That smell from an overloaded motor: what is it?

Old question, with I’m sure with easy answer.

I put too much ice in the blender and gunned it. The engine was running but the blade didn’t spin for a second or two. Up came that recognizable odor of burnt electric something.

What is that?

That’s from the material used to make the brushes (graphite) vaporizing; I assume you mean the smell you get if you short two wires together and make sparks. Of course, the insulation could also overheat, but that is unlikely if it started right away (unless things were really wrong, like a shorted motor).

Ozone. The electrical arcing between the brushes and the commutator of the motor produces ozone.

Michael63129 is probably right for a blender that isn’t spinning. Something is overheating in the motor to the point of burning. Many blenders spin at a high rate of speed and emit an odor. That odor is ozone created by the electric motor. It’s not really harmful as in the buildup of ozone in the upper atmosphere.

Ok, let me get this right. R. P., you’re saying that during normal operations high speed motors emit ozone–which smells different.

I guess I never have the blender on at high speed for me to notice that smell.

The smell after the blades buck up, and the motor strains, is the brush graphite vaporizing.

How much of that, or for how many few-second bursts, can I do that (not that I want to)? That graphite doesn’t replenish itself…

Doesn’t graphite just vaporize into carbon dioxide?

IANA chemist or an engineer but I did take some interest in ozone when I took high school chemistry. Here’s my take on it and anybody with more knowledge can CMIIW.

Ozone is O3. A combination of 3 Oxygen atoms. It is unstable.

Oxygen as we typically know it is O2. Two oxygen atoms.

The operation of an electric motor will generate ozone which has a distinct odor. Low speed, high speed, whatever. Depending on the size and the speed of the motor it may produce enough ozone that you can smell it. But, ozone is unstable. The extra oxygen atom will combine with another extra oxygen atom and become just more oxygen. Therefore, the smell doesn’t last long and wont stink up your house.

A few years back there were shysters that were selling ozone generators as air cleaners. I guess the idea was to produce more odorless oxygen. I don’t hear much of them anymore.

Again, none of this is to be construed as having to do with the ozone layer in the atmosphere. That’s a whole different subject. If the production of ozone by electric motors were an environmental hazard we would be extinct.

Sorry if this is somewhat of a hijack. If the motor isn’t turning it is overheating and burning something within the motor, not producing ozone. Not good. If that is the case you are shortening the life of the motor and an electric motor can usually last a long, long time.

I guess the “burning something in the motor” (monitored via smell) is what I am wondering now, in the abstract.

Surely engineers have worked out tables for mass-loss in the brushes vs. efficiency for motors, right?

Yes Ozone for sure, plus overheated insulation, if you could go to a light switch, flick it on and off repeatedly, while you nose is up against it, thats ozone, it’ll remind you of a good (or bad) thunderstorm.

new appliances (that aren’t ozone generators) in good shape would likely not produce much ozone. as the contacts wear and you might run at high speed then devices might.

one device that produced a lot are toy electric trains. with the electrified rails and the need to operate with much vibration then these produced ozone. you might remember a characteristic smell when operating these.

The bumper cars at the carnival really put out the smell, iirc.

The OP’s description is of a stalled motor, not a running one. A stalled motor will overheat, and whilst it is not rotating won’t produce ozone, since the contacts are not commutating. So the smell is much more likely to be hot insulation, or I[sup]2[/sup]R smell as it is sometime known :D.

The motor is wound with magnet wire, which is polyurethane laminated. When it gets hot, the lamination cooks and gives off that characteristic burnt electronics smell.

Ditto Vaughn and o’Furniture. Certainly not ozone in a stalled motor. Ozone smells sweet, and wouldnt remind anybody of something burning, if that is any help. Also I am pretty sure that of the things that happen to brushes, vaporizing due to heat isn’t one of them. I think brushes melt at around 5000 C, a good deal higher than anything else in the motor and in fact higher than any other substance. Vaporized carbon in a hot environment would probably oxidize rapidly to CO or CO2 and not have a smell, anyway.

I think the wire is dip coated, not laminated. Only a few kinds of wire are insulated by lamination. Dip coating is more typical for varnishes.

Thanks to everyone.

So, during the stall, it isn’t the graphite, but the insulation. Presumably if the insulation smokes away completely, the wiring will short. Ignorance fought.

But I’m still kind of wondering: what is the rate, and under which conditions, can that burn-to-failure process occur–presumably with various thicknesses of insulation and with varying loads.

I’m wondering if that’s been doped out (no pun intended for SD).

Insulation isn’t going to overheat if as you say it stalls for a “second or two” unless there was a major short, then it wouldn’t work very well (to say the least) even when not stalled (you need a LOT of power to make something the size of a motor, even a smallish one, get hot enough to cook insulation in that time).

But that was the thing: the insulation did overheat/burn, on the logic of (smelled) smoke there’s fire…

Yeas, but I have has plenty of experience (electronics is my hobby) to know that it just can’t heat up that fast unless serious power is being dissipated, unless it was stalled for a minute or more. For an example, check out how long it takes for a motor to fail with the field windings bypassed (which greatly increases the power from anything resembling normal operation, even if stalled).

Motors vary in how much excess heat they can dissipate. It is typical of high speed brush motors to have pretty high power densities, and a cheap one in a home appliance is all the more prone to this. I wouldn’t be surprised if it could smoke in a few seconds. Some other motors, such as some small shaded pole induction motors, can sit stalled indefinitely and only run a little warmer. Indeed, torque motors in the form of high slip induction motors are intended to operate in the stalled condition or even while being spun backwards.