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Re: (ET) Motor control

Allen and others,
    Strangely enough GE reversed some of their tractors with shunt motors
(E15 and E20) by reversing the field and others by reversing the armature.
Maybe they reversed the armature so that they could use the same circuitry
and power relays as on the larger (E12) permag motor powered tractors.  
I don't think so.  Read on.)
    There is one reason why you'd like to reverse the armature versus the
field.  What many don't know is that most shunt wound motors don't have
totally separate armature and field.  A high current capable series coil in
series with the armature, usually called a stabilization coil, is wound in
with the field windings.  Why that is done follows
     When your shunt wound motor is heavily loaded it tends to slow down.
This is because not all of the armature voltage is used to generate the
armature magnetic field, but rather some of the voltage is lost
parasitically as resistive losses.  As the armature current goes up, a
larger percentage of the voltage is lost as an "IR" drop.
    I = current, R = resistance, and because V = I x R, and IR drop is
actually a voltage drop.  This voltage drop is the voltage not used to
develope power, and only the armature voltage less this IR drop actually 
generate power.
    The full armature current then goes through the stabilization windings
opposite to the field current, thereby weakening the field magnetic field.
When the field magnetic field becomes weaker, the motor tends to speed
faster.  Now you have two competing and opposite effects at high motor
loads, one of which slows the motor, one of which speeds it up.  By
adjusting the sizing of these opposing magnetic fields by sizing the
windings when the motor is designed you can actually get the motor to slow
down, not change speed, or even speed up under load.
    Here is why one might want to reverse the field instead of the 
When you reverse the field, now the stabilization winding is no longer
reverse in polarity to the field winding, but of the same polarity.  A 
that might exhibit very good speed regulation forward might be unstable in
speed under load in reverse because the stabilization winding is now in
reverse a destabilization winding.
    In our tractors speed regulation under load is probably only important
forward because when in reverse loads are usually much lower.  Actually,
forward speed regulation is only really important for heavy traction loads,
like plowing, and when mowing we don't really care.
    If I were you I'd reverse using the field windings.  This way you won't
have to bridge your armature FETs or maintain your reversing relays.  This
will result in lower voltage drops to the motor, which will improve 
both your battery life and speed regulation.  For really smooth control you
would want to bridge your field.
    Hope this helps.

Steve Naugler
----- Original Message -----
From: "Allan" <allanhv worldnet att net>
To: "elec-trak mailing list" <elec-trak cosmos5 phy tufts edu>
Sent: Thursday, March 02, 2000 11:06 AM
Subject: (ET) Motor control

> I am trying to build my own FET based controller for my E-20.  (I'm
> hoping it will work on E-15's as well.)
> Is it possible to reverse the direction of the motor by reversing the
> field rather than the armature?  I can't figure out why GE wouldn't have
> done this if it would work.
> Any comments would be much appreciated.
> Thanks,
> Allan