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Some DC motors have a characteristic smell when they run, and today I started wondering what it would be. I noticed it many years ago for some train scale models (probably Märklin), some years later on my father's power drill (handheld, AC powered). Back then I assumed it had something to do with the sparks in there. Now my brushless (and sparkless) cordless drill, a Bosch GSB 10,8, has the same smell when running; another one, DRC 18/4 from Festool, much less so.

The smell is hard to describe, it is not something like a solvent or grease or something that is melting. I’d also say it is strongest for the first few seconds after some idle days.

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    \$\begingroup\$ Burning dust, most likely. \$\endgroup\$
    – Hearth
    Commented Sep 17, 2022 at 17:22
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    \$\begingroup\$ Does it perhaps smell like a that rainy day smell? If so, that's smell of ozone generated by sparks in the air. \$\endgroup\$
    – DKNguyen
    Commented Sep 17, 2022 at 17:29
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    \$\begingroup\$ It sounds like the lubrication used in the unit, oil or grease. \$\endgroup\$
    – Gil
    Commented Sep 17, 2022 at 17:56
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    \$\begingroup\$ @DKNguyen ozone is the answer combined with hot varnish on the coils. \$\endgroup\$
    – D Duck
    Commented Sep 17, 2022 at 18:16
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    \$\begingroup\$ A beleive brushless does not normally have sparks or an arc of any type to generate Ozone. \$\endgroup\$
    – Gil
    Commented Sep 19, 2022 at 14:23

2 Answers 2

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O3 - ozone. All dc motors produce this gas, lightning also makes ozone. Same smell.

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    \$\begingroup\$ Why would brushless dc motors (without a commutator) as explicitly mentioned in the question produce ozone? \$\endgroup\$
    – user1937198
    Commented Aug 6 at 9:10
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Possibly shellac

Shellac was used for fixing inductor, motor, generator and transformer windings. It was applied directly to single-layer windings in an alcohol solution. For multi-layer windings, the whole coil was submerged in shellac solution, then drained and placed in a warm location to allow the alcohol to evaporate.

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  • \$\begingroup\$ I've worked with shellac many times, and it smells differently also when burning. One can smell it e.g. for sealing wax which is often made of shellac (or, nowadays, of cheap wax), and it has a resinous, armoatic smell, quite different from what I smell on the motors. Also, factory new motors have that unknown smell as well, and I'm sure they don't use shellac anymore because it is organic, expensive, slow-drying etc. for this purpose. \$\endgroup\$
    – Simon A. Eugster
    Commented Sep 17 at 8:57

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