Re: (ET) Re: new electrics

[Rob Brockway <getractor yahoo com>]

The critical amps are usually limited by design resistance and max voltage.  The PM design must tolerate locked rotor current every time the motor starts (and with the electrak mower when it stops). 

 

The Nauglers <snaugler earthlink net> wrote:

> With all this talk about new ETs and better or cheaper ways to make them
> please remember that due to the limited energy storage on board our tractors
> efficiency must be paramount.
>
> V belt drives tend to be only 90-95% efficient when everything is in good
> condition and clean.
>
> Friction drives are 80-90% efficient.
>
> Chains, again in good condition, are 98 % efficient, but high maintenance to
> keep them in good shape in a tractor application. Notice that they are not
> used except in high torque attachments like the snowblower. A worn, dry,
> and/or rusty chain can have efficiencies as low as 60-70%. The wasted
> energy turns to heat and destruction of what is left of the chain and
> sprockets.
>
> Direct drive, like the mower decks, are 100% efficient.
>
> GE used VX series V belts for the traction motor because the VX series V
> belts are more
>  efficient than the old A/B/C/D series V belts. If the new
> Poly V belts, which are like the serpentine belts on modern cars, had been
> available when the Elec Traks were designed they would have been used
> preferentially over VX belts.
>
> What GE did was make a whole series of design choices to keep efficiency
> high. Where they lost efficiency was in the motor controls, and that was
> mainly a limitation of affordable DC drive technology of that time. If you
> were, in your new tractor, to use a friction drive for the traction motor
> and belts in the mower deck, and then allowed for lack of maintenance, you'd
> find extra losses of 10-25% after 1 or 2 years of service. I mean 10-25%
> higher losses than in a tractor with direct drive to the blades, and a high
> efficiency belt somewhere in the traction drive.
>
> With respect to protecting the mower motor with a circuit breaker: This
> will not work well with a permanent magnet motor. In permanent
>  magnet
> motors there is some critical armature current above which you will
> demagnetize the magnets. The friction washers, while crude, act as a torque
> limiting clutch. By limiting torque, you effectively cap the armature
> current. If you use a circuit breaker alone the current can, and most
> likely will, pass well above that critical current before the breaker trips.
>
> Steve Naugler
> snaugler earthlink net
>
>
>
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> Elec-trak cosmos phy tufts edu
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