Not too long ago, we were lucky to have on National Public Radio the radio program “Car Talk.” There is still a website for that here.
People would call in with various questions to which Tom and Ray would provide their answers along with much humor and side commentary. However, one caller had an issue which I think Tom and Ray got wrong.
The issue at hand was a dashboard blower that had stopped working. Not only was there no fan action, smoke had been rising up from a dashboard vent.
After this had gone on for some while, the blower started working again. The question was whether this situation needed any repair attention now that things were back to working. Tom and Ray both said that nothing needed any attention. They thought that something had somehow gotten into and jammed the blower and then gotten dislodged, which allowed the blower to resume working. As long as the blower action seemed normal, the blower should be left alone. I had some qualms about that opinion.
It would be my guess that the blower was driven by a permanent magnet DC motor. When the jam occurred and the blower action was halted, the motor ceased generating its back electromotive force or back EMF. With no back EMF, the armature current of the motor would have risen very high and would have led to overheating of the armature conductors. If those conductors got hot enough, the insulation material around those conductors could get burned and up would come that smoke.
When normalcy returned, my further guess would be that the armature conductors were then being mechanically supported by the charred remains of whatever insulating structure had been holding them up. I would have some misgivings about the stability of such a structure but Tom and Ray never pointed out any such possibility.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).