Electric Motor Horsepower.

[mscher]

On my new trade, an 80's Sears air compressor, there is a 2HP motor, that is 230 volts only. It's got a fairly good sized twim cylinder compressor, belt drive.

Currently dont have, 230V AC in the garage, but I was wondering about electric motor HP in general. Somewhere I read where there is a practical limit, on how much power an electric motor can produce, on 120v.

Do newer electric motors accuracte reflect rated horspower?

Can I get 2HP, on a 120v motor? Will eventually install 230vAC (for wleder, etc.), but would rather, in the short term get the compressor running off 120V.

 

[bruceb58]

Re: Electric Motor Horsepower.

Almost all ratings of an electric motor's Hp are overrrated by the manufacturer. Often they rate it based on the current of a stalled motor. I HP is equal to 746 watts so a motor that is 80% efficient drawing 15A at 110V is only going to be able to deliver 1.8 HP.

 

[bigdee]

Re: Electric Motor Horsepower.

http://www.grainger.com/Grainger/MARATHON-ELECTRIC-Air-Compressor-3K787?Pid=search You need a motor that is specified for an air compressor and yes 2 hp is the maximum hp you can get at 120 vac. Keep in mind that you will need 20 amp circuit wiring (12 guage minimum) fed by a 30 amp circuit breaker to handle the starting current.

 

[hrdwrkingacguy]

Re: Electric Motor Horsepower.

All motors have a FLA rating at a given voltage...That motor might be 8a @ 230v but if you pull the same load at 115v it might be 19 or 20...The pump and pulley determine the load on the motor...You want to keep the FLA of the new motor close to the amp draw of the compressor @ 115v...unfortunately there isn't an easy way to guess what the FLA will be...Get a 2hp motor and just check it to make sure @ 115v it isn't over-amping > service factor of the motor...

http://www.grainger.com/Grainger/MARATHON-ELECTRIC-Air-Compressor-3K787?Pid=search You need a motor that is specified for an air compressor and yes 2 hp is the maximum hp you can get at 120 vac. Keep in mind that you will need 20 amp circuit wiring (12 guage minimum) fed by a 30 amp circuit breaker to handle the starting current.

IMHO This is only good advice if you want to make an insurance claim due to a fire...10ga wire for a 20a circuit 8ga for a 30a(length matters but I wouldn't risk it)...That motor draws 17.8a @ 115v...On the motor there is a MOCP max over current protection rating and a minimum fuse size...It seems to me that if you are running a 230v circuit eventually anyway + $300 for a motor + all the wire and conduit + the fact that you don't have a standard outlet you can just plug that into, it seems easier to use what you have and run 230v and do it right...

PS I know according to minimum code you can use 12ga on a 20a circuit up to a certain length, but I wouldn't do it...I just had to pull 300' of 12ga out cause it was all melted do to drawing 17a constantly with a 20a breaker...It wasn't 300' in one run...All the fan coils at a school each had their own breaker and the furthest run was 75'...5 units all had burnt and discolored wire...

 

[bigdee]

Re: Electric Motor Horsepower.

IMHO This is only good advice if you want to make an insurance claim due to a fire...10ga wire for a 20a circuit 8ga for a 30a(length matters but I wouldn't risk it)...That motor draws 17.8a @ 115v...On the motor there is a MOCP max over current protection rating and a minimum fuse size...It seems to me that if you are running a 230v circuit anyway $300 for a motor + all the wire and conduit + the fact that you don't have a standard outlet you can just plug that into, it seems easier to use what you have and run 230v and do it right...

On a dedicated motor circuit you can absolutely oversize the breaker if the motor has overcurrent protection (thermal overload. While this is the minimum size conductor you could use with this motor it is perfectly legal and safe. If you use fuses instead of a circuit breaker you can fuse 175% ABOVE the conductor current capacity or 35 amps.

 

[Fishing Dude too]

Re: Electric Motor Horsepower.

