How much can a motor handle

[Horshu]

I'm building a motor from scratch but I'm curious how to determine how many batteries I can throw at the thing before risking burning it up. Is there some standard method for calculating a motor's voltage/amperage rating?

[Turnination Guy]

You need to stop posting and read these threads first.

http://tinyrc.com/forums/showthread.php?threadid=15964
http://tinyrc.com/forums/showthread.php?threadid=15428

Then, before you post again, go here:

http://www.starterupsteve.com/swf/posting.html?

After this, you may post again.

[Horshu]

Actually, until a moderator tells me otherwise, I can post whenever the hell I want to, but that is between the moderator and me. I have read both of those threads in full, but the reason I posted separately is because I am not asking how to wind a motor or what kind of gauge to use. The threads you mention are specific to certain voltages and certain windings; I am not asking about that. And neither am I asking for smartasses who assume every post they don't like violates some rules they adhere to to tell me how they think I should post.
What I am asking for is a more general methodology for determining what fries the motor such that I don't determine if my motor can handle 15V by frying several. Rather, I am looking for rules of thumbs such as X gauge wire multiplied by Y turns divided by Z winds = <some derivative of maximum voltage>. This isn't "can my 130 handle 4 lith-ios" or "how many turns should I put on my XMods," but rather, "what factors destroy a motor, in what manner, and how can the effect of the factors be estimated prior to an applied test".

[Turnination Guy]

Well clearly you should have been more specific in your first post then. You probably should have titled this thread “Motor Failure Modes”.

But the stock electronics will give out far before the motor does. Motors burning out are ALMOST a non-issue, because of the electronics limitations.


The main thing to kill motors is friction, and its effects on the motor. Friction causes heat, which will cause moving parts to heat up and eat through whatever they are in contact with faster. As you should have noticed from reading the forums like I told you to, heat is best eliminated by properly ventilating the motor can. The armature will circulate air like a fan, and keep the motor as comfortable as it can be. Ultimately, the friction based wear on the motor is what will kill it.

For example, on a motor being fed by direct power, the first thing to go might be the brushes, which would get ground down because of the increased wear at high speeds. This would keep the rest of the motor safe, assuming it actually is the first thing to go. Fortunately, the brushes are replaceable parts.

Of course, this also depends on whether you have bearings or bushings inside the motor can and end-bell. If you have bushings, they could very well be ground down enough that the commutator timing is thrown off, which would cause some very strange motor behavior. The motor can and end-bell can be replaced with ones that have bearings in them, to prevent that problem.

Nothing is going to happen to the magnets (unless they are Neodymium; they will die if they are overheated, or take a sharp blow of some kind), and I doubt anything will happen to the armature; and I don’t think we are dealing with wattages that would overpower our wire gauges, so that should be fine too.


If I am right (and I would certainly appreciate a second opinion on this), for our purposes it is not the electrical limitations of the motor that would make it fail, but rather the mechanical ones. The voltage and current constraints, fundamentally, are caused by the problem of loading up the car with battery weight. Unless there is some kind of freakish battery breakthrough tomorrow (and there isn’t), you won’t find a practical way to overload the car with electricity when building for RC purposes.

Alternately, if you are trying to figure out what kind of windings of what gauge of wire are safe to use, I already showed you where to get that. Anything outside of the range that those guys use probably isn’t of any use for RC purposes at all.

[Horshu]

Ahh, OK. Very good info. Thank you very much. The basic scenario I'm looking at is: external turbo w/ regulated voltage to keep the board stable at high voltage, vented ball-bearing can with possibly extra scoops drilled in, thick silver wire with multiple turns to allow for higher voltage, carbon brushes for hardness lubed with comm drops to aid friction. I'm additionall going to wrap my turbo board in copper, ala hi-perf RAM heatsinks that connect to the motor for heat dissipitation. When I start building some specific configurations (waiting on a few different components to build a few motors), I'll feed it some different voltages and post my results in the Winding thread.

[Turnination Guy]

Your end bell has a bearing too, yes?

You seem to have neglected the magnets. Using better magnets will outright add torque to a motor without significant penalty (maybe it adds negligible weight?). This also lets you trade your newly gained torque for more speed by changing the windings on the armature. Neodymium magnets ARE recommended, with some precautions:

You can get more powerful Neo magnets, but as you go up the power scale, their heat tolerances will go down. So the more precisely you know how much heat your setup will generate inside the motor, the better magnets you can get away with. But you’d have to be REAL careful to not operate outside of your estimate; as I said before, too much heat can kill the magnets. However, there ARE Neo magnets that are plenty temperature safe for all your racing needs.

Also, all magnets suffer from a permanant loss of power if they are ever dropped or jarred. This is mostly an installation problem, since the cars are plenty cushioned by suspension and giving materials. But if you are using a custom chassis, jarring collisions may be a concern.


There is also a very cheap source of improved magnets, the Spinbrush. By dissecting a Spinbrush toothbrush you can get at the better magnets inside the motor that powers it.

I don’t know just how much heat sink you are adding, but remember that weight is a factor. Effective use of ventilation can reduce temperature and save you weight. If you can, I suggest mounting a plate of fins on the underside of the car, thermally connected to the motor. It’s probably the ONLY use of ground effects in our scale, using them to draw heat away.

[moebrown20]

You must use high-grade bearings also.
Some cheap sets are rated @ 60k rpms, some @80-85k, the more expensive ones have the best ratings. They keep the heat down as well.

[Horshu]

Yeah, I'm still waiting on my parts (been almost a month) so I can order bearing, as I'm not sure what size I'm going to need. But right now, all the bearings in my car are rated at 100k. I couldn't find ratings on the motor-sized bearings, but once I get a better idea of the size, I'll research it some more.

[moebrown20]

it depends on what can you're using. Most are 2x5x1.8 or 2x5x2.5