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Posted by Daniel Rudy on October 17, 2008, 7:52 am
At about the time of 10/16/2008 6:26 PM, IanM stated the following:
> Daniel Rudy wrote:
>> At about the time of 10/16/2008 4:41 PM, Dave Plowman (News) stated the
>> following:
>>>> I have some additional information. The black and white wires are
>>>> connected to brushes. The DC resistance between the two is about 2.7
>>>> ohms. The red and yellow wires have a DC resistance of 7.3 ohms. So I
>>>> guess they can be wires either series or parallel. What I don't
>>>> understand is if the red and yellow wires are for the field, then why
>>>> bring them out? Why not just connect them internally?
>>> Think the way they are connected - series or parallel - makes a difference
>>> to the motor characteristics. Also separate access to the field can make
>>> speed control easier.
>> Sorry, but I'm not up on that theory. How can the field make speed
>> control easier? For DC motors, I usually use a PWM scheme for speed
>> control.
> You cant reverse it if the field connections are internal. Reversing
> the polarity on both windings will maintain the same rotation direction.
> You need to reverse one winding relative to the other to reverse the
> direction.
>
> REDUCING THE FIELD CURRENT *INCREASES* THE NO LOAD SPEED.
> (dont take it too far, you wont be happy)
>
> Field loss is a critical failure and if unloaded its likely to
> over-speed till it grenades, otherwise the armature current will
> increase till it melts. Its therfore advisable to have a contactor with
> a low impedance coil in series with the field winding to cut power to
> the armature if the field circuit fails. At the minimum for bench
> testing, switch the armature to reverse, not the field.
Well, I just finished doing a bench test on this motor. The results
that I have are listed below. The power source that I used was 4
standard C alkaline cells wired in series for 6v. I removed the gear
reduction and tested just the motor.
field armature
red yellow white black result
-------------------------------------------------------
+ - open open no rotation
open open + - no rotation
+ white yellow - CCW rotation, high torque
- white yellow - CCW rotation, high torque
+ black - yellow CW rotation, high torque
- black - yellow CW rotation, high torque
+ - + - no rotation
Rotation was observed facing the motor. That last one I'm not sure
about...maybe I don't have enough power. But this gives me a few ideas
on power control. I'm thinking of placing a DPDT relay in series
between the armature and field windings for direction reversal, then use
a the standard PWM control from a microprocessor to control speed.
Plus, this will also protect the motor too in case any winding fails it
will open circuit.
There are absolutely no numbers or any other markings on this motor
besides a warning about it being hot. But from what I have been able to
scrounge up on the web. It seems that this motor was manufactured by
Magnetek and is rated at 24v. Stall current is 20. The lead wires are
only 16 AWG, so at stall, even the high temp wires would melt...the
wires are rated at 200c.
--
Daniel Rudy
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> connected to brushes. The DC resistance between the two is about 2.7
> ohms. The red and yellow wires have a DC resistance of 7.3 ohms. So I
> guess they can be wires either series or parallel. What I don't
> understand is if the red and yellow wires are for the field, then why
> bring them out? Why not just connect them internally?