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Re: (ET) Diagnosing delay error on E15 board

   Reversing field polarity is a commonly used means to reverse rotation
and current flow.  
Remember, the speed (and torque) is produced by field flux times
armature amps.  When a DC motor rotates, the field induces a voltage
into the armature winding and current is "pushed or pulled" depending
upon what the applied voltage is compared to the internal voltage (CEMF
- counter electro motive force for us motor guys).

When you slam the E-15 throttle to "neutral" and reverse, the field goes
through a polarity change and is affecting the internal flux.  As
armature voltage is decreased by the contactors opening, field is still
there and causes the motor to regenerate which is why you'll see the
ammeter go "negative".  If you just open the armature contactor, all the
inertia of the tractor is generated through the motor and flashes across
the contactor (or across the motor itself which is caused a flashover
which can lead to severe damage).

So, field is important.  If your ammeter pegs and the tractor motor
won't run, it's probably because you have no field to allow the motor to
produce torque to make it turn and generate CEMF.  With no CEMF the
armature is a low resistance "short".

And, if you continually jam the throttle from forward to reverse, the
little red VDR-1 blows up because it can't handle the abrupt changes in
field current and resultant voltage spikes which are created by the high
field inductance and time lag for it to collapse.  This can also cause
the SCR's to turn on which is not good and upsets the control logic.

The PC card "monitors" the motor voltage and allows contactor closing to
attempt to minimize current spikes and tractor jerks.... actually a good
system for 1968 technology.  Today, we have golf cart drives that you
can program with a laptop for motor parameters and application.


-----Original Message-----
From: David Roden [mailto:etpost drmm net] 
Sent: Friday, October 28, 2011 9:34 AM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Diagnosing delay error on E15 board

On 27 Oct 2011 at 16:29, CZ wrote:

> Reversing the field was probably the stupidest idea they could have 
> come up with ...

Actually, that's an entirely reasonable way to reverse a separately
motor.   I think (but am not 100% positive) that's the way the Alltrax

As I understand it - and warning, I'm not an EE - because the field
operates at relatively low current, it makes more sense to reverse it
with light, inexpensive relays or semiconductors, instead of using
expensive, heavy high current contactors or semiconductors to reverse
the armature's polarity. 

I guess it's just the GE didn't design proper field handling into their
E15 controller.  Maybe it's easier to do that when the field current is
controlled by a microprocessor?

David Roden - Akron, Ohio, USA

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