Yes you can put an oversize fuse or breaker than the wire can handle, but if a fire happens your insurance will not pay for your loss. The load limiting device is there for one reason and that is to save the wire. How far is your garage from your house and main breaker box? Can run 4ga wire in conduit for a good enough price, then put a small load distrabution box in your garage. JMO it's up to you check the cost differance between motor and wire, might be cheeper to do the wire.

 

[bigdee]

Re: Electric Motor Horsepower.

On my new trade, an 80's Sears air compressor, there is a 2HP motor, that is 230 volts only. It's got a fairly good sized twim cylinder compressor, belt drive.

Currently dont have, 230V AC in the garage, but I was wondering about electric motor HP in general. Somewhere I read where there is a practical limit, on how much power an electric motor can produce, on 120v.

Do newer electric motors accuracte reflect rated horspower?

Can I get 2HP, on a 120v motor? Will eventually install 230vAC (for wleder, etc.), but would rather, in the short term get the compressor running off 120V.

Give us details on how your garage is currently wired. Is it a detached garage? And if so what size wire is feeding it and where does it originate.

 

[mscher]

Re: Electric Motor Horsepower.

The wiring is nothing spectacular.

Old one car garage, about 70' from the main. Overhead wire from the 60's. Inside is a mix of old/new wiring (new stuff is 12ga) with 30 amp fuse box.

Maybe I'll just run 220v. I need more projects.

 

[Tim Frank]

Re: Electric Motor Horsepower.

On a dedicated motor circuit you can absolutely oversize the breaker if the motor has overcurrent protection (thermal overload. While this is the minimum size conductor you could use with this motor it is perfectly legal and safe. If you use fuses instead of a circuit breaker you can fuse 175% ABOVE the conductor current capacity or 35 amps.

:facepalm:

Why not just replace the fuses with copper pennies and get it over with?

 

[Tim Frank]

Re: Electric Motor Horsepower.

http://www.grainger.com/Grainger/MARATHON-ELECTRIC-Air-Compressor-3K787?Pid=search You need a motor that is specified for an air compressor and yes 2 hp is the maximum hp you can get at 120 vac.

Wrong. I've installed 5HP 115V motors.

Keep in mind that you will need 20 amp circuit wiring (12 guage minimum) fed by a 30 amp circuit breaker to handle the starting current.

Wrong again...and very dangerous.
Intentionally install undersized wiring,,,and then intentionally under-protect it???

I'm sure you mean well, but do you actually consider what might happen if someone were to follow this [plan] before you post this type of advice?
I guess the fire won't happen at your place....

 

[bigdee]

Re: Electric Motor Horsepower.

:facepalm:

Why not just replace the fuses with copper pennies and get it over with?

And what does that mean? That would be a code violation for sure. mscher, I believe it would be cheaper to convert to 120/230 rather than buying a new 120vac motor. The cost of running a dedicated 120 vac circuit would be about the same as converting to 120/230 if you can DIY. And for those that disagree with what I said about 12 gu. wire fed by a 30 amp breaker read 430.32 of the NEC.....it states that you can use up to a 50 amp circuit breaker!! Also read 430.22. IF the motor has a overload protector (thermal overload) that is the OVERCURRENT protection for the branch circuit wiring....the circuit breaker ( in this case only)is for SHORT CIRCUIT protection. If you think this is confusing read the code requirements for welders. I am a licensed electrical contractor and have also taught classes on the NEC and code enforcement.

 

[bruceb58]

Re: Electric Motor Horsepower.

Wrong. I've installed 5HP 115V motors.

Would love to see what you bought that said it was 5HP!

