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Re: (ET) Motor Swap & kill a watt
As a GE Motors Engineer for the past 28 years and an ex-DC motor
designer, I can offer some comments on this thread and motor design theory.
- Brushes are angled to the commutator to provide brush stability in
either direction (called a reaction brushholder). On uni-directional
machines, it is better for brush wear if the holder is angled with
rotation.
- Neutral is the brush yoke position for optimal commutation and most ET
motors were tested in both directions. I'm not sure if these can even be
changed or adjusted.
- Motor theory
Torque = Armature Amps x Field Flux x Machine Constants
On the compound or stabilizing shunt wound drive motors, main field
flux is provided by the shunt field power (low amps) that comprise of many
turns of small wire around the main poles. When volts are applied to the
armature (A1-A2), rotation is provided by the electromagnetic forces
created by having a wire run perpendicular through a flux field (provided
by the main field). These moving conductors in the armature create a
voltage known as EMF (Electro-motive-force) at the commutator under the
brushes. Each segment on the commutator connects to an armature coil
(which is a loop of wire down the core and back to the next segment) and
all armature conductors are in series. Current through the brushes
provides continuous current to these coils in series so they can produce
torque by going through the main field flux. The brushes also short out
the current in individual coils as they pass underneath to allow it to
reverse to keep up with the other coils ahead of it. Higher armature
voltage allows more speed with same torque but exceeding the design
voltage causes commutation issues and possible flashover as the brushes
cannot "commutate" (transfer current fast enough). If you reduce the main
field current (field weakening) the motor will run faster but loose torque
(again, within limits).
Now, as current increases, armature amps passing through the armature
tend to weaken the main field flux and the motor speed will rise with
load....not desirable. So, a few turns of "series" is wound on top of the
main field to boost it as load goes up as the same current runs through
this series as the armature so they "cancel" amp turns....result - motor
speed more constant with load.
Mike in KY wants to power a Zero-turn mower with 2 compound motors but
the series field needs to be "switched" to remain cumulative (add) and not
differential (subtract) from main field flux when running one motor in the
opposite direction. The drive motor is reversed by reversing the field
which makes the series field differential but the designers felt it would
not be for long enough periods to be a problem. Remember, reversing the
armature OR field polarity reverses the DC motor.
Series motors (snow blower) do not have a separate main field and are
designed for continuous loaded operation. We've made 3,500HP drilling rig
thruster motors and it would be a real mess if the propeller came out of
the water or the shaft broke....here's why. Series wound motors generate
main field flux as armature amps pass directly through the series field
wound on the main poles. At light loads, speed is high because there is
less flux....as load increases, flux increases and the motor slows down.
That's why the ammeter pegs when you turn the snowblower on and comes down
to low amps with the load of the drum spinning. When you hit the snow
bank the motor slows down drawing a lot of amps to provide maximum
torque/power. If you "2 speed it" by voltage, you will lower the speed
proportionally and burn the watts in the resistor. You can still reach
high loading but I think it's better to leave it as-is for cooling and
performance.
The mower motors are permanent magnet....main field flux provided by
magnets which can become discharged if you pull the rotor in and out too
many times. Then, the motor will run faster with less torque.
Hope this rendition helps....I am trying to get more motor details from
the old files but they are hard to find within GE if they even exist.
...Walt
Erie PA
-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu
[mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Jerry Rhodes
Sent: Tuesday, December 26, 2006 9:27 PM
To: ralphgv talkamerica net; elec-trak cosmos phy tufts edu; David C
Robie
Subject: Re: (ET) Motor Swap & kill a watt
Dave the brushes in the 20 motor on my bench are preset for fwd operation.
The brushes an holders are angled to the com
Jerry
----- Original Message -----
From: "David C Robie" <mycroftxx1 juno com>
To: <ralphgv talkamerica net>; <elec-trak cosmos phy tufts edu>
Sent: Tuesday, December 26, 2006 4:56 PM
Subject: Re: (ET) Motor Swap & kill a watt
>
> If motor has to run in reverse, remember that the brushes may be factory
> set for forward operation, not neutral position. This, of course, means
> inefficient operation. It would pay to check this - but wait for another
> answer on this board someone may know whether 20 motor brushes are
> factory set advanced for fwd direction or not. Any sharp electomechtech
> or even some degree engineers (most of em seem to get 'dilberted' a lot)
> would advance em to begin with.
>
> Dave
> Weymouth MA
>
> On Mon, 25 Dec 2006 21:55:03 -0500 Ralph & Elaine Vogan
>
> <ralphgv talkamerica net> writes:
>> David C Robie wrote:
>>
>> >(power). It would be nice to have 'variable power' - using toggle
>> >switches instead of a cam - so you could choose how far to throw
>> the
>> >stuff, or use minimum power for light snowfalls higher for heavier
>> ones.
>> >
>> Good idea. And easy to do. Even one resister and switch would give
>> me
>> two speeds. I noticed that the motor has to run in reverse to drive
>> the
>> thrower the right way. But I don't think that will be a problem.
>>
>> >Why not breadboard the thing and temp tie it on the dashboard (good
>> ole
>> >Duct tape) to see what develops. I have a 20 motor here and if you
>> find
>> >that vari power works dcent and useful, I might consider a motor
>> switch
>> >on my blower just to have the added feature.
>> >
>> Ralph Vogan
>> ralphgv talkamerica net
>>
>
>
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