# Gasoline versus electric horsepower



## BBQ

In general, at these HP levels it takes a larger rated gas engine to do the same work as an electric motor.

If you got a 10 HP electric you could probably get by but you might have to under drive the compressor a bit.


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## BBQ

You really need jref to chime in.:thumbsup:


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## wdestar

I'm not a mechanical engineer, so I can't verify the conversion for gasoline HP to electrical HP (probably little difference in that true HP = torque x speed), but I can verify this - never go backwards.

So if he has a 16 HP gas engine, it certainly makes sense that a 20 HP electric motor should safely cover the bases. Secondly, you need to consider the fact that if the compressor is in a cold location - a barn or garage - the oil viscosity will increase, increasing the load (demand) on the motor. Now you have to figure in Service Factor - In this case, I'd recommend 1.25. A good safety margin.

And last, will it be INT or CONT duty. I suggest CONT as their needs may change as time goes by.

My 2 cents


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## stuiec

School last year told us that a variable torque consequent pole motor is desirable for centrifugal compressors, and constant torque consequent pole motors are better for piston style compressors. Real world I have zero experience here.

Edit: and they are larger, noisier, less efficient, have lower power factors and higher currents (compared to single speed motors)..... I'll just shut up and learn now....

Edit: _and_ OP stated single phase... I just kant brain todaay


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## nolabama

Handy. 
http://www05.abb.com/global/scot/scot259.nsf/veritydisplay/bf88560fb1b0335cc2256fc6003e4e05/$file/motor%20guide%20gb%2002_2005.pdf

Some debated rules of thumb ( found a cranky Mech engineering site like ours) 1 hp electric = 3 hp gas. 

Also heard 3/5. 3 hp electric = 5 hp gas.


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## snorky18

Air consumption determines pump size.

Pump size determines motor size. Specs on the pump could help, but I’m assuming since it’s salvage you probably don’t have specs.

For reciprocating piston compressors running 240 Volts, you occasionally see 7.5 HP, maybe at the most 10 HP, but it’s _rare_ to see over 5hp motors being used (~23 amps or so). If you go to price a new motor that large you’ll find out why.  Don’t even think about a 3450RPM motor for any sort of large compressor. Stay with 1740 RPM and down.

If you can compare the number of cylinders, layout, and physical dimensions of the pump to a new air compressor of the same configuration, that should give you a really close starting point for what motor to use. If you undersize the motor, you can reduce the shaft pulley size (to a point) to increase torque output to the pump without any negative effects (besides the obvious lower pump RPM, resulting in lower CFM output, and it would start a bit harder).

5HP and up you generally need magnetic starters. 3HP and down you are generally OK with just the pressure switch.

If they got it salvage, the "correct" size motor for that pump may be different from 16HP, so I wouldn't get too hung up on converting the gas HP to electric HP.


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## JRaef

nolabama said:


> Handy.
> http://www05.abb.com/global/scot/scot259.nsf/veritydisplay/bf88560fb1b0335cc2256fc6003e4e05/$file/motor%20guide%20gb%2002_2005.pdf
> 
> Some debated rules of thumb ( found a cranky Mech engineering site like ours) 1 hp electric = 3 hp gas.
> 
> Also heard 3/5. 3 hp electric = 5 hp gas.


That's about right. The reason is, an electric motor develops shaft torque much faster than a combustion engine can. The engine must be over sized then (in comparison) to provide accelerating torque, even if it needs less once operating. An electric motor is there instantly with the accelerating torque, so it can be sized more closely for the running load.

I learned this the hard wy once in replacing a portable rock crusher engine. It was 160HP diesel, but low RPMs. They wanted a direct drive electric motor, no transmission, so I calculated the engine torque based on the HP rating at the speed it was running, determined an electric motor that would provide that running torque, then used a VFD to slow it down.

Twisted the shaft off of the fricken crusher... 

After that I got involved in a lot of farmer projects converting diesel pumps to electric because of new air quality laws here in California that restrict diesel engines to 1000 hours per year. The pump guys helped me learn the truth of this. 2:1 minimum, sometimes you can get away with 3:1.


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## nolabama

snorky18 said:


> 5HP and up you generally need magnetic starters. 3HP and down you are generally OK with just the pressure switch.
> 
> .


Gonna go with a 5 horse motor. Now I get to plumb the controls. 

Pressure switch. Low pressure start contactor high stop contactor. ? Really that simple?


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## socalelect

nolabama said:


> Gonna go with a 5 horse motor. Now I get to plumb the controls.
> 
> Pressure switch. Low pressure start contactor high stop contactor. ? Really that simple?


Set your pressure switch to whatever cut in and out you want usually 175 out 150 in. Run control power thru pres switch and run contactor with that by using 24 volt the pressure switch contacts will never be an issue


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## Peewee0413

Hp is Hp, Unless Maybe I slept during that part of physics. 3Hp is not 5Hp, 1 is not 2, and black is not white.

