What's up with these horsepower ratings on electric motors? 115 VAC 60 Hz. 15A = 3 HP?
I don't think so.
http://www.aircomparts-air-compressors.com/j5042/

I have an air compressor that claims 115 VAC 15A 5.5 HP
AO Smith motor K56A71B23

Somebody want to tell me about these ratings?
Or is this forbidden as, "over unity"?

 

What's up with these horsepower ratings on electric motors? 115 VAC 60 Hz. 15A = 3 HP?
I don't think so.
http://www.aircomparts-air-compressors.com/j5042/

I have an air compressor that claims 115 VAC 15A 5.5 HP
AO Smith motor K56A71B23

Somebody want to tell me about these ratings?
Or is this forbidden as, "over unity"?

There was an old thread here about shop vacs with 5 "peak" horsepower and other outrageous claims. Search "shopvac" and @strantor .

Apparently the current surge was measured the instant it was turned on.

Who knows, I buy by the amperage rating and, all other things being equal, the gauge of the power cord.

 

I buy by the amperage rating

I just do the math on the volts and amps, then subtract about 20% for efficiency losses, but my question is about where these obviously false horsepower labels come from. Are they using tiny little Shetland ponies for their "horse" power? Is there some air pressure x CFM rating system? Is it simply outrageous, shameless lies?

 

There was an old thread here about shop vacs with 5 "peak" horsepower and other outrageous claims. Search "shopvac" and @strantor .

Cute, but no cigar. Apparently the local denizens of AAC haven't figured this one out. I present another opportunity in case better information has been found since that 2013 Thread.

 

Horsepower
This is a rating that was used primarily for marketing purposes and had little or no relation to vacuum cleaner performance in the real world. Thankfully, it has been largely done away with as a vacuum cleaner specification.

Often referred to as "Peak Horsepower" these ratings were obtained by removing the fans from the vacuum motor and subjecting it to the maximum load possible before the motor burned out. Then, a complex formula was applied to come up with a Peak Horsepower rating. Again, this specification is meaningless in terms of evaluating the cleaning ability of a vacuum cleaner.

 

this specification is meaningless in terms of evaluating the cleaning ability of a vacuum cleaner.

Apparently it is also meaningless in evaluating an air compressor.

I guess I should feel lucky that air conditioning compressors obey The Law rigorously. If you could send 1725 watts into an air conditioner and get 5.5 HP of cooling, you could cool a 1000 square foot residence with a 20 ga. extension cord (on 240 volts).

 

There was an old thread here about shop vacs with 5 "peak" horsepower and other outrageous claims. .

Motor HP is has a rpm component (time element) Vacs have typically 20krpm's..
Max.

 

In some the specs are duty cycle adjusted. Amp rating is for the typical continuous duty mode and the HP is its peak intermittent duty cycle mode.

A typical 2 HP induction motor can run off a 120 VAC 20 amp line but just barely and will handle a 5+ HP load for a few tens of seconds before it starts to overheat if it's connected to a line that has the current capacity to support it.

It's the same principle as to how a common garage door opener motor can put out 1+ Hp despite having a motor frame size that wouldn't pass for a 1/4 - 1/3 continuous duty one anywhere else.

 

Apparently it is also meaningless in evaluating an air compressor.

I guess I should feel lucky that air conditioning compressors obey The Law rigorously. If you could send 1725 watts into an air conditioner and get 5.5 HP of cooling, you could cool a 1000 square foot residence with a 20 ga. extension cord (on 240 volts).

If you think hard enough and work long enough, anything is possi (never mind).

 

Apparently it is also meaningless in evaluating an air compressor.

I guess I should feel lucky that air conditioning compressors obey The Law rigorously. If you could send 1725 watts into an air conditioner and get 5.5 HP of cooling, you could cool a 1000 square foot residence with a 20 ga. extension cord (on 240 volts).

The rough rule for air compressor capacity I have always used is 3 CFM per HP @ 100 PSI for a single stage unit and 4 CFM per HP @ 100 PSI for s two stage. So if your compressor is a single stage and rated at something thing like 7 CFM @ 90PSI your dang close to it being an honest 2 HP unit.

 

From the Shop Vac website:

Horsepower Rating
**"Peak Horsepower" (PHP) is a term used in the wet-dry vacuum industry for consumer comparison purposes. It does not denote the operational horsepower of a wet-dry vacuum but rather the horsepower output of a motor, including the motor’s inertial contribution, achieved in laboratory testing. In actual use, Shop-Vac motors do not operate at the peak horsepower shown.​

The single asterisk explains that the rated capacity (in gallons) is the volume of the tank but operating volume is less to accommodate the filter. Weird.

 

if your compressor is a single stage and rated at something thing like 7 CFM @ 90PSI your dang close to it being an honest 2 HP unit.

Mine says 5.8 CFM @ 90 PSI. Your estimate (300 cfm-psi per HP) would suggest an honest 1.74 HP and that resembles what's coming out of the wall outlet (-25% for motor and compressor efficiency losses) but the label says 5.5 HP.
Anyway, you presented the closest thing to a useful answer but the only way I'm going to consider this to be "case closed" is to see the equation that converts motor horsepower into compressed air horsepower. (Googling did not help.)

