Got Carbon Brushes, Dummies?

Spring loaded carbon brushes for an electric motor

The radial arm saw in the woodshop at Antisocial Network HQ has been acting up: the blade wouldn't start, started slowly, or would jam easily. The chief carpenter, having 1) worked in a factory that makes electrical motors and 2) seen his father perform this job, is pretty sure that the brushes failed in the machine, which is almost thirty years old. No one uses a radial arm saw any more, eh? No problem: brushes are designed to wear out and are (relatively) easy to replace. The owner's manual for the saw having finally disappeared, the carpenter tried looking for online help in performing the repair. That's when he accidentally ran across Imani Angulo at – where else? – eHow.com, valiantly attempting to explain to her readers "What are the Functions of Carbon Brush?" (now residing at ItStillRuns.com, decorated by what appears to be a photograph of a carburetor).

Any grammatical failings of the OQ notwithstanding, our carpenter read through Angulo's little article. At the same time, he also read through some content posted by a person who actually knows what carbon brushes are. The difference is... let's just say, "massive"!

For all we know, Imani may be a music savant (though we doubt it), but it's a pretty good bet that she had no inkling that carbon brushes even exist before taking on this title. We say that because she did little more than carelessly reword the Wikipedia article on the topic, starting with her introduction: "Carbon brushes are electromechanical conducting devices that connect to moving parts to provide an electric current. They are typically used in motors, generators and alternators. " In fact, carbon brushes don't "connect to moving parts" -- they contact a rotating motor part, not connect.  Brushes are spring-loaded to maintain that contact and are much softer to prevent damaging the rotor or commutator, but they are wired to the motor or generator's stator. The softness of the material (graphite in some sort of binder) used to make the brushes is the reason that brushes wear out.

Angulo is clearly parroting more authoritative sources when she says things like

"Carbon brushes are a common component in household appliances... Micro-carbon brushes... are precision parts designed for toys, electric razors, and audio and video equipment... Cars generally use small and auxiliary carbon brushes for DC motors, starters and alternators... Automotive carbon brushes are used to supply [sic] the current to the wire coil in the alternator... Industrial carbon brushes are used on large commutator machines, in which the current between the rotor and external circuit is periodically reversed. These include AC and DC motors [sic]..."

Perhaps you noticed that while formulating a rather extensive list of devices that use carbon brushes, Imani only rarely approaches the discussion of their function. When she does, she typically gets it wrong: she has the function of an alternator backward: the brushes take power from the commutator, not supply it. Angulo also doesn't know enough about electrical generators to differentiate DC from AC: "the current between the rotor and external circuit is periodically reversed" only in an AC generator, not in DC -- that's where the "A" in AC comes from: alternating current!

No, it's clear that here we have another example of a humanities graduate (BA in Jazz? give us a break!) nattering on about technical stuff after having assiduously avoided taking any science courses. And now you know why Imani is picking up a Dumbass of the Day award!

copyright © 2016-2021 scmrak

SE - ELECTRICAL