view, at least, there is no better advice to the original poster than to supply a link to a manufacturer who builds a saw of the same type and hp as the OP's. That manufacture has direct experience indicating that a 5 hp electric motor is a good choice for such a saw. I would not want that advice to be drowned out by insults and innuendo, and have the OP go away thinking that a 1:3 ratio is valid for his application. A certain amount of perseverance in advancing a point is worthwhile.

But I agree with you. There comes a time when this is no longer fun or productive.

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#38
In reply to #32

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 2:20 PM

Hi Old Salt.

Against my better engineering and human judgement, I am sure I will accumulate several "off topic" votes...

I understand that you are new here, and may be unacquainted with the ability to mark your own posts off topic. Your recent post offers nothing of use related to the thread, and like my off-topic post above, should be selected as off topic, so that in the future, viewers of this thread who are looking for an answer to the title question "Gas vs Electric Motor -- equal HP?" will have less to wade through.

I was extending you an olive branch (no, not literally -- it's a figure of speech) because you seemed unusually offended by my use of the word bunk. I offered a couple of synonyms, hoping that they would seem less offensive. I notice that in the post to which I am replying, you still seem hung up on the word bunk. I hope you'll forgive me if, in the future, I forget that sensitivity and characterize another of your nonsensical or over-generalized posts as bunk. Perhaps your dad used the word in a way that hurt your feelings (??)... rest assured that I do not want to open up old wounds, so will try to remember to use some other word, perhaps choosing from the list of alternates you have provided.

Your post (to which I am now responding) is a little hard to follow, but it appears that you think I am picking on you but not on the others. Perhaps I am jumping to this conclusion, but I am drawing that conclusion from your text: "Also, others have have made other postings of "rule of thumb" answers such as the "The common conversion is 3/5." made by TCMtech. What about them?"

It seems I have written words to this effect already, but the difference is that his contention is correct, and my compressor example (and numerous other compressor examples) shows that his contention is correct. Several other posters also have said that 5 hp would be a good bet. Further, TCMtech suggested, correctly, that if the OP anticipates frequent overloads, that 7.5 hp would be a good bet. I have no reason to challenge his contention because it falls under the heading "good advice." I challenged yours only because it erodes the overall quality of CR4 as a place to find good guidance -- it's nothing personal, and in fact, I was generally unaware of any prior posts you've made.

You might be whining to yourself: "Well what about posts 1 and 2? Why didn't MB find anything to criticize with them." That's because there is nothing to criticize. HP is HP. An electric motor of 5 continuous hp produces exactly the same power as a gasoline motor of 5 continuous HP. Real engineers will say that, beyond that, you must look at the nature and magnitude of expected loads and consider the quite different torque curves of a potential motor vs a potential engine. When real engineers design power washers, the power washers end up with the same HP ratings, electric vs gas, for reasons that would be obvious to real engineers. So TCMtechs rule of thumb does not apply to power washers, fans, etc. but is much closer than your theory, which has no utility at all in such cases. His answer is right on the money re the OP's particular application. It also has the advantage of being to the point, rather than rambling on about how some MIT guys are wrong, and really impressed with your "solution".

Sad to say, but all this picayune stuff is the stuff of real engineering. A saw is a different machine than a power washer. The same rule of thumb cannot apply to each, and it happens that your a rule of thumb applies to neither. No big deal. It does not mean you are profoundly idiotic, just that you are wrong in this one case.

That your first response to me was rude and that subsequent ones have become increasingly rude, may be a matter of some pride for you, just as you seem to take pride in your mastery of profanity. To each his own. I am sure that you might make some useful contribution here, even if I find your approach off-putting.

And no, I will not refrain from commenting on the un-useful ones, because, as I have implied before, if this site gets overrun with a bunch of poorly-supported inapplicable hunches presented as if they are generally applicable, the site uses its utility. For a while not long ago, we had loads of over unity promoters, who, when challenged, would turn beligerent. The site is a better place without them -- although I'd have to admit their ability to undermine their own arguments can be a hoot.

