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).
That insulation is enameled wire wrapped in a powder painted rotor. If some of the smoke was let out, it could have come from the insulation, it could have come from the brushes, it could have cooked off some grease in the sleeve bearings. If the motor is still running, there isn’t enough of a short to prevent it.
And just what do you think would happen if that insulating “structure” broke down? The blower motor would stop running and THEN could be replaced. Blower motors are cheap. Installation labor can be a little or a lot depending on the car. Why not run 'er 'til she stops?
Yes, overloaded electric motors can get very hot, enough to make the wire insulation smoke, or create a burned motor smell, even though the motor is still working. Sometimes, they are just a little worse for the wear and tear but still run ok.
On permanent magnet DC motors, overheating can weaken the field magnets resulting in a motor that makes less torque and is overloaded by a “normal” load. Sometimes the heat melts the epoxy that holds the field magnets in place and then the motor really crashes.
So, what’s fundamentally wrong with the idea of carrying on with the situation until the motor finally gives up? If it’s working now, and one day it will die, then one day the owner will have to replace it or what? Buy a new car? That’s no different that the situation today, so if the owner wants to play the waiting game there’s no good reason why they are wrong.
I suppose an electric motor that was damaged in a prior overheating experience might be more prone to failure and then one day burst into flame and burn the car up; but it seems pretty unlikely. For one thing, the blower motor circuit is protected by a fuse or circuit breaker. The most likely scenario is that when it eventually fails, it just won’t have enough ummpth to turn the fan blade, or the circuit protection devices will turn the circuit off. At which point the motor can be replaced or repaired.
Before any of that happened, the ballast resistor would smoke and/or the fuse would blow. Regardless, I would never NOT investigate when the magic smoke is released from something electrical…
Well maybe a mouse or a snake got in there and jammed it for a while. Normally whenever you let the smoke out of a motor, it is on its way to motor heaven. So pay me now or pay me later. I’ll have to admit though that I let a little smoke out of my wood router and it still seems to function, although I bought another one for the heavy lifting. They’re never the same IMHO.
Some of the electric fans for home use I’ve taken apart to repair use a thermal cut-off fuse to break the circuit in case the blade gets jammed. Often I find that’s the reason why the fan stopped working. I wonder if car blower motors use that?
Those work great in AC motors where the winding is stationary but in a permanent magnet DC motor, the windings spin with the armature.
On a series wound DC motor, the thermal cutout can be in contact with the field winding. If the armature draws too much current, it will also overheat the field windings.
Tomorrow is Saturday, and at 10 am Saturday, that’s when Car Talk used to air on the local NPR station. That was before the program was replaced with another of NPR’s never-ending news and analysis programs. No worries, no news-analysis for me on Saturdays mornings. Tomorrow at 10 am I’ll be playing the weekly podcast I downloaded to my mp3 player.
