Watts to Horsepower

bigtiller · Oct 24, 2014

I was reading yesterdays comics when I got to "Non Sequitur" it read - 745.7 watts equals one horsepower. Which is about 6.21 amps. It didn't seem right so I googled it. Well, it looks like it is correct.

So why does my 1hp bench grinder have a decal on it that says it is a 10 amp motor? And the same with my 1.5hp table saw with an 18 amp motor?

Who's right and why do the rules change?

bcp · Oct 24, 2014

Some are rated by how much power they are putting out right before they burn up.

Others are rated at maximum steady work.

From:
News from the Northwoods: Electric motor horsepower

"National Electric Manufacturers Association (NEMA) has a conservative system for rating electric motor horsepower. The horsepower rating is for continuous duty, such as you get turning a fan, or a water pump. For this you get a pretty beefy motor. A NEMA quarter horse motor is the size of a five pound sack of potatoes and weights two or three times as much. NEMA ratings are customary on stand alone electric motors.

For appliances with built in motors, blenders, vacuums, skil saws, and the like, the maker is under no compulsion to use the NEMA rating system. The marketing guys demand the highest possible advertised horsepower, which is the power the motor can deliver in a very short burst. This can be ten or twenty times the conservative NEMA rating. This is how you get a six horsepower shop vac."

Bruce

dave1949 · Oct 24, 2014

The ten amp rating can also be the maximum draw at start-up or complete stall, and may have nothing to do with actual HP.

bigtiller · Oct 24, 2014

I think it is odd that there isn't some type of uniform measurement.

I mean, it's pretty easy to tell that a 1 horse motor on my bench grinder that is bigger than a football, has more power than the 6.5 horse motor on my shop-vac.

bcp · Oct 24, 2014

Your shop vac at 120 volts would use a little over 40 amps when putting out 6.5hp.

Bruce

SPYDERLK · Oct 24, 2014

bigtiller said:
I was reading yesterdays comics when I got to "Non Sequitur" it read - 745.7 watts equals one horsepower. Which is about 6.21 amps. It didn't seem right so I googled it. Well, it looks like it is correct.

So why does my 1hp bench grinder have a decal on it that says it is a 10 amp motor? And the same with my 1.5hp table saw with an 18 amp motor?

Who's right and why do the rules change?

746 Watts is a horsepower, but you have to put more than a HP in to get a HP out. Motors are not 100% efficient. Also the motors have a Service Factor rating. The SF times the nameplate HP is the max continuous power the motor will put out w/o damage. I think the amperage stated will be that drawn at the SF adjusted [max safe] output.
larry

dragoneggs · Oct 24, 2014

Yeah Hp ratings on electric motors/equipment has always been frustrating for me. To the extent that I don't give much notice or respect at all to that number.

LD1 · Oct 24, 2014

Ditto what Larry said. Need more than 746w in to get 1hp out.

At 10 amps and 110v, that's 1100 watts. Figure 80% or so efficiency and you are nearly spot on.

Also keep in mind that it may not be exactally 1 HP. They ARE alloud to round numbers.

I have no clue why shop vacs are rated the way they are.

JimRB · Oct 24, 2014

746 watts is one horsepower. That might explain why the power meter on my bicycle says it is rare I get over 800 watts. I am not a horse. There are folks who can do twice the watts as me which is not surprising as I am not a powerful fellow. I can't even round up to claim more HP.

buickanddeere · Oct 24, 2014

Electric motors are not 100% efficient. Electric motors use real current but also circulate lagging reactive current.

SPYDERLK · Oct 24, 2014

JimRB said:
746 watts is one horsepower. That might explain why the power meter on my bicycle says it is rare I get over 800 watts. I am not a horse. There are folks who can do twice the watts as me which is not surprising as I am not a powerful fellow. I can't even round up to claim more HP.

There are highly conditioned athletes who can do multi HP for quite a while. The guy who self powered an ultralight [English Channel?] used both legs and arms. I think he hit a peak of 6hp on takeoff.
A horse can do MUCH more than a HP for a short time, but 1HP is what it can do all day with a break for lunch

buickanddeere · Oct 25, 2014

Maybe for a sprint . Even the doped up Lance Armstrong was limited to 2/3HP continuous for hour while biking . Best I ever did was 2.21HP in a sprint up stairs in high school. Did 1/2 HP for 10 minutes a couple of times on the bike but was awful tired and rubber legged afterwards.

SPYDERLK · Oct 25, 2014

buickanddeere said:
Maybe for a sprint . Even the doped up Lance Armstrong was limited to 2/3HP continuous for hour while biking . Best I ever did was 2.21HP in a sprint up stairs in high school. Did 1/2 HP for 10 minutes a couple of times on the bike but was awful tired and rubber legged afterwards.

I guess Im wrong about the HP peak. Cant find anything. I think it was in a long ago Popular Science article on the human powered flights. ... Anyway, I came up with this - it has a video:
Aug. 23, 1977: Pedal-Powered Gossamer Condor Flies Into Record Books | WIRED

CobyRupert · Oct 25, 2014

What? Watt? Nobody's mentioned volt-amps or vars (reactive power), power factor,? The old: Yes you have a beer mug that accepts a certain quantity, but only some of that input (volt-amps =volts x amps) is beer (real power/watts), some is foam (reactive power/vars). The relationship of the mug size to real beer quantity is your power factor.

buickanddeere · Oct 25, 2014

Coby I mentioned reactive power in a prior post

CobyRupert · Oct 25, 2014

Yes. I just saw and was typing: LD1 & BuickandDeere are correct. A motors efficiency in turning the electrical input power into real shaft output power. Some of the input power goes into overcoming friction ( bearings and air gap) and reactive currents flowing around the coils. Wherever you have coils the magnetic fields which run around the conductors interact and cause s resistance. Technically, it's called an impedance.

doxford jim · Oct 25, 2014

There is also the consideration of starting current for motors - which is usually in the area of six times that of the running current. This is needed to get the motor to turn and get it up to rated speed - once there the current then drops to the rated value. Therefore there must be allowances for this higher starter current or the fuses would blow or breakers trip.

buickanddeere · Oct 25, 2014

CobyRupert said:
Yes. I just saw and was typing: LD1 & BuickandDeere are correct. A motors efficiency in turning the electrical input power into real shaft output power. Some of the input power goes into overcoming friction ( bearings and air gap) and reactive currents flowing around the coils. Wherever you have coils the magnetic fields which run around the conductors interact and cause s resistance. Technically, it's called an impedance.

Capacitance, resistance and reactance all impede current flow but they are not the same.

nova3930 · Oct 30, 2014

The ratings could also vary based on the difference between peak and RMS voltage in AC electricity

Soundguy · Oct 30, 2014

your question is missing some important variables.

theoreticle vs real world, and friction... and then you get into efficiency.

that's why when you see pto generators for sale. most specify about 2hp per kw as a rule of thumb.

bigtiller said:
I was reading yesterdays comics when I got to "Non Sequitur" it read - 745.7 watts equals one horsepower. Which is about 6.21 amps. It didn't seem right so I googled it. Well, it looks like it is correct.

So why does my 1hp bench grinder have a decal on it that says it is a 10 amp motor? And the same with my 1.5hp table saw with an 18 amp motor?

Who's right and why do the rules change?