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#49
In reply to #8

Re: Gas Motor vs. Electric Motor - Equal HP?

12/08/2010 3:27 PM

Thanks, your info is great and to the point. Doug

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#9

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 3:05 AM

1 hp = 746W, but of course that doesn't tell the whole story.

The replacement motor will have a different torque/speed curve, different load factors, a different inertia and is replacing an engine that is probably over sized (for longevity).

Electric motors are cheap, so buying one that is too big wont hurt. 5kW should be fine.

If you want to get all technical you could put in a motor, measure the load current during operation and use that info to correctly size a new motor.

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#10

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 5:42 AM

I suggest you use a 3 phase motor and a variable frequency drive to convert the single phase power to three phase for the motor. This will allow you to dial in the best operating speed for cutting different wood types. The correct speed and feed will increase quality. And a 3600 RPM 3 phase motor is cheap on ebay. If you cannot find a VFD drive designed for single phase power, you can use most any drive of the correct voltage by de-rating the output. A three phase in 5hp drive can run a 3hp motor off single phase power without problem. That's what I do around the farm.

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#11

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 8:30 AM

I have done more than a few converions each way with all kinds of hydraulic equipment. In the less than 40hp group it is 1.25hp electric to 3hp gasoline, with diesel being 10% better. So my bet goes with the guy that says 2:1. If you have a practice of jamming the saw blade give the motor some breathing room. Since you are going single phase you may want to do it at 5hp. How is the saw driven? If it is a belt your possibilities are endless (pardon the pun). A torque limiter would be a nice touch with any electric drive saw. The motor will cost much less to run than the engine. An amp gauge you can see while you operate would be a nice touch too. A gear belt drive is more efficient and sized correctly it is long lasting and trouble free.

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#12

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 8:31 AM

Small gasoline engines are usually rated at 3600 RPM, which is full throttle under load.The torque varies with rpm, usually increasing with rpm.

Electric motors have an entirley different torque curve,being more linear.Choose a 3600RPM motor and use pulley ratios to change speed to desired output. A 5 step V belt arrangement will allow a good speed range that is very simple and easy to adjust.Just be sure to use at least a "B" belt width to deliver enough torque at low speeds.An "A" belt just won't cut it.

As far as HP, choose a motor with a service factor of 1.15 or greater, and a 5 hp should suffice.

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#14

Re: Gas Motor vs. Electric Motor - Equal HP?

06/16/2010 11:33 AM

If you want a comparison of gas hp and electric hp equivalents just look up generators and compare the KVA output to gas hp producing it.

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#18

Re: Gas Motor vs. Electric Motor - Equal HP?

06/17/2010 6:33 AM

the 3/5 rule is a very common ratio mentioned in many mechanical application field guides and reference books.

A 5 Hp gas engine can do 5 HP but do not expect it to have a long service life while doing it. Running at the 3/5 ratio tends to put the engine under a moderate load but not so much as to over work it to the point of a greatly reduced life expectancy.

As far as individual engine design or manufactures specs industrial application engines and general purpose single cylinder lawn and garden type engines are not of the same specs for the same applications.

Sure you can use more engine than you need and likely have even greater engine life from it but whats the point or limit then? Many of the common small engine driven machines do use slightly over sized engines above the common 3/5 ratio in order to achieve greater running lives out of the engines rather that working them in their upper power limit ranges thats all.

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#19
In reply to #18

Re: Gas Motor vs. Electric Motor - Equal HP?

06/17/2010 9:55 AM

The other point is that it IS possible to oversize the engine too much, leading to tearing the rest of the mechanism apart when an overload condition exists. one must make sure that there is some kind of predictable sacrificial failure part in the system in order to protect the rest of the machine. Such as a key or drive pin that will shear out at a known value and protect the rest of the machine.

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#43

Re: Gas Motor vs. Electric Motor - Equal HP?

06/20/2010 7:19 PM

The 3/5 ratio is a common and typical ratio used mainly for smaller non continuous duty application engines when comparing them to continuous duty application electric motors but is by no means an all encompassing ratio for all mechanical applications great or small.

