[stirling]

I've read all over the place (threads here and elsewhere as well as various docs on and off-line) about motors getting hot (or not) - some people seem to think its a must and others see it as a problem. Sooooo - should they or should they not get hot? Does it depend on what type of driver is used? does it depend on whether the requirement is for high or low speed? Is it all mythical/personal opinion/witchcraft? - will I ever find out the facts?

[mcpltd]

Motors get hot, its what they do, but it comes down to a number of factors like.
Duty cycle, current settings, voltage and ambient temp.

I've read all over the place (threads here and elsewhere as well as various docs on and off-line) about motors getting hot (or not) - some people seem to think its a must and others see it as a problem. Sooooo - should they or should they not get hot? Does it depend on what type of driver is used? does it depend on whether the requirement is for high or low speed? Is it all mythical/personal opinion/witchcraft? - will I ever find out the facts?

[ImanCarrot]

I have an air bearing spindle and heating can be a problem- remember that a 100mm iron rod will change length by about 0.012mm for a ten degree Celcius change.

I liquid cool the spindle to stop this, but I do notice that the machine performs to specification after a weekend lying doing nothing only by performing a 1/2 hour warmup cycle.

I am working to very high tolerances though- not sure if it would be a problem with more relaxed tolerances.

[stirling]

Motors get hot, its what they do

I've built many machines over the years and I've never had any of the motors get anything more than slightly warm. Hence my question.

[mcpltd]

If you push the motor hard, with lots of volts they can get hot.
If it is way overpowered and you are not pushiong lots of volts and Amps through it, it will not even sweat.
Dont get scared if it is getting to 65-70 degrees, this is normal.

I've built many machines over the years and I've never had any of the motors get anything more than slightly warm. Hence my question.

[stirling]

thanks mcpltd. I'll just take this a little further if I may. The main reason I'm asking is that I've come accross statements on the zone of the type "if your motor isn't running hot you're not getting your money's worth".

So in my case(s) where none of the motors ever get more than warm @ an over-driving factor of ~20, it would seem that the suggestion is that I would be better off with a less powerful and cheaper motor which probably would have got hot and hence be better value for money.

However I have trouble with this not least because I'd have thought that a cool motor would last longer than a hot one and so in the end that would be better value for money.

[mcpltd]

You are right, a cooler motor will last longer that one that is being worked like a dog.
Motors are the lower cost part and the differnce in cost between a motor that has a torque of 1.5 Nm and one of 3 Nm is about £10, so why skimp to save £30 on the machine that may not work, when you can sped a bit more money and having a better chance of getting it right.
AS i have allready said, if you dont know what size motor to use, either copy from another machine, or use the biggest motor you can afford and fit.

Hope that helps.

thanks mcpltd. I'll just take this a little further if I may. The main reason I'm asking is that I've come accross statements on the zone of the type "if your motor isn't running hot you're not getting your money's worth".

So in my case(s) where none of the motors ever get more than warm @ an over-driving factor of ~20, it would seem that the suggestion is that I would be better off with a less powerful and cheaper motor which probably would have got hot and hence be better value for money.

However I have trouble with this not least because I'd have thought that a cool motor would last longer than a hot one and so in the end that would be better value for money.

[stirling]

You are right, a cooler motor will last longer that one that is being worked like a dog.
Motors are the lower cost part and the differnce in cost between a motor that has a torque of 1.5 Nm and one of 3 Nm is about £10, so why skimp to save £30 on the machine that may not work, when you can sped a bit more money and having a better chance of getting it right.
AS i have allready said, if you dont know what size motor to use, either copy from another machine, or use the biggest motor you can afford and fit.

Hope that helps.

Thanks - as no one else seems to want to offer a counter view on this I'll be sticking with my present setup and going for cool running.

[pminmo]

Heating is also relavant to heat sink. If the motor is attached to a large aluminum frame, metal to metal, motor heat is transferred to a much larger area, thus will run cooler.

[mcpltd]

As they say "If it aint broke why fix it"

Thanks - as no one else seems to want to offer a counter view on this I'll be sticking with my present setup and going for cool running.

[stirling]

Heating is also relavant to heat sink. If the motor is attached to a large aluminum frame, metal to metal, motor heat is transferred to a much larger area, thus will run cooler.

