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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