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Re: Performance with transistor controller - was (ET) Traction, etc



At 08:32 AM 1/28/05 -0500, Markus Lorch wrote:
I was thinking that there is nothing
better than having the battery directly connected to
the motor and the motor drawing as much current as it wants.


Not necessarily true. There reaches a point for the motor where any increase in current is simply dissipated as heat since the magnetic field (armature or field) saturates. Whether the battery voltage is sufficient to reach that point is another question. Another thing that can happen, depending on the controller, is that the voltage drop across the controller is less than the drop across a pair of contactor tips. While true with a high capacity controller I don't know of any situation where it's significant enough to make a difference.

If we use a transistor controller I think we always loose some
power due to the voltage drop (0.7V I think) at the transistors,
right? (280W at 400A)


Not with a MOSFET based controller. MOSFETs are resistive not voltage devices as far as the load they represent to the battery. For this power class I'd expect the drop across the MOSFETs to be on the order of 2 - 10 mOhms (probably on the lower side of that).

Also the overheat protection/current limiting of the controller
probably helps prolong the live of the batteries (and the motor brushes)
but may limit us on the torque end as the motor can't draw as high currents
(which
is probably less of an issue with the 300A alltrax controller than
with the 120A 4QD controller). Right? So I am hoping that some day
I will be able to take my E20 to a tractor pull, then I'd guess a
controller version may be at a disadvantage?


Properly sized a controller will not significantly limit the torque (unless that's what it is meant to do) but will offer some protection for the motor by limited time at full current. There is also potentially another reason for limiting current in PM motors. At least theoretically it's possible to have a large enough magnetic field generated by the armature to destroy the field magnets. The armature resistance might, however, be large enough to prevent that.

For a tractor pull a larger controller might be needed than for regular use. One advantage a controller might have over simply shorting the battery across the motor is the controlled ramp time will cause the torque to increase more gently so that there is less jerk and potentially less chance of the tractor simply breaking free.

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