Of course. But remember that the issue I'm trying to bottom is that some say motors SHOULD be run hot and that ONLY if they run hot are you getting your money's worth. In a bizarre way that suggests that heat sinking is actually a bad idea which is clearly silly (at least I'd have thought so). This I suppose is why I'm trying to nail this one.

[stirling]

As they say "If it aint broke why fix it"

Well I suppose it depends on whether you just want something to work or whether you want to understand why and how it works as well. Then the next time maybe you can make it work better or perhaps make it work the same but cost less.

The story of flight...
Wilbur : Well that went well.
Orville: Yup.
The end.

[mcpltd]

If you are building a new machine, you may be able to save some money, but without knowing the axact power you need, would you risk buying smaller motors if you know that what you have now works well?
If it were me i would stick with what i know works.
All i was saying was that motors can get hot, but that does not mean they should.

Well I suppose it depends on whether you just want something to work or whether you want to understand why and how it works as well. Then the next time maybe you can make it work better or perhaps make it work the same but cost less.

The story of flight...
Wilbur : Well that went well.
Orville: Yup.
The end.

[pminmo]

Of course. But remember that the issue I'm trying to bottom is that some say motors SHOULD be run hot and that ONLY if they run hot are you getting your money's worth. In a bizarre way that suggests that heat sinking is actually a bad idea which is clearly silly (at least I'd have thought so). This I suppose is why I'm trying to nail this one.

"should" is meant as "it's ok, they normally run hot don't be alarmed". If they run cooler because of attachment to a metal body that spreads heat dissipation, thus the motor isn't as hot, maybe a positive thing for longevity. As to temperature and getting moneys worth, this is not an absolute correlation.

[NC Cams]

Motors are designed to operate at an accepted level of temperature rise. Depending on the company, the motor may be rated "cold" or at normal operating temp. Reason: as temp rises, the motor efficiency can decrease and the power going to it gets more readily turned into heat instead of power.

Motors can not stay at room temp due to internal friction as well as losses in the iron and the conducting elements - even pure copper has resistance and power is lost/turned into heat as a result. Friction, iron losses further contribute to energy being wasted by being turned into heat. Higher loads cause more heat. If the loads force the motor to operate outside of its designed load curve, you pay for it in higher, lower efficiency and deminished life.

The ACCEPTABLE temp rise is established by each manufacturer. This is why they usually post rated curves on/for their motors. The more efficient the motor's design, the less power it will lose/turn into heat as it operates. Thus, a 25 deg temp rise for one motor might be "normal" yet a bit high or low for a similar HP rated one from an totally different manufacturer - it all depends.

The "facts" are often truly ellusive but they are attainable. I'd suggest contacting your particular motor maker and obtaining a spec sheet. They useually publish power curves and duty cycles for the motor and acceptable temps that the motor should tolerate.

Sorry for no specific answer but the above should show you how to find it for your particular motor. Essentially there is no hard and fast industry standard rule to support the "...if your motor isn't running hot you're not getting your money's worth..." statemen.

That is essentially an urban legend used by those who justify the practice of running motors somewhat to highly overloaded.

There is some charm in/to the statement "If it is way overpowered and you are not pushiong lots of volts and Amps through it, it will not even sweat". Motors that are NOT overtaxed will usually last a lot longer than one that is punished with repeaded overloading.

The thing that guys who overload motors fail to consider is power consumption.

Buy a cheap, under powered motor and you will be constantly pulling high current thru it. Current costs money. You'r buying power ostensibly as Kwhours but in reality, you're paying for AMP/minutes. The more current you pull during any time interval, the more you pay. You'll either pay for the motor via high maintenance or purchase cost or pay the cost monthly when the power bill comes. Waste power in an overloaded, "hot is how to get your money's worth" motor and you waste money.

The TOTAL "cost" to run the motor and keep it alive needs to also be looked at when you over size a motor or decide to run it hot to "get your money's worth". Do ALL the math and let the figures talk. It may not be wise to listen to and follow the sage wisdom/urban legends....
l

[stirling]

So.... it would seem that the conclusion is that motors don't HAVE to be run hot but that with various considerations they CAN be run hot. Cool...