Motor horsepower an Apples/Oranges Question

[Andy Sowers]

I've been using a Bosch jobsite tablesaw for -2-3yrs now (model 4000)... looking to potentially upgrade it hopefully sometime soon.

I'm currently limited to 110V, and all the available spots in my breaker panel are already taken, so upgrading to 220V isn't really in the cards right now. Thus, my choices in a tablesaw seem to be limited to contractor/hybrids.

One question I have regards to the motor horsepower. My Bosch purportedly has a 15A/4.4hp direct-drive motor... but the biggest motor I've seen in other contractor/hybrid models tops out in the ~2hp ballpark. What gives? Is Bosch grossly overstating the horsepower of their motors, or is this a apples/oranges situation. I realize that there could be some variability from manufacturer to manufacturer, but this seems to be too extreme...

FWIW, I've never had my Bosch bog down in a cut (probably because I generally saw stock that's ~1" or less). I just want to make sure I look at my upgrade choices appropriately...

Thanks

Andy

[Jason Beam]

Is Bosch grossly overstating the horsepower of their motors, or is this a apples/oranges situation.

Yes, and Yes. The apples/oranges is universal motors vs. induction motors. The overstatement is that just about everyone overstates the HP rating on universal motors. It's usually called "maximum developed horsepower" - which means it'll never put out that many horses (physics deem it pretty tough to do) except at the last second before completely frying the motor. Induction motors are a more accurate (though can still be futzed with) statement of power.

A horsepower is somewhere in the neighborhood of 750 watts in a perfect world. I usually estimate 800 watts per HP to account for efficiency and it's easier math. A 120v device drawing 15a can only do 1800 watts - just over 2hp. That's watts, and watts is watts, don't matter what the motor plate says at that point. It's not a perfect equalizer, but it's certainly better than trusting what those crooks claim their motors do. :D

[Rod Sheridan]

A horsepower is a horsepower, period.

It's 33,000pound feet per minute, or 746 watts of work.

Your saw is using a marketing method of stating horsepower just before the motor stalls or burns out etc.

Typically the largest induction motor you can run from a 120V 15A circuit is aproximately 1 1/4 horsepower.

You could put in a 20 or 30A circuit using the existing slot in your panel if you wanted to purchase a larger saw.

You could also install a sub-panel by moving one of your existing circuits and your saw circuit to the sub panel. This would allow you to feed the sub-panel from the two spaces, and would allow you to use 240 volts in your shop.

Regards, Rod.

Regards, Rod.

[Chris Friesen]

One question I have regards to the motor horsepower. My Bosch purportedly has a 15A/4.4hp direct-drive motor... but the biggest motor I've seen in other contractor/hybrid models tops out in the ~2hp ballpark. What gives? Is Bosch grossly overstating the horsepower of their motors, or is this a apples/oranges situation.

Basically they're overstating the horsepower of the motors. Often for universal motors manufacturers will quote the peak HP generated when the motor is almost stalled. (This is how you get "6.5HP" shop vacs that run on 120V circuits.) What they don't indicate is that it can't actually maintain this for any amount of time or else the breaker will trigger or the motor will melt.

A 15A 120V circuit can only generate 2.4HP continuously assuming 100% efficiency (which is impossible). A more realistic value is 1.5-1.75 HP of useable power.

[glenn bradley]

I would take a hybrid over almost any contractor saw. Smaller footprint, better dust control and greater stability . . . JMHO. I run a 22124 and find is very satisfactory on a 110v dedicated circuit. You mention having power issues so if you cannot dedicate a circuit to your saw YMMV (mine did till I dedicated).

Don't be too hard on Bosch or others (like my Ridgid 6.5HP vacuum . . . yeah, right). They are just following "accepted" methods. Very much like stereo amplifiers that used to be rated at high amounts of power that turned out to be peak bursts at 20% distortion but, accepted. We all talk about 2.25 and 3.5 HP routers and compare them as such even though it is inaccurate. We are all a little better educated on these things now and the markets sometimes follow the knowledge level of their audience ;-)

[Lee Schierer]

One more thought. Adding to what the others have said and answering your un-asked question. 220 or 120. In terms of horsepower, it makes no difference what the volatage is. In terms of wiring, you can use lighter gauge wire for 220 circuits than you can for 120, but you'll not see any more horse power from a 2 Hp motor runing on 220 than you will on 120, assuming the wiring in both cases is sized correctly.

