RE: (ET) ET power

[Larry Elie <lelie ford com>]

Steve commented;

----------
From:   Steven Naugler[SMTP:snaugler earthlink net]
Sent:   Tuesday, June 08, 1999 9:26 PM
To:     Larry Elie; 'Dean A. Stuckmann'; elec-trak cosmos5 phy tufts edu
Subject:        Re: (ET) ET power

;Larry,
;    I only agree in part with your reasoning about why an electric motor
;needs to have not as much power as a gas engine to do the same job.  I 
think
;that there is a more important effect than starting torque, and that is 
the
;fact that when an electric motor is loaded and hence slows a little (or a
;lot for series wound) the available torque goes up somewhat, unless the
;controller prevents that effect.  With a gas motor, as it slows down, the
;available torque goes down.  The gas motor therefore needs a higher rating
;than an electric for otherwise the same job.  This could vary depending
;where the gas motor is operating on it's torque curve.
;    Now why the rule of thumb is 3 to 1, I am sure that there are more
;effects than starting torque and gasoline motor torque drop off with speed
;reduction.  I am not even sure where I came across this rule of thumb, but
;seeing identical machines powered with both gas and electric motors, air
;compressors being a good example, I've seen gas to electric hp ratios
;ranging from 2 to 1 up to 3.5 to 1.  And with improvements in small 
gasoline
;engines (better ignitions, carburators, and improved valve train), the 
rule
;of thumb may no longer be accurate and 2 to 1 may now be more accurate.

;Steve Naugler

I'm not sure it isn't the OPPOSITE.  Ratings of motors are, in a word, 
bad.  I have seen a Sears 
hand saw rated "2.5HP" yet it is only 9.5 Amps!  I have a Sears table saw 
rated 3/4HP that is 14 Amps.
The former is rubbish, the latter is probably correct.  All that "Peak HP" 
is garbage.  It used to 
mean take the Peak (not RMS) current and multiply by the peak voltage, for 
a peak power.  Never 
mind that this was insane; even with a square wave input you can't get 
away with that.  The problem 
is that you could never get more than a square root of 2 error on either 
current or voltage, so you 
couldn't lie more than a factor of 2 total.  Somehow, the managed to up 
the lie to better than 2.5 to 1!
At this point, all bets are off.  DC motors are better for ratings, but 
not immune to dishonesty.

Engine ratings are strange too.  There is shaft HP (reasonable), brake HP 
(not as reasonable), and 
actual HP (correct).  You are right about the way the torque curve works 
for a gas engine, you 
drop significant HP with RPM.  Diesels have a better torque curve.  The 
biggest difference in my 
opinion is more honest ratings in the gas engine industry.

Larry Elie