AC Electric Motor Question

[Jeff]

I have a Bosch 4000 table saw whose manufacturer's specs quote 15A at 120V and a horsepower rating of 4. I've read many comments about ignoring the manufacturer's motor hp ratings because it is mainly a marketing gimmick which applies for a split-second only at motor startup. The true hp rating is usually much less under typical load conditions.

I calculate the following for my motor: (15A x 120V)/746 W/hp = 2.4 hp. At an operating efficiency of 85% this drops to about 2.0 hp.

I've had no problems using this saw and it seems to have plenty of power even when outfitted with a 1/8" thick blade to rip or crosscut 8/4 red oak and cherry.

How did the Bosch folks arrive at a 4 hp motor rating? Is it really a gimmick or a typo in their spec sheet? I assume that it's a single phase motor. Correct? :dontknow:

Thanks in advance.

 

[Gofor]

Wow, that's almost as good as my 6.0 peak hp Ridgid shop vac that only pulls 11 amps at 120v!!

I THINK the hp in yours and mine is the point at which the motor self-destructs, not start-up. I always thought 15a/120v was about 1.5 useable hp. The only way my vac would achieve 6 hp is if I threw it off a high "peak" full of rocks!! :rolf:

I guess where they made them, the horses are a lot smaller, so it takes more to do the same job.

Go

 

[merrill77]

How did the Bosch folks arrive at a 4 hp motor rating?

Marketing: Hey guys, sales are down. But we can fix it all - by selling a 4hp saw that costs the same as the other guys 3.5hp saw.

Engineering: You can't even build a 3hp saw for that money.

Marketing: But we NEED a 4hp saw, or else we can't say ours is bigger than theirs. What if we just SAY it's a 4hp saw?

Engineering: You can SAY whatever you want, but it's still a 2hp saw.

Marketing: Perfect! When can we ship the new 4hp saw?

(no offense to marketers - I find myself doing quite a bit of it these days)

Chris

 

[DaveO]

Horse Power is really a very varied term. It originally was coined in comparing the then modern steam engine to the common horse drawn method. In some very backward ways it was determined that a horse (size, age and species not factored in :roll can create 33,000 ft/lbs. of force per minute. And that is how it all started. Horse power means different things for the different machinery it is being rated for. Obviously mechanical is the above number. Electrical is 746 watt at 100% efficiency, and boiler output is rated at 33,475 btu/hr.
So it is easy to say that with such varied definition of horse power that one could devise a formula that would make 15A table saw motor produce 4 HP :roll::roll:


For more confusion/information check here - http://en.wikipedia.org/wiki/Horsepower


Dave

 

[FredP]

hmmm..... I been thinkin....:icon_scra[uh oh] my 3 HP cabinet saw is having a very hard time ripping 16/4 hard maple so......:widea: I could replace the motor with my 16HP shop vac motor and replace the blade with one off the lawnmower and If I feed very slow should be able to resaw some telephone poles to usable size in short order!:gar-La; And I could do it without running another 220 line!:rotflm: anybody want to help? we could test it with a cucumber cause I gots no hot dogs unless someone wants to donate a finger.:dontknow: anybody?:dontknow::help: jeff...? DaveO can bring the beer!:gar-Bi

 

[DaveO]

I'll bring the beer for that one. Make sure you balance that lawn-mower blade on a nail before you mount it. You wouldn't want an out of balance blade being turned by such a powerful motor. I think I'll bring some popcorn too :eusa_danc
:gar-La;:gar-La;


Dave

 

[jeff...]

hmmm..... I been thinkin....:icon_scra[uh oh] my 3 HP cabinet saw is having a very hard time ripping 16/4 hard maple so......:widea: I could replace the motor with my 16HP shop vac motor and replace the blade with one off the lawnmower and If I feed very slow should be able to resaw some telephone poles to usable size in short order!:gar-La; And I could do it without running another 220 line!:rotflm: anybody want to help? we could test it with a cucumber cause I gots no hot dogs unless someone wants to donate a finger.:dontknow: anybody?:dontknow::help: jeff...? DaveO can bring the beer!:gar-Bi

You may have just thought of a better way to flatten boards and I thought Earl had it going on dragging them behind his pickup. This sounds like a real challenge, quick someone do the math figure out how we can get that lawnmower blade spinning about 7000 rpm, my Granville county math ain't so good you know :icon_scra

I'll bring the beer and a few gallons of gas

 

[Jeff]

Thanks for the input. Gofor is correct in that a 15 amp/120V motor produces about 1.5 hp IF the operating efficiency is only 65%. (15 x 120 x .65)/746. Merrill 77 puts a good spin on the topic; the approach makes me believe it's a smoke in mirrors Marketing idea. Thanks again.

 

[Glennbear]

hmmm..... I been thinkin....:icon_scra[uh oh] my 3 HP cabinet saw is having a very hard time ripping 16/4 hard maple so......:widea: I could replace the motor with my 16HP shop vac motor and replace the blade with one off the lawnmower and If I feed very slow should be able to resaw some telephone poles to usable size in short order!:gar-La; And I could do it without running another 220 line!:rotflm: anybody want to help? we could test it with a cucumber cause I gots no hot dogs unless someone wants to donate a finger.:dontknow: anybody?:dontknow::help: jeff...? DaveO can bring the beer!:gar-Bi

Thanks for the idea Fred, my 3.5 peak horsepower rated Craftsman TS was bogging down and now I know how to increase the HP. Please post your test results with the motor swap out :rotflm:

 

[Bas]

I think that when calculating the horsepower for certain smaller tools, they use Shetland Ponies... Harbor Freight and Craftsman have been known to use Sea Horses.

