TESTING THE TRANSFORMER
There are several tests that can be made to deter- mine the condition of the transformer. A simple test for grounds, shorts, or opens can be made with an ohmmeter, Figure 19–26. Ohmmeter A is connected to one lead of the primary and one lead of the secondary. This test checks for shorted windings between the primary and secondary. The ohmmeter should indicate infinity. If there is more than one primary or secondary winding, all isolated windings should be tested for shorts. Ohmmeter B illustrates testing the windings for grounds. One lead of the ohmmeter is connected to the case of the trans- former and the other is connected to the winding. All windings should be tested for grounds and the ohmmeter should indicate infinity for each winding. Ohmmeter C illustrates testing the windings for continuity. The wire resistance of the winding should be indicated by the ohmmeter. Each winding should be tested for continuity. If the transformer appears to be in good condition after the ohmmeter
test, it should then be tested for shorts and grounds with a megohmmeter or “megger.” A megger will reveal problems of insulation breakdown that an ohmmeter will not.