Re: (ET) field rev.

["Konstanty, Walter \(GE Indust, ConsInd\)" <Walter Konstanty ge com>]

Some Design theory inputs for a Friday morning:

- It's less costly to reverse the field polarity to make the motor 
reverse.  Smaller contactor required.

- DC motors need field amps and armature amps to produce torque.  Less 
field amps creates higher speed and less torque.

- Reversing the field polarity OR armature circuit polarity makes a DC 
motor reverse direction.  If you reverse both, same direction.

- Compound winding means there's a "light series" winding on top of the 
main shunt field winding which adds flux proportional to armature amps to 
"help" the main field produce torque as load goes up.  Unless you change 
it's polarity in reverse, it will do the opposite with load - decrease 
main field flux and motor speed increases.  That's why no field weakening 
in reverse and higher speed under load.  The reason it's used is to 
compensate for "armature reaction" (current in the armature "makes" flux 
which cancels field flux and causes speed to rise inherently under load).

- You always want full field when running less than rated armature volts 
for maximum torque on a DC motor.

- If you "lose" the field (bad contactor, shorted turns in the winding, 
bad wiring), the motor will draw very high amps and try to "run away" but 
load prevents that.  Ammeter pegs.

- Series DC motors do not have field leads as such and produce high torque 
at low speeds as the armature current excites the fields.  If unloaded, it 
tries to run away so series motors are always loaded (like car starters, 
ship propulsion, locomotive traction motors) and do provide inherent load 
sharing capability between multiple motors.

.....Walt
GE Motor Division
Erie PA

-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu
[mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Christopher
Zach
Sent: Thursday, July 13, 2006 7:40 PM
To: Ferguson Apiaries
Cc: elec-trak cosmos phy tufts edu
Subject: Re: (ET) field rev.


Ferguson Apiaries wrote:
> I have a New Idea 15 that I converted.  Got fed up with the controls so 
> made my own.  I have a simple controller for the armature and 4 switches 
> hooked up to the original resistors for the field.  My question is when 
> I have it in high and least resistance to the field and put it in 
> reverse it just speeds up by about 50%, why?  Nice feature though.

As discussed, compound motor. This is designed to provide more field 
(and thus keep the motor from blowing up) as the load on the armature 
goes up.

However it does not reverse with the main field, and the result is that 
it counters the main field in reverse. This is why E20's lock out field 
weakening when going in reverse (and probably E15's too)

Chris


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