Re: (ET) field weakening on E15 and moving dirt

["Christopher Zach" <czach computer org>]

Well, the weakening control on the E20 is basically nothing more than a set
of resistors in series with the field, bypassed by the not of the throttle
switches for 5-8 speeds. Thus as the throttle is opened up to 5,6,7,8 the
switches are opened and the resistors are effectively "cut in" thus 
lowering
the voltage on the field.

The E20 keeps from melting the armature under periods of high load with FW
engaged by comparing the amp current going to the motor's armature using a
small shunt in series with the armature supply line. If the voltage drop
across the shunt exceeds a certain value, it gates a transistor pair which
basically shorts all the resistors in the FW circuit (thus raising the 
field
current to full power) which brings the motor to most efficient mode (and
lowers the current draw in the armature).

Side note: When you engage cruise control you're in the most efficient 
speed
for that gear. When you accel the CC relay is dropped, and you lose your CC
upon decel. Also in reverse FW is disabled by default. This is a good thing
since the motor would probably blow up if you reduced the field current 
with
a reversed armature already. Remember that the compensation windings act as
a bit of a field generator, and when you have an E20 in reverse, the
compensation windings are also reversed (being in series with the 
armature).
Thus the compensation windings are acting directly OPPOSITE to the field
current and thus are reducing the overall field intensity. More so under
load; that's why the torque goes bye-bye in reverse: As the armature draws
more current the compensation coils provide a stronger opposing field WRT
the main field. If you ever released a high load in reverse (say the belts
broke) my guess is the motor would explode if there was any FW.

E15's have a more complex FW system; aside from reversing by swapping the
field current (instead of the E20's swapping the armature polarity with 4
big interlocks) I don't know what other advantages there are.

Chris

----- Original Message -----
From: <agp@???>
To: <elec-trak cosmos phy tufts edu>
Sent: Saturday, March 30, 2002 7:02 PM
Subject: (ET) field weakening on E15 and moving dirt


> I have been doing more investigation on the capabilities of my new ET.
One
> unfortunate thing about the control modifications made by Mark Hanson:  
> No
> field weakening, therefore top speed is only 4 mph.  :(
>
> I have examined the circuit that GE used for field weakening and it
appears
> simple enough.  But I think it would be better to use some kind of
> semiconductor switch for my 48V system.  Do any of you Trakkers know how
> golf cars have their field weakening set up?
>
> I'm thinking of using a FET (in series with the field) that will be in 
> the
> linear region until the motor gets very close to full speed.  Then
initiate
> a change in the input to the FET so that it will transition to a state
> where it is dropping 36 or 40 Volts at a reasonable current level 
> (between
> 1/4 and 1/2 Amp).  The transition should take place in an amount of time
> not so short that the acceleration would cause undue force on the driver.
> But quick enough that the wait time for the speed change is not 
> noticable.
> Maybe between 1/2 to 2 sec would be good.  I think of it as a sort of
> "overdrive" where the result (in gear "D2") would be infinitely variable
> control of speeds between 0 and 4 mph, and then a jump to the 8 or 9 mph
> "warp speed".  (I might consider a variable speed control for the "warp
> drive" if someone points me in the right direction.)
>
> One other note.  The Club Car controller that is being used has an input
> for a Tachometer.  The tach was apparently functional at one time.  But
the
> "studs" that were glued on to the pulley of the motor have fallen off.  I
> thought of having a machine shop just use some small bolts as the studs.
I
> believe the output of the tach would then be a pulsed waveform with
> rectangular shaped pulses of magnitude no greater than 5V.
>
> On the positive side:  I was able to push some dirt today!  I had been
> digging a trench in the back yard to put in an electrical cable out to 
> the
> water pump down by the creek.  When I was done digging I laid the cable
and
> then used the Elec-Trak to move the dirt back into the ditch.  The only
bad
> thing that happened was that the strap on the front lift broke twice.
(And
> was repaired twice.)  I think I'll just buy one from Bill Gunn with the
> order I'm placing on Monday.  Does he accept VISA?  (Prior emails in the
> archive said he only takes checks or COD.)
>
> That's all for now,
> Andy Poush
> Andrew Poush
>
>
>