Hp is a unit of measurement. Findings of James Watt ( his last name is not a coincidence).

hp is used to determine a force needed to move an object. Don't remember the number, but who cares really. I'm sure someone will Google it and try and look smarter than they really are. 

So like I said 3Hp is not 5Hp. 

Read up on the guys who gave you all a job.

Tesla(God)
Franklin(even though he was not so right on flow, not easy being one of the first)
Faraday
Newton
Watt
Ampere
Maxwell 
Edison
Hertz
Bell
Westinghouse(Sugar Daddy)
Marconi(theif)
Etc.
Bet you learn a thing or two.


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## nolabama

Whatever. Three horse electric does the work of five horse gas.


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## Peewee0413

How is that? It may work, but it does not perform the SAME WORK!. Unless you made some new physics discover Mr Scientist.


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## union347sparky

Peewee0413 said:


> How is that? It may work, but it does not perform the SAME WORK!. Unless you made some new physics discover Mr Scientist.


Because small gas powered engines are over rated anyway. There was a big lawsuit over that a few years back. Just because it says 5 HP on the shroud it's more like three.


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## Peewee0413

Its kinda like an engine in a vehicle the hp is not constant but limited at a certain rpm.


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## socalelect

I have never compared gas to electric but I know from experience their is a difference between diesel and gas engines and hydraulic motors ,of the same horsepower rating . its in the torque not the horsepower , horsepower sells engines torque does the rest


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## Peewee0413

Yes your exactly correct and just like someone mentioned bout a shaft breaking above . Torque was the culprit.


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## Peewee0413

At the same angular velocity a 5hp gas is the same as an electric. Unless Mr. Scientist above states otherwise.  physics is physics my friends.


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## mbednarik

A 16 HP gas is 16 HP at its peak, not all of the way through its powerband. Do some research before you start typing


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## nolabama

Yeah I didn't do real good in my mechanical engineering classes in college. I do know I am gonna replace a 16 hp gas compressor motor with 5 hp electric. The motor change should not be noticed. Cept on a chalkboard. Unfortunately I live in the real world.


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## mbednarik

Actually the torque rating of a 16 HP briggs is 7 ft/lbs. The torque rating of a 5 hp electric motor is 7.2 at 3600 rpm (2 pole), and 14.4 at 1800 RPM (4 pole).


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## Peewee0413

mbednarik said:


> A 16 HP gas is 16 HP at its peak, not all of the way through its powerband. Do some research before you start typing


I said that 4 posts before yours.


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## Peewee0413

mbednarik said:


> Actually the torque rating of a 16 HP briggs is 7 ft/lbs. The torque rating of a 5 hp electric motor is 7.2 at 3600 rpm (2 pole), and 14.4 at 1800 RPM (4 pole).



3600-7.2
1800-14.4
Do u see a pattern in the numbers 
Hp= (torque * rpm) / 5252
Unless I forgot over the years. 
U left out the rpm of the briggs where it is 7 lbs of torque. Hp is just a number of torque and rotational speed. ( in simple terms)


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## Peewee0413

Im being annoying on purpose. Just because somebody said 5 hp with gas is 3 with electric, and its really not. You can't swap them based off of the number of hp alone. Say you have a pump that said it needed blank ft lbs to break and then a lesser to maintain rotation. Didn't say the hp required. I could of put a 2 pole, 4 pole, 5 hp, 15 hp. Just need to punch numbers. You can use a 1/4 hp motor to rotate 4 tons of steel too. But torque is a factor. (Gear reduction). I'll stop picking on u all.


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## mbednarik

Every brings I have seen is rated at 3600 rpm.


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## micromind

The HP rating of an electric motor is just that.....HP at the shaft.

There are many ways to rate the HP of a gas engine. Most smaller ones are rated in gross HP; this is the HP the piston produces. To get net (or shaft) HP, you need to subtract taxable HP. Taxable HP is the HP needed to turn the engine; bearing friction, etc. 

This is why there's such a huge difference between the ratings of gas vs. electric. 

Another way to look at it is if you use the idiotic metric electric motor ratings, you'll see a KW rating. This is the HP converted to KW that's available at the shaft. The actual power input KW will always be higher that the KW rating on the nameplate. 

P.S. I stated 'idiotic metric' because in my mind, KW is a unit of electrical measurement, not mechanical. HP is a mechanical unit. Just like a meter is a device to measure something, not a unit of measure.


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## Peewee0413

If you break it down watt does not mean electrical. Its used in electrical but its just a measurement. A watt is a measurement of joule and joule is work done in applying a force of one newton through 1 metre. Newton which is the force to move mass 1 kilogram 1 meter per second sq. 