[Will Blick]

My question is.... do those shop Vacs actually have 6.5HP rated motors in them? I don't think they could output 6.5HP even for a split second....they are not physically big enough to have the number of windings to produce that amount of energy. My guess is, the Shop Vacs ratings are total BS.... no policing of claims....

While the HP rating on a saw makes more sense, although its clearly pushed for marketing purposes also..... remember, you can run a 4HP motor on 120V, just be careful to keep the load on the motor below the threshold of your circuit breaker.

I wonder if some of those saws actually have higher rated HP motors, as a means of over sizing for longevity ?

[Andy Sowers]

Thanks everyone... What everyone has said is what I suspected was going on. That gives me a little more confidence looking at tablesaws in the ~2hp range...

Offhand, I cannot remember what circuits I have running to my garage... I know that by default, it only had only 1-2 outlets originally (not including those for the washer/dryer). And those are shared with other outlets thoughout the house. I did add two circuits about 1.5yrs ago... I think those were both 20A 110 outlets (at that time chose 110V over 220V since essentially all my tool were 110).

My DC and jointer can be rewired for 220, so maybe its time to look into revisiting that breaker. If I remember correctly, my main panel for the house is already at the rated amperage (even though we aren't using one of the breakers at all). So adding a subpanel, probably wouldnt work without upgrading the main panel. Sounds like way more money that my wife will "approve."

I'm not an electrician, and don't pretend to play one on TV...

andy

[Alan DuBoff]

One more thought. Adding to what the others have said and answering your un-asked question. 220 or 120. In terms of horsepower, it makes no difference what the volatage is. In terms of wiring, you can use lighter gauge wire for 220 circuits than you can for 120, but you'll not see any more horse power from a 2 Hp motor runing on 220 than you will on 120, assuming the wiring in both cases is sized correctly.
This is true, but the amount of amps that are used on a 220 circuit are less than 110 in most cases, *or* you have more wiggle room by running the same saw on 220 as the amps have more headroom on the specific circuit. In most cases the saw that is up against the ceiling, say a 15 amp motor on a 20 amp circuit at 110, would have a lot of comfort running at 7.5 amps on a 220 volt circuit that also has 20 amps.

[Chip Lindley]

....remember, you can run a 4HP motor on 120V, just be careful to keep the load on the motor below the threshold of your circuit breaker.

A 4hp/120V motor (if it existed) would have a FLA around 40A!! That would require (at least) #6 wire! To just *run* a large motor on a small circuit IS Possible, but not much *work* can be accomplished unless the motor is under Load!

What is the Point? ....WHY?

[Tom Veatch]

My question is.... do those shop Vacs actually have 6.5HP rated motors in them? ...

Short answer is "Absolutely NOT!"

Motor probably has a "Locked Rotor Amperage" of about 40 amps. Convert 40 amps at 120 volts to watts to HP and you get about 6.5. But with a locked rotor, the motor may be consuming 4800 watts (6.5 HP), but with zero RPM, it's actually producing ZERO hp.

Look HERE (http://www.kevinsbrady.net/motors.pdf) for one of the best explanations of what's going on that I've seen.

Quote from the article:
"The author used to be an electrical engineer many years ago and is currently an attorney practicing in Minneapolis, Minnesota, advising clients in various technology law matters"

[hank dekeyser]

My Father had an old Delta 9" Contractors saw 1HP- I upgraded the motor to a 1-1/2hp motor 110/220 - I can rip darn near anything with that saw. Yes 1-1/2hp and it performs flawless. BUT that is a "real" motor (one with starting capacitors) that delivers what it says it will. It's unfortunate that companies no longer use the same standards any longer - pick any 3 machine companies and compare the Amperage on their supposed 2hp motors - one will be 8.6A @220, another 12A @220, etc. I was told that "they rate motors differently in China"

OP I would replace some breakers w/ double breakers (2 breakers in the space of one) to free up room, or preferably put a sub panel in to feed just the shop stuff.

