Can a dc motor spin faster, or use fewer amps, if cooled ?

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 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 room temperature..

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

------------------ 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 overheating. 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.

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 characteristics). 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.

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.

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....

...................

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....

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.

It was an attempt at humour, for Heaven's sake..

I gave the only information that really matters - use the thing within its specification.

It was intended to emphasise the importance of running the thing within its specified environment.. Running without cooling *can* cause a speed change.

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.

etc..

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..

Was that as spectacular as the 750KW Shunt wound DC motor that tripped out it's field controller while running at full speed?

Ok Point takn I guess i did not see the humour in your text.

Tom

Understandable. If I had any skills at writing humour, I would be writing equipment manuals, rather than trying to understand them.. ;)

----------------- 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 current, and possibly a slightly speed (due to increased resistance) than before. If the temperature becomes too high- 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.

Palindrome schrieb:

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...

- Udo

Don Kelly schrieb:

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

Udo

Of course it is.

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 beneficial.

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 an oven.

------------------- True there will be some change in efficiency. Just think, with a temperature change from 90 to 20 C (pushing the insulation) the resistance of copper changes to about 80% of normal resulting in a"Not to be sneezed at" efficiency gain of about 1 % (assuming 5% copper loss). However, to get this cooling will likely result in the need for more energy spent than saved and, typically a physically larger and more expensive motor. In addition, without any adjustment, the field current will rise as the resistance decreases so field losses (excepting PM fields) will pretty well wipe out the gain from lower armature resistance while increased cooling (bigger fans and other means) will eliminate the gain.

In practice, the effect of temperature (within the thermal limits of the insulation ) has only a minor secondary effect on motor performance. Cooling is used, not to improve efficiency but to protect the motor insulation.

Certainly there are many factors entering into the design and operation of motors, DC and AC, and trends in motor design have been in the direction of better insulation allowing higher operating temperatures and physically smaller machines so it is evident that designers consider the gains from this trend to overcome any advantages of cooling below the maximum allowable temperature.

I must apologise to Sue for not catching her joke.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

Absolutely no need - my ability to construct a joke is legendary. ;)

Luckily I am a tad better at engineering... honestly!

Dear Readers,

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

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 :-)

daestrom

Except that windage losses and bearing friction increase in a 'cold' environment, possibly negating any gain from conductor's conductivity improvement.

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 temperature.

daestrom

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I believe you- honestly!