We're in the final section of 7.1, and we'll be looking at troubleshooting inductors. There are only three possible defects probable with an inductor. The first is an open in the windings. That's depicted right there, actually. A short between windings, and a short to ground or another component. Here we have an overview of all the things that can happen to an inductor. We show the defect here, and then what we would expect to see with an ohmmeter.
First of all, if there's no defect and we connected our ohmmeter into the circuit we would measure the resistance of the coil winding. The thing is that this is typically very low. We're talking about with an ohmmeter you're only going to be measuring maybe two to three ohms, so it's a very small value. Remember, you're just looking at a piece of wire. The next one would be open windings. In this case here, we have an open, and in this case, the resistance would be infinite. We would have infinite resistance here. If you read infinite resistance in a coil you have an open. Then, there is the potential of shorted turns. This would be the scenario and it'd usually be shown closer, but this one is maybe shorted to this one. In that case, we would have a lower than normal resistance. The thing is with an ohmmeter this is really hard to see because the normal reading is extremely small, so the shorted turns are often hard to pick up with an ohmmeter. Then, coil-to-core short, and that's the case where you have the coil is shorted to the core material which should not be.
Here, let's see, a common fault with inductors is a shorted coil. That's the one we just mentioned. Usually, it is the result of insulation that has melted due to overheating. It is often very hard to detect the problem with an ohmmeter because of a good reading maybe only two to three ohms. The failed reading may only vary by a fractional portion of an ohm. There are meters which can test for inductance. In fact, your textbook has a picture of an inductance meter, and this type of situation that's probably what you would need. In this graphic here, you see this is a cutaway of an inductor. All of these wires there's insulation wrapped around there. You can see here two wires the insulation has melted and they are actually shorted together and this would cause a failure in an inductor.
This is a depiction here of how to test for the last fault we mentioned was the short between the core and the coil. In this case, we have our ohmmeter connected and this is to the core material. The other end of the ohmmeter is connected to the actual coil itself. Under normal circumstances, there should be no connection. The normal resistance here would be infinite. We would expect to have an infinite reading in this situation. In this situation here it's the same configuration, only now we're reading zero resistance. In this case, zero resistance indicates that there is a short between the windings and the core and this indicates a bad inductor.
This has been a brief lesson looking at procedures you might to troubleshoot an inductor.
Video Lectures created by Tim Fiegenbaum at North Seattle Community College.