Non continuous duty engines are usually of the smaller lawn and garden and vehicle propulsion applications types of engines. True industrial duty application engines are rated for their normal long term load holding capacity but they often have a fair percentage of peak HP over that number which is sort of like the service factor rating on electric motors. Some have a lot some don't.

What that means is that the 460 Cu inch V8 Ford in my pickup is capable of over 400 Hp on a vehicle application. BUT, that same engine could also be put into an industrial application where it is required to hold a specific load for very long periods of time while maintaining a specific RPM without being overloaded. In that application that Identical 400 HP engine may now be rated at only 250 HP or possibly less if the RPM requirement for the intended application is low enough. Although this is not an exact or perfect example it is typical of many real life applications where the specific usage of the engine will determine its factory specified HP rating. One application is peak rated the other is continuous rated. Same engine yet two different, but still honest, HP numbers even though Horsepower is Horsepower is Horsepower Supposedly.

Any engineer with real life working experience would know these basic concepts. On paper and by the numbers Horsepower is Horsepower but in the manufacturing world Horsepower is whatever the company who built something chose to rate it by or for which means all horses are not created equal.

And yes you can use a bigger gas engine or electric motor than what the minimum power requirement takes but still that does not necessarily mean its the best, proper, most efficient, practical, or cost effective choice. It just means you have more than enough to do the minimum requirements for job.

Theory says theory and reality are the same. Reality shows they are different. Hence in this case 3/5 equals 1:1 more often than not.

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#44
In reply to #43

Re: Gas Motor vs. Electric Motor - Equal HP?

06/21/2010 12:03 PM

Yet another excellent answer.

I'd considered writing up a summary for the more generic (title) question, (as opposed to the specific saw question) but you've pretty well done it.

There are applications like fans, wind tunnels, and power washers where hp = hp and the real continuous rating may have to be "discovered" by the tunnel designer. Any power level that results in 2000 hr time-between-overhauls can be reasonably considered a good continuous rating. (Tell that to a power plant engineer, however, and she'll laugh -- in that setting, nobody wants to rebuild so often.)

At the other extreme are applications like log splitters where the peak torque requirements can be really high, and the continuous loading is very small. Then the high stall torque of an electric motor can be a big advantage.

In the electric car world (which has been mainly hobbyists, but this applies even to quasi-production cars like the Solectrias) the cars that result from 2:1 and 3:1 rules of thumb are absolute toads, resulting in (for the Solectria) a 65 mph top speed on a perfectly level road, and nearly useless top speed on any sort of grade.

Re the marketing influence on hp, you are exactly right. In fact, many small commercial engines were recently down rated for this reason (11 hp Hondas are now 9 hp) -- no change in the engine, just a change in marketing.

I've found that when I hammer together stuff like this, doing the math first leads to fewer disappointments.

Theory says theory and reality are the same

I simultaneously agree and disagree. In 60 years, I have yet to encounter anything that cannot be quantified to within a couple percent , given the right set of theories. CFD results are now as good as as wind tunnel data for most applications. GPS works reliably to parts per billion.

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#48
In reply to #44

Re: Gas Motor vs. Electric Motor - Equal HP?

09/07/2010 10:24 PM

I dont like you.

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#50
In reply to #48

Re: Gas Motor vs. Electric Motor - Equal HP?

12/08/2010 3:39 PM

The feeling is mutual, I'm sure.

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#47

Re: Gas Motor vs. Electric Motor - Equal HP?

08/13/2010 4:02 PM

For an electric motor efficiency of, say, 90%, you need 9X1.1=10HP. This is for a DC motor. You can use an Universal Motor which has a collector or a DC motor. The current will be

220 over 10X[750]=0.03Ampers. In fact a factor of square 2 multiplies the result, and you need also a rectifier AC to DC.

You can use also batteries, but only with an Universal Motor.

If you prefer to use an AC source, the eficiency will be lower by a factor of 0.75 aprox., and you can use an induction motor wich last longer than an universal motor because there is no collector.

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