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Here's some DC
motor basics:
- Maximum torque
is from armature amps and strong field amps/"flux"
- "Slapping" high
(36) volts across a stopped/loaded motor will probably cause a
flashover.
DC motor speed is dependent upon
armature conductors rotating thru a magnetic field (main field) to produce an
internal counter EMF. Brushes basically provide amps to drive internal
volts and switch current direction in the armature. As load "slows" an
armature down, it "draws" amps to maintain the CEMF. When you apply full
voltage it instantly tries to draw "infinite" amps to run base speed which can
cause a flashover/burned bars/bad stuff. That's why the control provides
stepping thru resistors to limit amps and come up to speed and it actually
monitors the CEMF before allowing the next relay to close. Modern controls
"chop" the voltage to perform a ramped-up voltage while maintaining less than
rated amps.
- So the trick to
make a puller is to optimize torque without drawing too many amps. Weak
field allows the motor to run faster but torque drops off. Ideally, you'd
want a solid state controller that goes to a set "current limit" allowing the
voltage to ramp up as quickly as possible to maximize torque and instantly start
weakening the field when motor volts reach 36.
Difficult to
maximize that with fixed relays. You either need timers on the relays or a
way to "fool" the control to close the next relay before the motor reaches that
stepped voltage in order to add acceleration without over-amping the
motor.
- Remember, GE
didn't want people to be thrown off these fang dangled electric
machines.....
...Walt
-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu [mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Jerry Rhodes Sent: Wednesday, July 26, 2006 10:24 PM To: Elec-trak cosmos phy tufts edu Subject: (ET) E20 info OK before I start this, I need to know if I'm
reading the schematic right. 1st I'm building a E20 puller, an I am stripping
the control circuit down to the bear bones, no PTO, no RTNs, no CC, etc... I'm
looking at a schematic for an E20 DA when I hit the foot feed the start swt
brings in 1F an 2F, I now have power to the motor a A1 an S2 thru R1, R2, R3,
wire #8(or #24) feeds a diode to wire #62 to Field F1..F2 draws from R4,
wire #37, FW2, wire #22 a complete circuit ( so far so good?)...so I want the
field strong? with only one resistor in the line and as I go up in speed ( 1A,
then 2A, then 3A, I still only have R4 in line if I go on up in speed I add R 5,
R 6, R 7 for a weak field?
All safeties will be in the line ...ie key swt,
seat swt, brake swt, CB 3 will be replaced by a deadman swt ( which will also be
in wire #15) ..diodes will be added for feedback control..
So am I way out in left field...it looks ok on
paper...an it looks to work with the testing I've done on a mockup
board..
I'm open to all input you got..thanks in
advance
it will only be running hard for about 90
sec
Jerry NW Ohio
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