Let's see...is there anybody on this site who is practically OCD about proper labeling of units?

 

Mine says 5.8 CFM @ 90 PSI. Your estimate (300 cfm-psi per HP) would suggest an honest 1.74 HP and that resembles what's coming out of the wall outlet (-25% for motor and compressor efficiency losses) but the label says 5.5 HP.
Anyway, you presented the closest thing to a useful answer but the only way I'm going to consider this to be "case closed" is to see the equation that converts motor horsepower into compressed air horsepower. (Googling did not help.)

Let's see...is there anybody on this site who is practically OCD about proper labeling of units?

Yep. took me a long time to get that answer as well and even then it's not always close being it's not to hard to make an air compressor takes even more HP to pump less air. It's a fair estimation point for the heavy low speed one and two stage type compressors but not so good for the high speed lightweight single stage ones.

The problem with most of those is they are built to be as cheap as possible which means undersized reed valves trying to feed cylinders with pistons moving way too fast to get a decent air charge into and out of them between suction and compression strokes. I figure some of them to be around 2 CFM per HP in many cases. Lots of frictional heat being made for what air the do pump as the trade off for being cheap and lightweight.

 

sounds like marketing scam. maybe showing how much power it may consume before melting

 

Well, I've never paid for a computer, an air compressor, or an arc welder. If I had, I would have done the research before I laid my money down. That's why I'm here asking questions...I didn't do my homework because I had nothing invested. The compressor works very well with it's 20 gallon tank storing lots of energy. I just got curious about the fact that you can't get 5 HP out of a 115 volt wall outlet, but the label on the compressor says it does. It's either a magic compressor or a pack of lies, and I believe we know the answer to that one.

I don't expect anybody is going to find the equation for a pack of lies, so consider this closed.

 

I don't expect anybody is going to find the equation for a pack of lies,

Well... A Pack of Lies can still make a good story, even if there's no truth in them... this thread has been both fun and informative... many thanks!

 

How to convert watts to horsepower
How to convert power in watts (W) to horsepower (hp).

Watts to mechanic / hydraulic horsepower
One mechanic or hydraulic horsepower is equal to 745.699872 watts:

1 hp(I) = 745.699872 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 745.699872

Example
Convert 10W to mechanic horsepower:

P(hp) = 10W / 745.699872 = 0.01341 hp

Watts to electrical horsepower
One electrical horsepower is equal to 746 watts:

1 hp(E) = 746 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 746

Example
Convert 10W to electrical horsepower:

P(hp) = 10W / 746 = 0.013405 hp

Watts to metric horsepower
One metric horsepower is equal to 735.49875 watts:

1 hp(M) = 735.49875 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 735.49875

Example
Convert 10W to metric horsepower:

P(hp) = 10W / 735.49875 = 0.013596 hp

(source: http://www.rapidtables.com/convert/power/how-watt-to-hp.htm )

 

How to convert watts to horsepower
How to convert power in watts (W) to horsepower (hp).

Watts to mechanic / hydraulic horsepower
One mechanic or hydraulic horsepower is equal to 745.699872 watts:

1 hp(I) = 745.699872 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 745.699872

Example
Convert 10W to mechanic horsepower:

P(hp) = 10W / 745.699872 = 0.01341 hp

Watts to electrical horsepower
One electrical horsepower is equal to 746 watts:

1 hp(E) = 746 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 746

Example
Convert 10W to electrical horsepower:

P(hp) = 10W / 746 = 0.013405 hp

Watts to metric horsepower
One metric horsepower is equal to 735.49875 watts:

1 hp(M) = 735.49875 W

So the power conversion of watts to horsepower is given by:

P(hp) = P(W) / 735.49875

Example
Convert 10W to metric horsepower:

P(hp) = 10W / 735.49875 = 0.013596 hp

(source: http://www.rapidtables.com/convert/power/how-watt-to-hp.htm )

Preaching to the choir, Tony? ... ... just kidding ... I'm sure your post will come in handy to those who stumble upon this thread, and are unfamiliar with power conversion.

 

It's chinese horses...

 

Manufacturer ratings depend on the method they choose to take their measurements. I was somewhat surprised years ago when I learned that light bulbs that were rated to last a certain number of years was based on testing at lower voltages. Sorry, I can't give you any specific examples, just that the testing involved using unrealistic circumstances to get the more favorable numbers. Somewhere on the packaging you might find some sort of statement that the bulb is rated for - oh, lets say 130 VAC. By changing the ratings they can make the bulb LOOK like it lasts longer than it will in yours or my house. I'm guessing here, but I bet the motor manufacturer is using some other factor to get the higher rating for their motor, making it look like the more desirable product.

Briggs and Stratton just got busted for over-rating their gasoline engines. A six horse motor didn't really produce six horse power. I'm not familiar with the goings on in that matter, but manufacturers are apt to fudge the numbers to make their product look more desirable.

[edit]

Found the lawnmower story. Older than I thought, but shows the practices.

http://www.yourlawyer.com/topics/overview/lawnmower_horsepower_fraud