 

[Jeff]

I can appreciate the humor, but flippant responses to a seriously intended question leave a lot to be desired.

 

[RayH]

I guess I need to get moderated. Or the thread closed. I believe that all the answers given are correct (except maybe about DaveO bringing the beer:gar-La. The ratings given on many tools and appliances are, IMHO, pure marketing BS. I believe all the responses are honest attempts to say, and explain, this while having a bit of fun.

Sorry if I offend.
Ray

edited: OOOPS: The automated moderator changed my "B##S## " to "bandsaw".

 

[scsmith42]

Jeff, according to my pocket reference, the average electrical motor (fan, furnace, appliance, pump, etc - normal duty) that draws 13 - 15 amps at 3450 rpm / 120V - FULL LOAD is producing 1 HP.

At 1725 rpm, full load, a 1 hp motor draws 13.6 - 16 amps.

Your conversion numbers are correct though, in that 1 hp is equal to 746 watts.

The bottom line, it ain't 4 hp. Best estimate is somewhere between 1 - 2 hp.

Scott

 

[jeff...]

Doing a little research on this thread - apparently there is such an animal as a metric horse power. I wonder if horses across the pond are over rated? You know a lot of stuff comes from over seas now, could be a metric horse to real American horse power question?

Does is say it's a metric motor or an American motor?

 

[Jeff]

Likewise, no offense intended on my part either. I'm just trying to understand the discrepancy. I've contacted the Bosch technical folks so maybe they can explain it to all of us. I'll post it when I hear from them. Maybe we'll all learn something about this stuff and be better informed about the issue.

I take your point about the member responses and appreciate their input and humor. No need to be censored or close the thread. After all, why have a discussion if everyone agrees/disagrees/or doesn't know? That's one of the great things about this site and its participants.

Thanks to all and Happy Holidays.

 

[mkepke]

Jeff: the formula for true^ horsepower is: (full load amps*voltage*efficiency*power factor) divided by 746

For small HP motors (like you have), efficiency is usually 80-90%. Power factor figure, say 80%.

The usual rule of thumb is 9 amps @ 120V = 1HP.

Assuming the Bosch 4000 uses a universal motor, then the "4HP" is likely the locked rotor torque. Basically the torque measured instantaneously during the time the power is applied, but the shaft is too loaded to spin (also the time right before the windings smoke). This is when any universal motor generates the max torque.

^ true horsepower = the max continuous horsepower available at the motor shaft to do useful work. If the Bosch 4000 was not a direct-drive saw, you'd have to derate the useful horsepower further to account for inefficiencies in power transmission from the shaft to the arbor, e.g. belt slip.

-Mark

 

[DavidF]

So what they might be using is the lowest current draw figure to avoid frightening people off using it and the highest hp figure so that people will think it's really powerful, but actually the two aren't related to the same testing conditions???

 

[Jeff]

I'm finally beginning to understand this hp mumbo-jumbo. Mark, thanks for the explanation; the "locked rotor torque" confirms what I've found along with this morning's response from the Bosch tech folks.

From Bosch: "The true measure of a motors power is the amp rating of the motor. Because this is a gear driven motor and not a direct drive, the testing is done at the blade. The 4.4 hp is reached at the "stall point". In other words, the tool generates 4.4 hp just before it stalls. These are tests approved by Underwriter Labs, and are commonly used in the tool industry."

Also, here's a supporting explanation that I found yesterday. It just happens to be written from a woodworker/small tool user perspective.

http://www.kevinsbrady.net/motors.pdf

The take-home seems to be "let the buyer beware". To wit, my "Contractor" Shop-Vac from Lowe's has the following very prominently placed label: "6.5 peak hp". Hmmm, suggests that the thing is a Tasmanian devil when it comes to dust cleanup. :eusa_clap

It's been an informative discussion and thanks to all for participating.

 

[mkepke]

So what they might be using is the lowest current draw figure to avoid frightening people off using it and the highest hp figure so that people will think it's really powerful, but actually the two aren't related to the same testing conditions???

Close but not quite. They represent two different points on the motor's performance curve but they are not the "lowest" and "highest", instead they are the "highest at max continuous design load" (aka Full Load Amps) and "absolute highest".

Lowest amperage would be when the motor was not loaded at all (free running). It's a fraction - 20-25% maybe - of the FLA.

FLA is the value used to size the supply wiring. I suppose since it represents max typical usage.

I suspect manufacturers would publish the free-running amps if they were not beholden to building codes.

-Mark

 

[Howard Acheson]

irst, routers and shop vacuums use universal motors which are not rated the same way as induction motors. Nor is there an industry agreed upon standard for measuring universal motors. Universal motors are integral with the tool and you can't purchase a universal stand alone motor. Manufacturers rate most universal motor tools at the highest number they can for marketing reasons. Routers are rated using the maximum amperage draw at full load amperage just before it burns up. They are not rated at the running amperage shown on the label. Not saying this is the right or wrong way to do it, but it is a valid and repeatable number.

Second, the formula many refer to is for a theoretical situation which does not take into account efficiency, service factor and other factors.

Induction motors are not rated using that formula either. The are measured using a dynomometer when running at a certain load factor and continuous temperature.

"Continuous Duty" is the key and it forms the basis for measuring HP in induction motors.