What temperature do motors operate best at?
- Thread starter NickF23
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TylerDurden
100 GW
The colder the better. Resistivity drops with temperature.
See Reid's thread:
http://endless-sphere.com/forums/viewtopic.php?t=525&start=15&postdays=0&postorder=asc&highlight=cooling
See Reid's thread:
http://endless-sphere.com/forums/viewtopic.php?t=525&start=15&postdays=0&postorder=asc&highlight=cooling
knightmb
100 kW
I might add, it's great to have a cold motor within reason. Meaning, don't think that cooling your motor with liquid helium will result in a super-conducting motor :wink:
You also start to compromise metal strength when the temperature reaches a far extreme below 0 (either F or C)
You also start to compromise metal strength when the temperature reaches a far extreme below 0 (either F or C)
Thanks guys. So its just down to the losses from the copper windings? If so its not such a big issue
http://www.cirris.com/testing/temperature/copper.html
I calculate from this that a rise in motor temperature from 20C to 60C gives about 15% increase in losses from the copper windings. I'm not totally sure what percentage of motor losses this consists of and it would of course depend on the motor.
The more serious issue is damage to the magnets which i heard Reid say starts at 80C. So all in all good to be cool but I think I'll maybe put away the liquid helium can :lol:
I did a quick test run with my new passive cooling on my motor and was pleased to see that even at 30 amps continous in didn't more than warm. Whereas before it would get hot at 20 amps. It was a good tip from Fetcher about the lousy cooling of this kind of axial flux motor, as its hard to know how hot it is without disassembling immediatly after a ride.
link to thread
http://endless-sphere.com/forums/viewtopic.php?t=401&start=15
http://www.cirris.com/testing/temperature/copper.html
I calculate from this that a rise in motor temperature from 20C to 60C gives about 15% increase in losses from the copper windings. I'm not totally sure what percentage of motor losses this consists of and it would of course depend on the motor.
The more serious issue is damage to the magnets which i heard Reid say starts at 80C. So all in all good to be cool but I think I'll maybe put away the liquid helium can :lol:
I did a quick test run with my new passive cooling on my motor and was pleased to see that even at 30 amps continous in didn't more than warm. Whereas before it would get hot at 20 amps. It was a good tip from Fetcher about the lousy cooling of this kind of axial flux motor, as its hard to know how hot it is without disassembling immediatly after a ride.
link to thread
http://endless-sphere.com/forums/viewtopic.php?t=401&start=15
roller
1 W
my experience has shown me differently, folks. it's not as black or white as hot and cold.
It's true that cold copper is less resistive than hot copper, of course, but a motor that's doing work, even at high efficiency will absolutely be warm. not hot to touch, but noticeably warm to touch (maybe 50C or about 30C above ambient on housing for BLDC, on shaft for brush PM)
why?
efficiency is powr OUT/pwr in. if you're motor isn't warm, then it is either not giving significant power out or is grossly oversized for the load. If it is oversized, you lose efficiency just moving the eccessive mass that is not needed.
look at a typical power curve (etek curve attached, I think). if this Etek is running at under 1HP, efficiency falls off fast . .. no work out. Stone cold as can be, but crummy efficiency. it's oversized for this application.
even a lowly 75% 1 hp motor does better than a mighty etek in this application.
===============
special note: bear in mind while 'feeling' the warmth of a brush motor, that all the heat is deep inside the motor, exiting through the shaft (and only through the housing via bearings) if it is not ventilated (or a hub motor). It may be a LOT hotter than what you feel at the surface.
It's true that cold copper is less resistive than hot copper, of course, but a motor that's doing work, even at high efficiency will absolutely be warm. not hot to touch, but noticeably warm to touch (maybe 50C or about 30C above ambient on housing for BLDC, on shaft for brush PM)
why?
efficiency is powr OUT/pwr in. if you're motor isn't warm, then it is either not giving significant power out or is grossly oversized for the load. If it is oversized, you lose efficiency just moving the eccessive mass that is not needed.
look at a typical power curve (etek curve attached, I think). if this Etek is running at under 1HP, efficiency falls off fast . .. no work out. Stone cold as can be, but crummy efficiency. it's oversized for this application.
even a lowly 75% 1 hp motor does better than a mighty etek in this application.
===============
special note: bear in mind while 'feeling' the warmth of a brush motor, that all the heat is deep inside the motor, exiting through the shaft (and only through the housing via bearings) if it is not ventilated (or a hub motor). It may be a LOT hotter than what you feel at the surface.
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safe
1 GW
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You are 100% correct.
I've created spreadsheets to compare the powerbands of various motors and discovered that on a big motor like the PMG 132 or Etek that you need at last 100 amps of current at 48 volts in order to get good performance. If you used only 40 amps the efficiency is actually worse.
