Re: (ET) ET power

["Steven Naugler" <snaugler earthlink net>]

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

----- Original Message -----
From: Larry Elie <lelie ford com>
To: 'Dean A. Stuckmann' <dstuck lakefield net>;
<elec-trak cosmos5 phy tufts edu>
Sent: Tuesday, June 08, 1999 10:02 AM
Subject: RE: (ET) ET power



> Now, to totally confuse things, people DO rate an electric motor at
roughly 3 X of a
> gasoline motor, but this is because of the STARTING TORQUE.  The same is
true of
> a steam engine; they produce gobs of torque at zero RPM.  At rated RPM,
all motors
> are the same.  BTW, most gasoline engines are also designed for as less
than 100%
> duty cycle, industrial engines being an exception.  If that isn't enough,
the ratings on
> lawn equipment is on the ENGINE not the TRACTOR.  Drop a good 20% or more
for
> reality.  A John Deere 12 Hp is doing real well to get better than 9 Hp 
> to
the wheels.
> I think this is part of the confusion as well.  The real POWER of the
motor is simply
> the rate of doing work; the torque times the RPM.  The real power 
> consumed
is the
> voltage times the current.  The real efficiency is the power output
divided by the power
> consumed.  If the traction motor can continuously consume 100 Amps at 36
volts, this
> is 3600 watts, and assuming 1000 (not the perfect 760) watts per Hp, this
is only 3.6 Hp.
> Now, I have seen over 150 amps (but not for long periods) on my E12, 
> which
is over 5 Hp.
> Not bad, but it doesn't prove much.
>
> Larry Elie
> Ford Scientific Research
>
>
>
>