Watts to Horsepower

[bigtiller]

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]

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]

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]

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]

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

Bruce

 

[SPYDERLK]

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]

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]

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]

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]

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