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Re: (ET) Mowing speed and switching from forward to reverse, etc.
To my surprise, replying to this appears to have sent to Peter rather than
to the list. Sorry about that Peter. I'm going to see if I figure out why
that's so, that doesn't happen on any other mailing list I'm on.
Robert
At 02:58 PM 11/6/04 -0600, you wrote:
Also, on a side-point: I'm in the electric forklift industry. We tell our
customers that it's better to use the motors to slow, stop and change
directions, rather than use the brakes. This is called "plugging". Now,
I'm in sales, so I don't have an extremely deep knowledge of electrical
concepts, but one thing I'm thinking may be the difference is that with
most forklifts (w/DC motors) they're not "permanent magnet", as I'm
surmising the ET motors are. Is that where the difference is? I've been
using my direction control to "plug" for changing directions and more
rapid stops because my brake doesn't work. There seems to be a delay built
in that if I do it too quickly it won't change directions. However, if I
pull it into 'neutral' for a second, then down to reverse it works nicely
for stopping quickly.
Any thoughts or input on this point?
Also being from the same industry (I used to design such controls) I can
offer some points.
- Forklifts use series wound (also called traction motors) {that's not as
true as it used to be, they are steadily switching to separately excited
(or shunt wound) and AC motors}. These motors are immune from some of the
problems that can effect PM motors, notably demagnetization under high
field and shock conditions, also the torque vs. current curve tends to be
quite favourable to the application. PM motors do have offsetting
advantages and the newer magnets are less susceptible to demagnetization.
- Plugging in the context of the forklift industry has a narrower
definition than used elsewhere and specifically referred to a style of
braking where field of the motor is reversed under load. A (large) diode
is run from the armature (rotor) on the side that connects to the field
back to the other side of the armature. When the field is reversed the
armature essentially becomes a generator braking the motor and dumping the
vehicles kinetic energy into the motor and the diode.
- During the active plug braking period the motor control is essentially
controlling the motors field and small changes in the voltage can make a
large changes in the current through both the armature and the field. This
makes electronic control essential.
- Generally this form of braking has been preferred for forklifts because
it is both stronger than the mechanical brakes and suffers from less wear
and tear.
- This is not regen braking, no energy is recovered back to the battery, in
fact additional energy (although not much) is taken from the battery to
sustain the field.
- There is a meta-stable state in plug braking where the current in the
field can get high enough that it becomes self sustaining. Not usually a
problem in well designed systems but can lead to excessive wear and heating.
As I understand it there are tractors out there with PM, shunt wound and
series wound motors. Only the latter would provide true plug braking
although with proper control any of them will be able to handle a direct
direction change at speed. (and I know for a fact that the Electric Ox will
operate that way w/o concern).
Bottom line, if the vehicle is not designed for it don't do it. Worst case
scenarios involve broken axles, bones and various other pieces. It is
conceivable that it will work simply do to such limiting factors as slip
and the internal resistance of the batteries.
Having said all that I don't know what tractors are capable of doing this.
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