Re: (ET) Ariens AMP Mower

[Christopher Zach <czach computer org>]

David Roden wrote:
> Not here!  My property is hilly, uneven, and broken up in to little islands 
> of grass.  I appreciate being able to creep the machine at times.  Am I 
> typical?  Probably not, but I'm sure I'm also not the only one.

Really? My yard is somewhat hilly and the shunt motor with cruise
control (full armature, full field) is the only way to go. If I go up
hill, it maintains speed as the motor pulls more amps. When I go down
hill it maintains speed as the motor *pushes* the amps into the pack in
regen. In all cases the tractor keeps a steady speed, switching speeds
is what de-stabilizes your tractor's traction.

> When you're designing an electric tractor from the ground up, I agree that 
> the cheap way to go is to bolt the motor into an existing design.  There are 
> advantages to using mass-produced, proven components.

Sure. Put a shunt motor in a Chaftsman tractor, gear it to run the motor
full speed/power, then belt it to the blades and transmission.

> However, an electronic motor controller is effectively a continuously 
> variable transmission without the inefficiency of hydro drive.  The cost of 
> a small DC motor controller adequate for a garden tractor is certainly not 
> prohibitive, and much less today than it was 30-35 years ago.

Sure. However for most mowing you really need a few speeds. Use the
transmission for that. Yes you will lose a bit of efficiency, but it's
just not worth adding hundreds to the cost of the tractor to squeeze a
bit more out of your T105's. The perfect *is* the enemy of the good.

> I don't understand why you suggest this.  Reversing the low current field in 
> a shunt motor is almost a trivial task, requiring only a small relay or 
> small semiconductors. It makes much more sense to reverse the field than to 
> reverse the armature. This is one of the advantages of a shunt or sep-ex 
> motor over a PM or series motor.

Um. Yes. However when you go from forward to reverse while the tractor
is still moving forward, it will beat the daylights out of that little
relay since it will get a very high voltage spike as it goes from F to
R. This will destroy a varistor quickly too.

If you could ensure that the motor stops every time or use a nice big
contactor pair to switch the field then this would be fine. However if
you use a little relay it's going to get beaten up.

This is what GE did with the E15 and one of the main reasons why it is
so bad. The E20 switched the armature and it's much more reliable and
tolerant of going from F to R.

An even better solution though might be to forget motor reverse and just
use the reverse gear on the transmission. Every transmission has it, so
it's not like you're saving money. Do what everyone else does.

Chris