If its only electrical then why can you convert watts to hp? Hp is not electricity. Its a measurement of energy. Energy is energy. There is electrical potential energy. gravitational potential . Chemical potential 

Physics can complicated , but its one of the only thing that never lies.


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## Peewee0413

How this electric motor question turned into a physics debate stumps me. Maybe we should let the topic end. End and allow people to resume thinking electricity is some invisible substance traveling through a wire.


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## socalelect

Peewee0413 said:


> How this electric motor question turned into a physics debate stumps me. Maybe we should let the topic end. End and allow people to resume thinking electricity is some invisible substance traveling through a wire.


Just make sure all connections are tight so the electrons don't leak , makes one hell of a mess


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## Peewee0413

Good one


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## wdestar

*What is HP?*

Ok

So I tell my students that HP = Torque x Speed. Simple enough to remember.

Now - 1 HP = 550 foot-lbs/second. Again, not hard to remember.

At what point did Briggs & Stratton get away with calling a 5 horsepower motor 16? I'm confused.

Are there different standards for horsepower?


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## Peewee0413

They state their max output of the curve.


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## Peewee0413

Just as 10 hp electric motors are rated at 60 hz, Hp differs at different rpm.


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## Peewee0413

Torque is the work potential. Hp is work over time. Or something like that.....


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## MetalMasher

Old thread, but let me see if I can clear some things up (and hopefully, it's useful):

First, consider a gasoline engine and it's power. Typically (this is not true in all cases as we see with the skeptical HP ratings given by marketing wanks), the HP rating of a gas engine is its maximum power capability. A 5HP gas engine will only make a maximum of 5HP, it won't/can't make more without some form of modification. Of course, we know that HP is a calculated figure, it is the relationship of torque and RPM, specifically, torque x RPM divided by 5252. Also, it is typical (again, not in every case, but for the sake of small gas engines, it tends to hold true) that maximum HP occurs somewhere above the maximum torque output. Maximum HP occurs at the point on the torque and RPM graph where the "rate" of torque decline matches the rate of RPM increase. That is theoretical maximum. Other factors come in to play, such how many RPMs the engine can take before it grenades, etc, but for the sake of the argument, the point is, a gas engine makes X HP and that's it. They also vary their power output across a much wider range of operating RPM.

Contrast that to an electric motor. They are typically (again, not always) rated at continuous power, for example, a 2HP motor means you can run that motor at 2HP output nonstop, also known as "duty cycle." This is, from my understanding (I may be wrong), because an electric motor will happily try to draw more amperage under load, to the point it will cook itself. Much depends on efficiency rating and the power factor capability of the motor, but since electric motors "produce maximum torque from 0 to operating RPM", their tendency is to get to and stay at its operating RPM. When it is running at less RPM, it will draw more amperage in an attempt to get there. What this translates into is an actual increase in power output under load. I don't know exact numbers, but I'm under the impression that a 3HP electric motor will actually make double that, or more if fed enough amperage. The trick here is understanding how long, or what duty cycle you can run the motor in that regime. This is also why many induction motors, typically at or above 3HP have capacitor starters, to keep from spiking amperage draw at start up and kicking your circuit breaker.

So, if you want to replace a gas engine with an electric motor, depending on the type of operation, you can use a lower HP rated electric motor and get equal operation. The key is the time that the electric motor must operate in that increased output mode. Too long and you chance burning it up. For machines that only need short cycles of high output, such as a hydraulic press, or maybe even a saw, it will work fine. Running say, a conveyer belt, or anything that needs a certain power on a continuous basis, then you may have to start looking at a 1:1 relationship.


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## drsparky

I thought horses were powered by hay and oats, not gas or electricity.


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## emtnut

drsparky said:


> I thought horses were powered by hay and oats, not gas or electricity.


Yep, and the turbo-charger is the little hand held whip :biggrin:


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## stevemark

I took in this the hard wy once in supplanting a compact stone smasher motor. It was 170HP diesel, yet low RPMs. They needed an immediate drive electric engine, no transmission, so I determined the motor torque dependent on the HP rating at the speed it was running, decided an electric engine that would give that running torque, at that point utilized a VFD to back it off.


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## CoolWill

stevemark said:


> I took in this the hard wy once in supplanting a compact stone smasher motor. It was 170HP diesel, yet low RPMs. They needed an immediate drive electric engine, no transmission, so I determined the motor torque dependent on the HP rating at the speed it was running, decided an electric engine that would give that running torque, at that point utilized a VFD to back it off.


Is that so?


