RE: (ET) My First Mow Job

["Humphrey, Timothy" <HumphreyT neads ang af mil>]

Guess what else happens when those diodes short.

When they short they short the armature, this explains the burnt runs. Now 
I
need to figure out why they are shorting.

These diodes are mounted on the card. But they have nothing to do with the
current sensing circuits, they just use the card as a convenient mounting
surface. 

On second thought it might have been 413 and 414 that shorted. When I get
home tonite, I'll verify which it was, unless someone has a card handy, it
was the two bottom most diodes they are connected to pins 17 and 19. 

Here's what happens in either case, if I'm reading the schematics 
correctly.

Electrons leave the N post of battery 5 travel and through fuse 3. Some of
them then go through the seat switch, the brake switch, CB-3 and to the
coils of 1A, 2A, and 3A,(these are the armature resistor contactors). If 
one
of the corresponding switches on the foot pedal are making contact, the
little electrons speed their way on over to card 4 pin 16, then through
diode CR402 or CR403 out card 4 on pins 19 or 17, repectively and down to
the armature where they pass through the closed contacts of either the
forward or reverse relay, at this point they are traveling along the same
route as the armature current through CB-1 and the meter shunt then into 
and
out of the line disconnector, and fuse 1 and finally in to the P post of 
B2.
Those were the diodes for the armature current control circuit.

Now how about the field weakening control diodes. Lets say were in forward.
Again, electrons leave B5 and go through fuse 3, but some of them decide to
go through FW-2, then into card 4 on pin 9, through CR-412 out pin 7,
through the field, back into card 4 at pin 18, through cr414, the closed
contacts of relay 1F and into the B2 as above. In reverse much the same
thing happens except the current flows through FW-1, CR-415, CR-413 and the
1R contacts. The FW resistors are disabled in reverse. 

If CR413 or CR414 are shorted, you will have no field current. You will 
also
lose the ability to switch out the armature resistors. Thus there will be
high current and the motor will overspeed (in neutral) and have no torque.
(my meter was pegged and I wasn't moving in LL)

If CR402 and CR403 are shorted, you will lose the armature control circuit,
but field weakening should still be available, depending on current draw,
resulting in low current, low speed.

What we want is low current, high speed.

Keep in mind when ANY ONE of these diodes shorts it shorts the armature but
only for a millisecond or two or at least until a run or lead burns 
through.

Stay Charged!

Hump 

> -----Original Message-----
> From: Christopher Zach [mailto:czach computer org]
> Sent: Friday, May 09, 2003 1:02 PM
> To: Humphrey, Timothy; Discussion List Electrac tractor
> Subject: Re: (ET) My First Mow Job
> 
> 
> > When they short you lose your upper speeds. I
> > haven't figured out why yet.
> 
> That's pretty easy to answer: That whole card assembly is 
> basically there to
> monitor current going across the shunt. As long as the 
> current is below a
> certain value the card does not short the field weakening 
> resistor banks and
> high-speed is availible.
> 
> As soon as the armature current exceeds the value (say you're 
> climbing a
> hill), the transistors turn off and the field weakening resistors are
> effectively shorted. This brings the field to full current 
> and slows down
> the motor. It also has a side effect of bringing the motor to 
> full power.
> 
> Really great idea IMHO.
> Chris
>