CHAPTER
4 Troubleshooting Dry-Type Transformers
D
ry-type transformers are a part of most electrical installations. They range in size from small doorbell transformers to three-phase 25-kVA units installed in electrical closets (Figure 4-1) to large, free-standing units rated at several hundred kVA (Figure 4-2). Electricians must know how to test for and diagnose problems that develop in transformers—especially in the smaller, drytype power-supply or control transformers.
Open Circuit If one of the windings in a transformer develops a break or “open” condition, no current can flow and therefore, the transformer will not deliver any output. The symptom of an open-circuited transformer is that the circuits, which derive power from the transformer, are de-energized or “dead.” Use an AC voltmeter or DMM to check across the transformer output terminals, as shown in Figure 4-3. A reading of 0 V indicates an open circuit. Then take a voltage reading across the input terminals. If voltage is present, this indicates that one 49 Copyright © 2007, 2000, 1996 by The McGraw-Hill Companies, Inc. Click here for terms of use.