There is no such thing as a true 5HP 115V motor. That may be how a manufacturer rated it when they are stalled but a true 5HP motor on 110V would consume over 40 Amps. Like I said before, manufacturers will label anything and people will believe it.

http://www.engineeringtoolbox.com/electrical-motor-horsepower-d_653.html

When a 115V motor is rated high like that it is usually the maximum "developed" horsepower which is a lot different than the continuous duty hosepower of a motor that is properly rated. When a motor is rated this high, all one needs to look at is the continuous duty current and do the appropriate calculations from those numbers.

http://www.onlineconversion.com/motor_horsepower.htm


Here is an example of one of these overrated 5HP 115V compressors. I laugh whenever I see these. Again...amazing what people believe!
http://www.nationalpneumatic.com/en/Omega5HP-115V1PH-BELT-DRIVEN-20-GAL-PROFESSIONAL/MGAPK-5020.aspx

 

[hrdwrkingacguy]

Re: Electric Motor Horsepower.

So what you are saying is that every data plate on every AC unit, every motor, and every compressor, from GE, AOsmith, fasco, copeland, bristol, and tecumseh is wrong and you are right...I guess my industry needs to hire you because we have been looking at it wrong for 80 years...

 

[Tim Frank]

Re: Electric Motor Horsepower.

Before this goes south...and as highlighted by QC's edit of my earlier post, I was a bit over the top, apologies to BigDee.
I was not questioning your knowledge base (in one of these forums it's simply impossible to know) just the wisdom of your advice in this instance....a residential installation of questionable vintage.

 

[bruceb58]

Re: Electric Motor Horsepower.

So what you are saying is that every data plate on every AC unit, every motor, and every compressor, from GE, AOsmith, fasco, copeland, bristol, and tecumseh is wrong and you are right...I guess my industry needs to hire you because we have been looking at it wrong for 80 years...

Show us an example with the specs. I am just a lowly electrical engineer that knows nothing about what electrical power is.

You mean like these Tecumseh motors? They are rated exactly as I said.
http://www.aosmithmotors.com/WorkArea/DownloadAsset.aspx?id=612

 

[j_martin]

Re: Electric Motor Horsepower.

Show us an example with the specs. I am just a lowly electrical engineer that knows nothing about what electrical power is.

Aw come on, stop it with the logical fallacies.

It's a fact that Harbor Crate, Sears and Sawbucks, etc will rate some things, such as saws and air compressors at breakdown HP, which is the peak transient HP developed when the motor is suddenly overloaded to breakdown torque. Obviously, being an electrical engineer all your life, you would understand that is not maintainable for more than a fraction of a second.

In fact, most devices so rated turn out to be quite inferior, and if you try to run them at even a reasonable level continuously, they tend to break down or burn up.

The 5 hp air compressor will say to be sure to plug it into a 20 amp circuit. You're the engineer, do the math.

 

[bruceb58]

Re: Electric Motor Horsepower.

I actually looked at Harbor Freight last night thinking that they would be one of the guys that overrate. They actually seemed to have honest HP rating on their compressors. No 5HP compresors...just 2HP ones.

 

[mscher]

Re: Electric Motor Horsepower.

Keep up the good arguing, boys, I'm learnings a lot here.

Probably a moot point anyway.

Wouldn't the pressure switch be 230V and would need to be rewired(if possible) or replaced also?

Also, when I turn the motor by hand (no belt), it turns hard for about 1/2 revoultion and then very easy, the rest of the way. Is this normal, or does the armature have rust, or something else? Ther motor did seem to run normal when we plugged it in at work.

I'm physicing myself up a re a wiring job, anyway.

 

[ondarvr]

Re: Electric Motor Horsepower.

I went through this a couple of decades ago when looking for a portable air compressor for commercial use.

I had seen HP ratings and amp draws all over the board and it just didn't make sense. Finally I was at a commercial tool show and ran into the rep that supplied the electric motors for Sears air compressors (one of the biggest offenders in the over rating scam). I asked him about the service factor rating you see on the motor itself and why the Sears models had no number (1.2, 1.75 etc), only the word "special". He just kind of laughed and said they do that so it can be over rated to the point of being ridicules, and the motors are special ordered that way. I believe a true fully rated 3/4 horse motor is the biggest you can run on a 15 amp circuit.