There seem to be:
1. Small Motors: 250 Watt - 1200 Watt (ebike size)
2. Large Motors: 5hp - 20 hp (motorcycle size)
And there are very few really good quality motors in between. I can't think of a really good quality "stand alone" motor that runs at peak at about 2 hp that is light weight and cheap. The Transmagnetics seems to be one of the better options, but theirs seems to peak at about 1.5 hp.
http://thunderstruck-ev.com/transmagnetic.htm
However, on the small motors heating is the number one problem because you tend to have to apply more current than you ideally should in order to get any real power out of them. So the heating issue is more significant on the small motors. They need to build motors to fill the "gap"...
I've created spreadsheets to compare the powerbands of various motors and discovered that on a big motor like the PMG 132 or Etek that you need at last 100 amps of current at 48 volts in order to get good performance. If you used only 40 amps the efficiency is actually worse.
There seem to be:1. Small Motors: 250 Watt - 1200 Watt (ebike size)
2. Large Motors: 5hp - 20 hp (motorcycle size)
And there are very few really good quality motors in between. I can't think of a really good quality "stand alone" motor that runs at peak at about 2 hp that is light weight and cheap. The Transmagnetics seems to be one of the better options, but theirs seems to peak at about 1.5 hp.
http://thunderstruck-ev.com/transmagnetic.htm
However, on the small motors heating is the number one problem because you tend to have to apply more current than you ideally should in order to get any real power out of them. So the heating issue is more significant on the small motors. They need to build motors to fill the "gap"...
safe
1 GW
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Heat FactorI added the "heat factor" as described in the link:
http://www.cirris.com/testing/temperature/copper.html
...into one of my spreadsheets.
The results are interesting:
At 15C (60F) the peak power is 1.31 hp @ 2513 rpm.
At 80C (175F) the peak power is 1.21 hp @ 2312 rpm.
Which translates to a 7.6% loss of horsepower.It's interesting how this heating actually shifts the powerband lower when it happens... this helps to compensate for the loss, but actually would encourage even more abuse. It's a "viscious cycle" of heat.
(don't forget the spreadsheet is in an old Microsoft Works format, you will need to convert it to use it... sorry)
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TylerDurden
100 GW
roller said:
Sure it is. Heat is wasted energy.
Heat begets more heat due to an increase in resistivity... ask Safe, he's the guy that most recently toasted his motor on a hill.
The etek datasheet shows where the losses are most/least: where do you think the power is going when the efficiency drops... light? Just because you can't feel it, doesn't mean it ain't there.
:lol:
safe
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TylerDurden said:
He didn't make his point clearly enough... but his point was valid.
The point he was making is that on a really big motor you have to push enough current through the motor in order to get the peak efficiency out of it. If you were to restrict the current heavily you would drop below the desired current that the motor needs to run.
You and I are so used to these tiny little motors running WAAAAAY above their rated load that we don't have much experience with big motors like the PMG 132 or the Etek that can be run at very low loads if you want to.
Basically the "bottom line" is that there is a "correct" current for any motor. If you go above that "correct" value you get excessive heat and performance losses. If you go below that level then the motor is "starved" for current and can't run at full performance.He had the right idea, but wasn't forceful enough in making it clear what he meant.
The PMG 132 is the "dream motor" in my opinion... resistance is 1/10 that of the motors we use and the weight is only 25lbs. You can get 20 hp out of them if you want... or 5 hp with very high efficiency. If I had $800 that "had" to be spent (which I don't) it would be my #1 choice.
TylerDurden
100 GW
safe said:
Resistance = heat
heat = losses of eff.
Are we through here yet?
safe
1 GW
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TylerDurden said:
You didn't understand the situation. You guys are talking about two things. You are BOTH right, but "right" about the answers to different questions.
Your "narrow" focus is on resistance and the fact that it's always a bad thing. This is true.
He was focused on the fact that reducing current too low on a large bike motor begins to cut into the efficiency and so you might be thinking:
"Gee, if I lower the current I will make less heat and therefore get better performance."
...which is NOT true for the bigger motors because other factors begin to come into play.
There is a "correct" way to run any motor... resistance is always bad, but running a big motor with hardly any current is bad too...
TylerDurden
100 GW
Oy vey!
Guess why...
:lol:
safe said:
Guess why...
:lol:
safe
1 GW
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TylerDurden said:
BackEmfThe BackEmf is a linear constant tied to rpm:
BackEmf = VoltageConstant * Rpm / RadiansPerSecond
A big motor has more BackEmf than a small one and so in order to get any reasonable performance out of a large motor you need to run enough current through it so as to get good efficiency out of it, but not too much current so as to produce heat related losses.
Again... the "bottom line" is that all motors have their "ideal" current and rpm. The more you can keep you motor in this "ideal zone" the better overall efficiency you get.
The first chart is a big motor running in it's "ideal" current. The second chart is what happens when you "starve" a big motor and suffer the losses that a large BackEmf produces. This was "pretty much" the point the guy was making... heat is a problem for excessive current, BackEmf is a problem for minimal current. (those bigger magnets demand more current to get them running at full steam)
