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Re: (ET) Something to ponder
At 10:50 AM 5/9/2006 -0500, Tim wrote:
Geof,
I think some of your ponderings are possible.
#1 E-Tec motors have very low inductance and need special controllers.
Running 2 motors in parallel will half the inductance and might cause
premature controller failure.
Also you need to watch the ripple current in the motor. If your switch
frequency is low you will lose a significant amount of energy in ripple
current. Most of that will be heating the motor and wiring some will be
emitted as RF. The frequency should be at least 15kHz, 20kHz or even
higher would be better.
#2 Running a motor on each wheel, while wired in parallel, will cause
difficult steering as the slower inside tire/motor will now try and draw
more current thus making the turn more difficult.
However they will also load share with the most loaded motor taking the
most current and thus providing the most torque, see next comment.
#3 I believe the motors in series is better. This would add the
inductance (more controller friendly) and provide an electrical version
of a differential making turning easier than the parallel motor method.
Negative of the series arrangement is that you need twice the motors
voltage rating...if each motor is rated at 24v then you need a 48v
battery supply. At times this arrangement can supply 48v to a 24v motor
but with softstart circuits in the controller, the motors should not be
damaged (an extreme case would be if one tire was kept from spinning and
the other tire was free to rotate then 48v would go to the free spinning
motor and might cause its armature to fly appart).
Actually, this is the real problem with a series connected PM setup. The
motors will be at the same torque and the torque will be that of the motor
with the least load. If one wheel is slipping or off the ground it will
effectively limit the torque to the wheel with the traction. Basically if
either wheel breaks free you are stuck.
#4 I don't believe the spinning blades would have much of an effect on
turning.
Only if you are using them to trim the sides of the hedges :)
Robert
" 'Freedom' has no meaning of itself. There are always restrictions, be
they legal, genetic, or physical. If you don't believe me, try to chew a
radio signal. " -- Kelvin Throop, III
http://www.aeolusdevelopment.com/