Last summer, our refrigerator stopped working. Alarmed that the entire contents of the fridge may be lost, we moved everything from the freezer to another freezer that was in the pantry. But the contents of the refrigerator were more of a problem. A couple of picnic coolers would suffice for a couple of hours, but not for days.
My son (a student in a university EE program) and I disassembled the rear of the freezer section, and we found the condenser coils frozen, so they were working correctly. The actual cause of the failure was that the circulating fan had stopped. Without the circulating fan, the cold air from the coils does not chill either the freezer or the refrigerator section below. The appliance was crippled.
Thinking the motor bearings might be seized, we took out the motor to lubricate it, and we found that the fan turned easily. It was not the bearings. When we checked the continuity of the motor windings, we discovered that it was an open circuit.
At least now we knew we had to replace the motor. It would take two or three days for the new motor to arrive, and, in the meantime, we would have no refrigerator. This is a big deal when you have four boys living at home.
While I went online to find a replacement for the motor, my son, not knowing any better, decided to unwrap the insulation tape on the motor and see what he could do. The motor has hundreds of turns of fine enameled wire, about 32 gauge, on a plastic core. I figured an open circuit was probably buried under dozens of layers. However, upon removing the tape, my son found an obviously melted wire on the surface of the windings.
I thought this would be a great time to explain to him what happens when a transformer gets one shorted turn. Magnetically, this one turn shorted out the whole transformer, which is why there was enough energy to melt the copper wire into metal balls. The enameled insulation was so thin that the motor vibration must have been enough to chafe the insulation of two adjacent or overlapping wires, causing a shorted turn, which then melted to form an open circuit.
My son did not assume my belief that the motor was ruined, and proceeded to unwrap about two turns in each direction. He asked me to solder it back together. Finally, a light bulb went off. I realized that you can repair small motors if you can access the bad windings, so we soldered the wires back together, and voila! The motor worked again. It had three fewer windings, but out of, say, 300 windings, this is a miniscule change.
We were able to reassemble the refrigerator/freezer and get the food chilling. The lesson? Don’t assume it’s impossible just because you've never done it before. The spare motor arrived three days later. It’s in the shop now, waiting for the next (unfixable) failure.
This entry was submitted by Chris Kelly and edited by Rob Spiegel.
Chris Kelly holds a BS from the University of New Mexico and an MS in computer science from Colorado State University. He has served as a design engineer with Agilent Technology since 1983, and he designs small electronic instruments, such as function generators, and DMMs. He has been named on six US patents in the areas of data acquisition and conversion, instrument calibration, and signal generation.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Click here to access the Sherlock archives.