Motor rating tag

[Logwilli]

I have an air compressor with an electric motor. The motor’s metal specification tag riveted to it reads 5.5 Hp and below that it specifies 120V, 15amp. It also has a 15amp plug. I am just a structural engineer, but even I believe this cannot be correct. Does this look suspicious to any of you electrical engineers?

 

[davidbeach]

There seem to be a lot of single phase consumer products out there with nameplates that come from the marketing department rather than from Engineering.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[edison123]

Post the nameplate and motor photos.

Muthu

http://www.edison.co.in

 

[waross]

The largest motor that you can legally run on a 15 Amp circuit is about 1.5 HP.
The nameplate is probably a misprint. You are correct to be suspicious.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Here’s how “marketing HP” works.

AC induction motors have a torque-speed curve. In the non-marketing / engineering mindset, HP is based on Full Load Torque (FLT) at full slip speed. Simple.

But a standard “B” curve motor, there is an added level of torque called Break Down Torque (BDT) or “peak torque” that occurs when a load causes the rotor to slow down a few percent. BDT can be as much as 180-220% of FLT, but only exists for a few seconds because it reaccelerates the load to a stasis point. When the motor produces BDT, it also draws 180-220% current, so it is immediately entering an overload state. So it cannot sustain BDT for more that a few seconds without risking damage.

The “marketing” viewpoint on this however is that if you apply the HP formula (Tq x RPM/5250) to the BDT, however brief it is, you can claim a higher “HP” value that they can use on their sales material. For years, I used to see it described as “develops xxHP”, but lately they seem to have even dropped that pretense. What they are legally REQUIRED to list somewhere are the actual watts consumed. If you convert the watts to HP (746W/HP) you get closer to the real HP number. If you find the watts, I think you will discover this is a 2 or 3HP motor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

120 Volts x 15 Amps= 1800 Volt-Amps.
Assume combind PF and efficiency of .83 x 1800 VA = 1494 Watts.
1494 Watts / 746 = 2 HP.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Logwilli]

I suspected it is some kind of marketing angle, but just for FYI hear is a pic of the label
Thanks for the feed back

 

[Logwilli]

 

[electricpete]

You've got good advice already.

I think you could use your X SCFM @ Y PSI numbers to compuate a fluid horsepower (under assumption that the input pressure is atmospheric). FHP forms a lower bound for the BHP (in the sense that BHP = FHP / CompressorEfficiency). That may or may not help you slightly piece together part of the puzzle if the basis of those numbers has something to do with a steady state power rating (rather than some other limitations... I dunno I don't work with compressors that much).

 

[waross]

It's easier to look at the nameplate, Pete.
120V/15.0A is probably correct.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

Haha, you're right there's probably not much need for additional poking around on that. I knew FLA was missing in the original post and but once we've got the nameplate we've got the FLA. Never mind!

 

[waross]

My treat in the Pub, Pete.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

That is 2 HP at max. And it's not some cheapass chinese make either. Wonder if that 1-800 number still works.

Muthu

http://www.edison.co.in

 

[electricpete]

My treat in the Pub, Pete.

If you mean Pat's Pub, I'll pass. I've been waiting for the political insanity to die down (in general, not just Pat's Pub), but it seems I'll be waiting awhile more. But if I'm ever in Canada or you're ever in Texas, I just might take you up on that, Bill.

That is 2 HP at max. And it's not some cheapass chinese make either. Wonder if that 1-800 number still works.

yeah, good point as waross already calculated 15 Amps at 120 volts with 0.83 product of (PF*Eff) would give 2hp. Even if PF*Eff was miraculously 1.0, that would only bring the calculated hp up to 2/0.83=2.4hp.

So maybe the FLA is the normal steady state thermal limit, and the HP here is the "marketing speak" calculated from breakdown torque as jraef mentioned which could approach up to 2.5 times higher than the steady state HP rating.

The phrase CV (Chevaux-Vapeur) reminds me of the original meaning embedded into the phrase HP (Horse-Power) that somehow doesn't jump out as much when it's in my native language of English. Here in the 3rd decade of the 21th century, we're still measuring our mechanical power by comparing to a standard horse. Maybe this nameplate is using miniature horses ;-)

 

[LittleInch]

Maybe it was originally rated on the 230V version of the motor...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[edison123]

pete - Though officially we are a SI nation, people still use HP, ft and inches etc here. Limey legacy, I guess.

Muthu

http://www.edison.co.in

 

[jraef]

Personally, I like the use of HP instead of W when referring to motors, mostly just because too many people equate the Watt value on a motor nameplate as being the ELECTRIC Watts, not the MECHANICAL Watts. HP is harder to confuse with an electrical value because it forces you to think of a horse, not a light bulb.

CV being short for Chevaux-Vapeur always makes me chuckle, because having grown up with horses, I think of the horse "vapors" that I had to endure whenever my sisters were riding ahead of me on the trail...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[jraef]

...I dunno I don't work with compressors that much.

I do and I will say that in the INDUSTIAL compressor world, nobody plays these kinds of nameplate shenanigans, this is a marketing thing targeted at retail consumers who will walk down the isle at Home Despot and glance at the price tag first, then compare the "HP" value to the next higher priced one and run a quick "$$/HP" valuation in their heads to make their decision.

Standardized NEC table values for a 2HP 230V 1 phase motor is 12A, 3HP is 17A. So at 15A this is either a hyper efficient 3HP motor, or a very INEFFICIENT 2HP motor. My guess would be the latter.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LittleInch]

I think the literal translation is Steam Horses.

Vapeur in French means steam.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jraef]

Sure, but on a cold day on the trail...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden