[Fwd: Electronic Dynos]

[Stephen Dubovsky]

At 12:12 PM 12/1/96 -0500, you wrote:
>Doug Robson wrote:
...
>> Ok so who want to discuss writing a program to convert the EMF against
>> an engine dyno to Horspower and torque. is it easy or am i silly
>> 
>> regards
>
>
>Hi Doug,
>
>I'm not an electrical engineer, but I think this is what you're looking for:
>
>	EI = T*2*Pi*n/60 or 
>	T= (60*E*I)/(2*Pi*n)
>
>	where:
>	E is voltage	(in Volts)
>	I is current	(in Amps)
>	T is torque output (in Newton - Meters)
>	n is speed of rotation	(in RPMs)
>	Pi = 3.14159
>
>An alternative method to measuring torque is using a load cell, which is
comprised of 
>two strain gages.  While the generator is supported by its bearings, the
load cell is 
>the only memberane that keeps it from spinning.  You can get a pretty
accurate reading 
>with a load cell within 0.5%.  This is the method most water brake dynos use.
>
>Best Regards,
>Mazda Ebrahimi
>

  I also agree that using a strain gauge is more accurate, but there are
'gotchas' when using a load motor like this.  Mazda is right about the formulas.
EI=P (in V,A,Watts respectively).  The only problem is measuring the Volts.
The load motor being used (as a generator) has some internal resistance.
When loaded, the I causes a V drop on this internal resistance that you cant
measure.  It causes a power loss in the motor (ever notice how it gets
hotter when loaded, and not just turning at close to no load?).  There are a
few ways to get around this.  1) Measure motor resistance and compute what
the real back EMF would be. 2) Map back EMF vs rpm and use that instead of
the measured E.  Both of these have accuracy problems in that 1) the rotor R
changes a lot w/ temp 2) the back EMF will change slightly under load due to
non-linearities in the B vs H cure of the iron in the motor (scarry
magnetics stuff here).  If your motor is a field excited machine (adjusts
the load) then all of this becomes more complicated.  The back EMF is also a
function of your applied field (not constant as w/ a permanent magnet
machine).  The first method would probably be the best way then...
  Or just go find a strain gauge from Omega;)

SMD - does it sound like I know too much about electric machines???

--
Stephen Dubovsky
dubovsky at vt.edu

95 Yamaha FZR600
83 Porsche 911SC
84 Jeep Cherokee