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

["Markus Lorch" <mlorch vt edu>]

Hi Robert,

thank you very much for your reply. This really helped me understand
the implications of using a electronic controller better. 

Thanks, I much appreciate the effort

Markus

> -----Original Message-----
> From: elec-trak-bounces cosmos phy tufts edu 
> [mailto:elec-trak-bounces cosmos phy tufts edu] On Behalf Of 
> Robert Adsett
> Sent: Freitag, 28. Januar 2005 09:03
> To: elec-trak cosmos phy tufts edu
> Subject: 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
> 
> 
> _______________________________________________
> Elec-trak mailing list
> Elec-trak cosmos phy tufts edu
> https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak
>