When translating amps to hp for a 120V appliance, is the formula essentially
120*amps/745.699872That would give a 15amp ryobi table saw a rating of 2.41hp which is ridiculously high, no? Is my formula not right, or is 2.41hp not ridiculous or is there something else I'm missing?
ridiculously high, no? Is my
missing?
Well if it was a Craftsman shop-vac, it would make perfect sense... they have no problem claiming they can get 5 or more horsepower out of a 120V outlet! That level of power is achieved by using an old sales technique called "lying" ;^)
Your calculation is fundamentally correct but leaves out the effect of efficiency. 80% is a reasonable, maybe a little optimistic level of efficiency for a small electric motor. So if you're putting in 2.4 HP of electrical energy, you'd get 0.8 * 2.4 = ~1.9 horsepower out of the motor. Then you're going to lose some more in the gears, bearings, etc. So you maybe get something like
1.5 HP at the blade?It's also probably not rated to produce that kind of power continuously. It can do it for a few seconds or maybe tens of seconds, but longer than that and you'd probably smoke the motor.
Eric Law
You're missing two things:
1) The amperage rating on any electric motor represents the locked-rotor current, i.e. the current it draws when the rotor is unable to move. This is *much* higher than the current the motor draws when it's in operation, and is in no way representative of the motor's actual power output. 2) You're also assuming 100% efficiency, which will never be the case. There will always be some losses due to friction in the bearings, etc.GIYF
Missing several things; Doug pointed out a couple (LRA and efficiency, specifically) -- there is at least the power factor in addition which will be roughly another 20% for this motor, probably (Ryobi is not particularly noted for high-end products).
So, that 15A wouldn't be running amps, but if it were the hp would be something otoo 120*15*0.8*0.8/746 --> ~1.5 hp. It's probably about 1 hp running at about 10A running under load.
There's a fairly nice discussion on the factors involved here--I posted a response not terribly long ago either here or in the rec.woodworking group that also had links to some motor spec sheets; don't have that handy but a search of any of the motor manufacturers will show representative numbers for various levels of quality/pricing motors. Most "run of the mill" consumer-grade/homeowner tool-grade are not the high efficiency types at all--they are more expensive than most tools use that are carried by the BORGs. In part this is price but it's also because they're relatively small as industrial tools/motors go...
--
Nope. The nameplate will have the running amps at rated HP. Overload protection of may motors is based on the nameplate current rating. (Many motors also have a "service factor" greater than one which means the motor can be operated at higher current (and HP) at a little shorter life.)
Motors typically have a code letter for locked rotor amps. Locked rotor amps is about 6x the running amps. Unlikely the running amps for a table saw is 2.5A. (An even 15A is probably not a motor current rating.)
The "lie" in motor HP *may* be the max HP you can get from the motor before stall (which which would be a lie), which you might very occasionally use. Wouldn't think that would make sense for a shop vac.
Quite right.
You are missing the advertisement junk. Many consumer products are way over rated. The motor may use 15 amps for about .05 seconds when first started, but will drop to way less when running. There is no real truth in advertisement.
Just as some computer speakers are rated from 50 to 100 watts and are powered by a small wall cube. If you open them up they will have marked on the speakers 2 watts or less. That 50 watts comes from a music power rating. It may hit 50 watts for .001 seconds when a drum is first hit. The average power will probably be more like a half a watt or less.
You really think that Aaron's Ryobi table saw pulls 15A while running??
Well, if the exhaust port of the Craftsman shop-vac is exposed to a vacuum, and the inlet of the Craftsman shop-vac is located in a hyperbaric chamber, then it could easily generate five or more horsepower.
Jon
Hi, You are right except not adding power factor(efficiency) to the formula. Do you have ammeter? Read the current while motor is running/working. There you'll see it. Ryobi? My idea power tool is to buy the best and keep it life time. Some times buying old used one is even better than new crap.
I would guess Aaron used an approximate number (exactly 15?) for the sake of discussion.
What I can say is:
- the NEC gives a table value (approximate) of 16 full load running amps for a garden variety 1 HP motor running on 115V.
- the current value on motor nameplates is running current
- locked rotor current is indicated on most motor nameplates as a code letter
- as I wrote before, if 15A was a locked rotor current the full load running current would be about 2.5A. Looking at the NEC table that is the value for a motor well under 1/6 HP
- my table saw motor is 1 HP with a full load running current of 14.0A
I don't see any possibility that 15A is an LRA.
In answer - yes I do think it is entirely reasonable that Aaron's table saw motor pulls *about* 15A when the motor is *loaded* to it's full horsepower.
one HP is 748 watts, if that's any use. I suspect the engineers and brainiacs out there have answered any possible slant on the question.
1800 watts / 748 = about 2.40. Works out OK if you figure
100% efficiency.
Uh, 120 volts x 115 amps = 13,800 watts. You must have meant 15 amps.
115 amps?
YYes, I ddon't know how I ddid that. SSorry.
Is it really that hard of a leap of logic for you to come to the conclusion that the OP meant "15", and that the "115" value is a simple typographical error?
Jon
One horsepower appears to me to be 748 watts divided by efficiency of the electric motor. At this moment, I like to say 900 watts for an electric motor to deliver "1 horsepower".
Please keep in mind that AC electric motors, especially single-phase (not requiring 3-phase power), and smaller than a horsepower or two,
appear to me to be likely to have efficiency at best in the 80's %,
and their power factor appears to me to be typically close to .8 or so, maybe .86 or so in some cases.
- Don Klipstein ( snipped-for-privacy@misty.com)
746 watts.
TDD
Watt did you say?
AC Volts times AC amps =3D volt-amps, not watts
Hp =3D (q*V*I*eff*pf)/746 N =3D number of horsepower q=3D number of phases V =3D voltage measured line to neutral I =3D phase current eff =3D motor efficiency pf =3D power factor of motor
Approximate Hp for typical single phase, 120V motor.
******** Hp =3D 0.15 * motor amps ********HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.