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

[Robert Adsett <subscriptions aeolusdevelopment com>]

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,
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