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
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 that not all electrical & electronic devices (if hot) work better if
cooled. If the device is designed to run hot, then cooling it to a
temperature outside its design limits could easily result in it becoming
non-functional or unreliable.
For example, an electronic timing crystal is often put in an oven
designed to heat it to a constant temperature. Attempting to cool it is
not going to be beneficial.. As another example, I have a small pump
designed to circulate molten solder - it won't even turn when cooled to
Not so--- 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
The current is dependent on load torque which may or may not depend on speed
All cooling will do is extract heat more effectively - it will not affect
the production of heat.
Cooling will not allow faster "spinning" nor will it reduce the current. It
will allow higher load torques resulting in higher currents without
excessive temperatures (and lower speeds depending on the motor
These are opposite to what you have said. In general at lower currents and
higher speeds, the need for cooling is reduced.
Motor considerations regarding performance and cooling are quite different
from those of solid state electronics. Tom Grayson has it right.
Don Kelly email@example.com
remove the X to answer
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.
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..
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.
Yes it will - compared to the motor in its failed condition, resulting
from overheating as a result of lack of cooling.
No. This doesn't apply once overheating has caused damage. See above.
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.
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
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
I would be very interested to hear an explanation of the Physics that
this statement for a normally operating motor, not a broken one?
There is another point that should be kept in mind when discussing these
Is the motor, in the discussion, a speed regulated motor driven from a
or is the motor simply connected straight across a fixed DC Source ?
These two types of systems will have somewhat different responses, when
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
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
Changing resistance will simply cause the field voltage to fluctuate, not
and thus FLux. So, with a cooler operating motor, you would not expect the
speed to change
if all other things are held constant.
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
but that is usually the sum of it.
The only times I have seen catastrophic failure due to operating outside
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.
I gave the only information that really matters - use the thing within
It was intended to emphasise the importance of running the thing within
its specified environment.. Running without cooling *can* cause a speed
Irrelevant. If you operate it outside its operational envelope there is
a risk of failure and a dramatic speed change.
I suggest that you are considering only minor excursions from the SOAR.
Think bigger excursions. Ones I have encountered have included:
1) a 12vdc fan that had been fitted as a replacement for a 240v ac one
that appeared identical.
2) A dc motor bilge motor in a small compartment that some bright spark
had filled with expanding poly foam. True, it didn't fail immediately
but lasted 20 mins.
3)A motor used as a liquid metal stirrer that had original been
thermally insulated from the tank using a ceramic sleeve/spacer. Another
bright spark replaced it with a steel one, when it broke. It lasted a
couple of hours.
4)A torpedo motor that relied on sea water cooling that was accidently
run up with the torpedo out of the water. That was exciting and
expensive. It lasted 7 minutes.
No one is disputing that. The question was " Can a dc motor spin faster,
or use fewer amps, if cooled?" The answer was, "Yes it can - if not
cooling it would result in overheating and failure".
Sorry if my attempt to inject some form of humour into the answer fell
on technical ears..
You are trying to compare a corpse to a live person-they may look similar
but behave quite differently. The cause of higher current in the case of a
failure has nothing to do with the normal operation of the motor but depends
on the failure mode. Its electrical and mechanical performance, prior to
failure (which may take appreciable time-rarely immediate) is not determined
by cooling but is dependent on the load so my previous statements stand.
If the motor is not cooled is that it will reach a higher steady state
operating temperature at the <same> current, and possibly a slightly
<lower> speed (due to increased resistance) than before. If the temperature
becomes too high- <then> failure or deterioration of insulation occurs-
rarely, if ever, a sudden situation for motors. Bearings could also fail if
overheated (actually bearing failure is the main caused of motor failure)
and effectively this would add mechanical load resulting in higher currents
and eventual winding failure.
The electromechanical behaviour of a operating rather than a failed/failing
motor is dependent on the motor type and the nature of the load. The
temperature depends on the thermal balance but the current and speed will
not. That is the point that I made and stand by.
A failed motor is just that- a corpse.
Don Kelly firstname.lastname@example.org
remove the X to answer
Aside from Sue's 'humor', I would add that some motors may actually be less
efficient if severely cooled.
Windage losses are a function of air density, which is higher for 'cold'
motors. And most bearing lubricants increase the viscous friction if cold.
And of course, to take Sue's point to the other extreme, some bearings will
fail if the lubricant is too cold to flow properly :-)
A cooled DC motor will have fewer copper losses resulting in a higher
effective voltage for the motor itself regarding the priciple schematic
of this kind of motor (R + L + generator in series).
Don't know what happens to other contruction parameters (motor contants)
in dependence from the temperature...
see above -> fewer copper losses, more efficiency
Sorry, that is not true.
It is much easier to produce heat than cooling. The temperature of a
crystal is held at a temperature that lies above the maximum ambient
temperature so cooling will never be needed to keep the temperature.
The intention is to hold the temperature at a _constant_ value because
of the temperature dependency of the crystal frequency. It would also be
satisfying to keep it at a low temperature with a good regulation concept...
OK, so attempting to cool a crystal that is in a heated oven is not
going to be beneficial. True?
Yep. But I specifically stated a crystal in a heated oven. And that
attempting to cool a crystal that was in a heated oven was not going to
be beneficial. As you state - the whole idea is to keep the temperature
constant. Adding cooling to a crystal in an oven is not going to be
Of course the same constant temperature could be achieved with a crystal
in a refrigerator - in which case cooling would not only be beneficial
but would actually be essential - but I specifically stated a crystal in
This question is quite interesting, Mr.Sue have given good response to
this but I like to add few more points to the discussion.
1. Conductor's condctivity improves by cooling thus reduces losses,
hence the capacity of motor will improve and hence at same loading
current will reduce slightly.
2. Regarding DC motor, it have commutator and carbon brushes. We know
that Carbon Brushes are desined to operate at certain Current density
& temperature & speed. The Platina formation also depends on temp &
humidity in Commutator, thus giving good brush life & low sparking.
May your point be cleared
Except that windage losses and bearing friction increase in a 'cold'
environment, possibly negating any gain from conductor's conductivity
Not to mention various atmospheric contaminants. Hydrocarbon fumes (diesel
fuel) is a common one that will add to the film, while some solvent fumes
will quickly remove all of the coating leaving shiny copper (a bad
situation). And of course, to some extent humidity is a function of overall
The limitations of a DC motor are a simple a function of a few different
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
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.
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