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