alt.engineering.electrical

Chea,
The limitations of a DC motor are a simple a function of a few different
things,

Speed is ultimately limited by how fast the armature can turn before it
physically fails to hold itself together,
however the real limitation comes before this in the form of the velocity of
the commutator segments past the brushes.
Successfull commutation involves reversing the direction of the current in a
particular coil segment from one direction, before
the brushes, to the other direction, after the brushes, If the motor goes
too fast, The current is not able to be stopped
while the two ends of the coil segment are shorted out by the Brush, When
this happens there is increased sparking at load. on the trailing edge of
the brush.
This increased sparking will lead to shorter brush and commutator life, but
of greater concern is the possibility
of a "flash Over" where a spark will jump from segment to segment until
finally a path exists between two adjacent brush arms.
At this time there is destructive failure of the motor. So it is the job of
us control engineers make sure that the motor does not
exceed it's designed maximum speed under load.
This speed limitation does not change with temperature.

Power of the motor is developed by the armature current that interacts with
the Motor Field Flux,
The resultant torque causes the motor to turn against it's load, the speed
of turning is a function of the motor armature voltage
and the motor field flux.
The motor field is a direct function of the current flowing in it, Reducing
the motor field current, will reduce the field flux. this
changes the torque/speed characteristic and is usually a controlled item, to
make the motpr deliver the correct combination of
speed and torque. There is no overall gain in reducing the field current for
any given operating condition.
Torque is a direct function or armature current, so reducing the armature
current does not produce any performance advantage.

Reducing the temperature of the motor will reduce the voltage necessary to
deliver the Field current, but the current must remain the same to
keep the motor operating correctly. Armature current is controlled by the
difference between the supply volts and the counter EMF of the motor
Changing the temperature does not effect either of these.

So what Does Temperature effect?
It effects the resistance of the field pole windings and the resistance of
the various windings in the armature circuit. Keeping the temperature down
reduces the rate of deterioration or the insulation around these components,
thus the need for proper cooling of the motor.
If there was no cooling, the temperaure would soar, causing rapid
deterioration of the insulation material.

The lower temperature may change the magnetic reluctance of the field
components but I have never seen any motor appear to have
different flux levels because of this, while operating in the normal
temperature ranges. 0 to 50 deg C

The motors I have seen in sub zero environments (down to -30 Deg C) have
all operated with their internal temperatures around 30 or 40 deg C.
Perhaps other engineers have experience with actually operating the motor
itself in the sub zero region. I would be interested to hear about that.

This should give you something to work with.

Regards
Tom G

"TE Chea" <4ws@gmail.com> wrote in message news:473c16ed_2@news.tm.net.my...
>I notice electrical & electronic devices ( if hot ) all
> work better if cooled.
> Thanks for any advice.
>
>