Daniel Rudy wrote:
At about the time of 10/16/2008 4:41 PM, Dave Plowman (News) stated the
following:

In article ,
Daniel Rudy wrote:

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.