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## nibiru hater

wdestar said:


> I'm not a mechanical engineer, so I can't verify the conversion for gasoline HP to electrical HP (probably little difference in that true HP = torque x speed), but I can verify this - never go backwards.
> 
> So if he has a 16 HP gas engine, it certainly makes sense that a 20 HP electric motor should safely cover the bases. Secondly, you need to consider the fact that if the compressor is in a cold location - a barn or garage - the oil viscosity will increase, increasing the load (demand) on the motor. Now you have to figure in Service Factor - In this case, I'd recommend 1.25. A good safety margin.
> 
> And last, will it be INT or CONT duty. I suggest CONT as their needs may change as time goes by.
> 
> My 2 cents


you always go backwards from mechanical to electrical.
1 electri hp. = 3.37 gas hp.
16 hp gas eng would require 4.47 hp electric motor/ here is were you move forward to a 5 hp electric.


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## nibiru hater

nibiru hater said:


> you always go backwards from mechanical to electrical.
> 1 electri hp. = 3.37 gas hp.
> 16 hp gas eng would require 4.47 hp electric motor/ here is were you move forward to a 5 hp electric.


if your going to tell the truth satan
tell the whole truth.


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## nibiru hater

nolabama said:


> Customer has a 16 hp Briggs and Stratton engine running an air compressor. The motor is on its last leg and they would like electric.
> 
> How do you determine equivalent hp. Is it the same? Briggs Rpm would need to be determined. I'm not sure of that.
> 
> Electric motor would need to be 120/240. Does that make matters worse?/same?/better?
> 
> What makes a compressor rated motor special? Duty rating? Bearings and such?
> 
> Would CFM requirements determine motor size ?
> 
> They got motor/air compressor from a salvage and they may not need to run anymore than a small garage-shop.
> 
> Thanks in advance.
> 
> Edit. Piston style compressor


i know what you really want to acomplish with your motor run generator.
1 elec. hp is = to 3.37 gas hp.
a16hp engine requires a 4.47 hp motor.
but you want to allow for heat and dependebility so you would move up one unit to a 5.5 hp motor to insure you are not over burdening your motor.
good luck hooking that motor to the rear of that gen head. they build it were you cant simply couple motors to generators.
but hers what my dad did on the farm 45 years ago.
he used a craftsman lawnmower engine with a transmission torque converter sprag clutch between a 5000 watt genhead he had a friend disasemble and extent the out put shaft through the rear bearing housing and coupled a five hp motor to wire in reverse with a light switch circuite.
when we cranked the engine up, and it even out we hit the switch and the motor didnt even draw a start load when the the engine was turned off, the sprag clutch let genhead spin with out turning the engine crank.
the motor pulled 115 v @ 13 full load
we used the 3500 watts, excess, free to use, fairly silent, unitruptable, electricty anywere we needed alittle power for skill saws camping lites or a few things in the house on power outs.
we never tried going of the grid cause he was a naval ltcd and new better than buck the system.
good luck on your conversion hope it frees you up inorder for you to free up another for another for another until the world is free from the devils that be smarter than we.


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## Mathew Molk

Old post but if anybody is actually interested I HAVE changed quite a few Gas/Diesel engines to electric and vise versa.

If you skip the pedantic nonseance of 550 LbFt/Sec and 746W you will find tif you go electric to ICE Hp for HP you are going to end up with a system they will NOT work at all and will be giving you rcustomer their money back and end up with an engine sitting you your shop with egg all over your chin. 

Same going down. A 20 HP gas compressor will work like a champ with a 7-1/2 HP electric motor. Ben there, done it. Depending on the pump you can even run it just fine with a 5HP electric on 240 single phase power if you use a good unloaded. (There are guys that say 5:3 but I'm 2 to 3 gas HP to 1 electric HP guy.

And think about an electric drag car. Close the contactor and it's instantly going as good as it will ever go,,,,but a gasse has to rev up to whatever his trans brake will stand and then take off. If they left the line at idle and simply let go of the brake and stompped tha gas you would need a calendeer to clock what is otherwise an 8 second car. (Not too many kids kow anything at all about drag racing these days. though, so that may be a waste of time.)

On the academic side, the physics major above needs to do a bit of studying beyond 5th period High School physics class. There are things like HP/Torque curves. World of difference between an Electric Mot\or and Internal Combustion Engine. BTW Horsepower is a unit of work. - Torque is a measurement of force and that is what turns the compressor shaft. HP has\ nothing to do with how much force the prime mover can generate. and that is what counts. Horseposer sells cars and motors. Torque is what gets the job done. - RPM at s given torque output is what horsepower is all about, nothing more.

Choisong the correct engine/motor is out of these guy's leagues. The guys that never had a pair of Kleins in their hands would do much better to debate Motor Vs Engine. .....Start with why it a steam "Engine" when the fuel is not burned within it ? - 

Then they might fine out what HP they are talking about

Break Horsepower
Indicated Horsepower
Mechanical Horsepower
Metric Horsepower
Net/vs Gross Horsepower
Drawbar Horsepower
Boiler Horsepower
and like 10 more,

And then look up the word "sophomoric"


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