alt.engineering.electrical

Sue,
With all due respect, If the only way you can give any sort of credence
to your argument is to say that
if the motor was cooled it would have not failed and in this working state,
it would turn faster then
it will in the failed state, then you really do not have much of an argument
at all.

Yes, in one sense, you are right, a working motor usually turns faster then
a broken one, Big deal.
I believe the original poster was after some usefull information, not some
cute logic.

You wrote....
>As for your motor, if cooling is required to keep its internal temperatures
>to within design limits, then indeed it is highly likely that it could spin
>faster and/or use fewer amps if cooled.
...................

I would be very interested to hear an explanation of the Physics that
supports
this statement for a normally operating motor, not a broken one?

There is another point that should be kept in mind when discussing these
things.
Is the motor, in the discussion, a speed regulated motor driven from a
controlled drive
or is the motor simply connected straight across a fixed DC Source ?

These two types of systems will have somewhat different responses, when
exposed to
different ambient temperatures.
Consider an unregulated shunt connected DC motor with fixed voltage applied
across it's field and armature.
With decreased motor temperature, The field resistance will go down, and
thus for
the same applied field voltage, the field current will be greater. From
basic DC Motor Theory
If all other things are held constant, and the field current is increased,
the speed will go down
not up !!!!!!!!!!

With a fully controlled motor, The field will be "current regulated" to some
set value,
Changing resistance will simply cause the field voltage to fluctuate, not
the current,
and thus FLux. So, with a cooler operating motor, you would not expect the
speed to change
if all other things are held constant.

you wrote....
> Read my previous paragraph, "Operating outside that range will generally
> result in immediate failure or a dramatic decrease in reliability".
>

This is not true.
in my 28 years of large DC motor Experience I have yet to see a motor
immediately fail when operated outside it's specified limits,
At the very worst, the commutator will spark excessively, and brush life is
reduced
but that is usually the sum of it.
The only times I have seen catastrophic failure due to operating outside
limits
has been when it has been run to say 150% of maximum rated speed or voltage
or been subject to a large disturbance whilst in heavy regeneration.

In general, DC Motors are very forgiving devices. They take punishment well.








"Palindrome" wrote in message
news:Kzb%i.209311$g82.109745@fe07.news.easynews.com...
> Don Kelly wrote:
>> "Palindrome" wrote in message
>> news:3ZU_i.152062$Nb1.132967@fe01.news.easynews.com...
>>> TE Chea wrote:
>>>> I notice electrical & electronic devices ( if hot ) all
>>>> work better if cooled.
>>>> Thanks for any advice.
>>>>
>>>>
>>> All equipment is designed to work within a certain range of
>>> environmental conditions. Operating outside that range will generally
>>> result in immediate failure or a dramatic decrease in reliability. Even
>>> within the range there can be wide variations in expected life and
>>> efficiency.
>>>
>>> As for your motor, if cooling is required to keep its internal
>>> temperatures to within design limits, then indeed it is highly likely
>>> that it could spin faster and/or use fewer amps if cooled.
>> ------------------
>> Not so---
>
> Of course so. If a motor overheats due to lack of cooling to the point
> where insulation fails - then it is highly likely to run slower (indeed
> actually stop turning) and take a great deal of current - Cooling would
> indeed have made it spin faster and use fewer amps..
>
> Read my previous paragraph, "Operating outside that range will generally
> result in immediate failure or a dramatic decrease in reliability".
>
> "If cooling is *required*..... it uses fewer amps is cooled. --- It will
> typically take enough amps to trip the breakers if not cooled, if cooling
> is a requirement.
>
>> all that cooling will do is allow the motor to be run at above its rated
>> load torque-either continuously or for a longer time without overheating.
>
> No. Not if cooling is required to run at its rated output. *Additional*
> cooling is required to allow it to run over its rated output..
>
>> The current is dependent on load torque which may or may not depend on
>> speed (voltage related).
>> All cooling will do is extract heat more effectively - it will not affect
>> the production of heat.
>
> No. The production of heat will change dramatically once the motor fails
> due to overheating. Cooling can prevent overheating and thus can affect
> the production of heat.
>
>>
>> Cooling will not allow faster "spinning" nor will it reduce the current.
>
> Yes it will - compared to the motor in its failed condition, resulting
> from overheating as a result of lack of cooling.
>> It will allow higher load torques resulting in higher currents without
>> excessive temperatures (and lower speeds depending on the motor
>> characteristics).
>> These are opposite to what you have said. In general at lower currents
>> and higher speeds, the need for cooling is reduced.
>
> No. This doesn't apply once overheating has caused damage. See above.
>>
>> Motor considerations regarding performance and cooling are quite
>> different from those of solid state electronics. Tom Grayson has it
>> right.
>>
>
> I didn't say that he had not. I was simply making the point that speed can
> reduce and current can increase as a result of a reduction in cooling.
> Thus, cooling can result in lower current and higher speed.
>
> --
> Sue