Newsgroups: rec.crafts.metalworking
Subject: Re: gas engine hp to electric motor hp
From: John De Armond
Date: Mon, 25 Jul 94 07:33:54 GMT

arf@MCS.COM (Jack Schmidling) writes:

>Vaccum cleaners now seem to be rated in HP with numbers that seem
>truly absurd.  We just bought bought one rated at 4.0 HP that is 
>about the size of the 1/2 HP motor on my drill press, I mean the whole
>vacuum cleaner.

>Furthermore, I find rating hp based on power consumption a bit
>absurd.  After all, a short circuit would eat  up lots of juice but do
>nothing but get hot.

This is absurd.  My "4.0 HP" shop vac has a nameplate rating of 8 amps
max.  That is about 1.3 electrical HP, probably 1 mechanical HP.  
A call to the Federal Trade Commission would probably clear up what 
loophole these people are being allowed to advertize under.  Certainly
nothing logical or scientific.  My guess would be that they're using
some sort of made-up "hp equivalence" based on how much air is moved
or vacuum created.

John

Newsgroups: rec.crafts.metalworking
Subject: Re: gas engine hp to electric motor hp
From: John De Armond
Date: Mon, 25 Jul 94 17:58:35 GMT

vincent.j.miller.4@nd.edu (Vincent J. Miller) writes:

>I think they all go by stall hp, which a previous post described better than 
>I can.  Anyway, its what an electric moter will produce just before the 
>windings turn to jelly.  Sears sells a 5hp shop vac, which as near as I can 
>tell, would be pretty tough to run on a 30amp 110, line.  Maybe it comes 
>with a special cable to connect straight to the breaker box? ;-).

The problem is, "stall hp" has no meaning because a motor is making no
power at stall.  Recall the formula for HP in conventional units:

hp = (RPM * torque in ft-lbs)/5252

When RPM goes to zero, so does HP.  And RPM typically drops under load
for universal motors faster than the increased torque can make up for
it.  Plus "stalling" has no meaning for motors driving centrifugal
fans, such as on vacuum cleaners.  I have no idea what this HP rating
really is, other than fraud, but I'm reasably sure that it isn't anthing
to do with what comes off the output shaft.

John