[Andy Sowers]

Now that I'm home, I took a peek at our power panel... as I thought ALL the available slots are occupied, and of those... all are already the double breaker style. Additionally, the total amperage on the entire house is only 125A! :(

So in short aside from a major electrical rework to jockey things around to add a new sub-panel, I'm going to be stuck with 110V equipment. The only "easy" way around this would be to share the dryer connection which is also in the garage. Certainly not a great solution, but my guess its been done before!!

Thanks to everyone for the insight on the motor hp... I feel better knowing that I can safely consider some of the hybrid saws without fear of getting a motor that is grossly underpowered.

--Andy

[Mike Henderson]

Take the double breaker slot used by the dryer and use that for your subpanel. Then wire your dryer from the subpanel.

None of that should exceed your 125 amp limit unless you put some really large equipment in the shop.

Mike

[Will Blick]

> A 4hp/120V motor (if it existed) would have a FLA around 40A!! That would require (at least) #6 wire! To just *run* a large motor on a small circuit IS Possible, but not much *work* can be accomplished unless the motor is under Load!


I fully agree..... however, its not that uncommon to oversize a component in a product.... it happens for many different reasons.... often, the maker can get a killer deal on larger motors if they are a higher volume item, etc. I did not think I needed to spell out that..... that if a maker did this, they would require a 120V / 40 amp service to run the saw....the saw would have an amperage cut-out, so it would generate no more HP than the mix of volts / amps / motor eff. will allow....




> Motor probably has a "Locked Rotor Amperage" of about 40 amps. Convert 40 amps at 120 volts to watts to HP and you get about 6.5.


makes sense.... you know there is something in those motors they have reverse calculated to make such hideous claims....

[Jeffrey Makiel]

Manufacturer's stated horsepower ratings have a lot in common with Wall Street and credit card companies these past few of years. :)

Caveat emptor ("let the buyer beware")

-Jeff :)

[Larry Edgerton]

So you guys are saying I really don't have a 5.5 Hp vacuum cleaner? http://www.sawmillcreek.org/images/icons/icon12.gif

[george wilson]

The rule is 3 amps per H.P. at 220 volts.

[Peter Quinn]

The rule is 3 amps per H.P. at 220 volts.

I wonder why then my old DeWalt RAS, with a 2HP motor and a motor plate rating of 17A@220 requires a 20A circuit? And why my new Grizzly 1HP feeder draws 4.7A@220? Or why my 3HP PM66 TS draws 22A and requires a 30A circuit per manufacturers recommendations? Seems like electricity might be a good place to require industry standards?

I can tell you that I doubt my 6HP shop vac motor could power my TS for very long, and that the motor on my TS weights more than a half dozen shop vacs combined. Some of that could be the cast iron mount, but no doubt some of it is all those pesky windings and magnets?

Start throwing in efficiency ratings, duty ratings, and motor class designations and a simpleton like myself starts to get a headache just thinking about these things.

[Tom Veatch]

The rule is 3 amps per H.P. at 220 volts.

George, I believe it would be considerably higher than that. At a power factor of 1 (almost as hard to achieve with an electric motor as 100% efficiency) 220v/3a is 660 watts. 1 HP is 746 watts.

Throw in reasonable values for efficiency and power factor and you're closer to a consumption of about 1200 watts consumed per HP produced.

I believe a better rule of thumb is 1 HP produced is about 240v/5a or 120v/10a consumed. Since the overall efficiency can vary significantly from one motor to the next, more exact values for a particular motor would be available from the motor's dataplate.

[Frank Hagan]

Doesn't NEMA publish standards for labeling of motors? I can't seem to reach NEMA.ORG this morning, but if the motor has a NEMA nameplate there are only a few "tricks" the manufacturers can play. One is with the service factor on the label; a 1 HP rating with a 1.5 SF (service factor) rating is the same motor as a 1.5 HP rating with a 1.0 SF.

I worked for a pump manufacturer, and our motors were from AOSmith, GE and Franklin. We had what we called "full rate" (service factors above 1.0) and "up rate" motors (service factors at 1.0). It wasn't as bad a marketing game as they play now, and it did serve a purpose. Homeowners would invariably choose the higher rated horsepower for their pool pump even if it wasn't needed.

What was interesting was that 1.5, 2 and 3 HP pumps were being used for pool filtration when a 1/2 or 3/4 would have been just as good. We sold two speed pumps for energy savings ... rotor speed would drop from the 3450 rpm standard in that industry to 1725, flow would be reduced to half, and energy use would be about 1/4 the total. The same effect could have been acheived with a 1/2 HP pump for hundreds less, but the homeowner buying the pool would often choose a "package" including the equipment, and the pool dealers trying to sell the right HP pump would lose out every time.

The reason we have 6.5 HP shp vacs is because none of us would buy a 1/3 HP shop vac that was next to a competing brand that was a 6.5 HP one.

[george wilson]

Actually,that might be for 3 phase. I can't recall now. That's what a machinery dealer told me. His old industrial machines are mostly 3 phase. Also,start up amps is a lot higher.

My 2 h.p. Bridgeport type milling machine,which is single phase,220 volts says it's 15 amps at 220 V. I just went out and checked it. It would be 31 amps at 110 V.,according to the motor plate. Maybe someone else can talk about 3 phase. I had a Weiler lathe,whose 3 h.p. motor was more the size of a 1 h.p.,but my 1 h.p. Hardinge HLVH lathe has an enormous 1 h.p. motor that must be over 12" in dia. X 18" long. Can't get down there to measure it.It is huge. The Weiler is no cheap lathe.Probably $50,000 new,as is the Hardinge HLVH. Why the huge difference in motor sizes? The Hardinge must have a very large overload capacity.

I do not know why a Dewalt 2 h.p. motor would need 17 amps,when my milling machine is 15 amps at the same power. My milling machine motor is a large motor for its power,probably easily weighs 2X the weight of a Dewalt RAS motor.

[Mike Henderson]

Doesn't NEMA publish standards for labeling of motors? I can't seem to reach NEMA.ORG this morning, but if the motor has a NEMA nameplate there are only a few "tricks" the manufacturers can play. One is with the service factor on the label; a 1 HP rating with a 1.5 SF (service factor) rating is the same motor as a 1.5 HP rating with a 1.0 SF.

I don't think that's correct. As I remember the definition of "Service factor" it's the amount of power the motor can produce as a multiplier of the rated HP (so for a 1HP motor with a 1.5 Service factor that would be 1.5HP), with a rise in operating temperature of 10*C, but the life is cut in half when operating at that level. [A SF of 1.5 would be quite high. SF is usually much smaller, probably 1.2 or less because of the operating temperature rise limit of 10*C when operating at the SF HP.]

So if you have a 1HP motor with a 1.5 Service Factor, you can get 1.5HP out of it, but the motor will fail much quicker.

A 1.5HP motor with a 1 SF would have a full life.

Mike

[See this discussion (http://findarticles.com/p/articles/mi_m0BPR/is_4_17/ai_62370062)of Service Factor.]

[Rod Sheridan]

Manufacturer's stated horsepower ratings have a lot in common with Wall Street and credit card companies these past few of years. :)

Caveat emptor ("let the buyer beware")

-Jeff :)

Jef, I disagree, any reputable motor (Leeson/Baldor/Westinghouse/Kato etc) will have accurate information on the motor nameplate.

I have however seen some motors from China labled as 7 amperes, 110 Volts, 1 HorsePower...An absolute miracle! Unity power factor, 100% efficiency........I should have purchased a boat load of those and retired wealthy beyond my wildest dreams.

Regards, Rod.