From Power Electronics Design: A Practitioner's Guide
7.7 Transformer Connections
Autotransformer connections are often a convenient way to obtain odd voltages from standard transformers. Figure 7.14 shows how a 1-kVA stepdown transformer can be connected as an autotransformer with the secondary connected in a buck connection for 360-V output and in a boost connection for 600-V output. Note how high the circuit kVA capabilities are compared to the transformer rating. Several caveats should be noted in connection with autotransformers, though. First, the isolation between primary and secondary is lost. Second, the secondary is now at primary voltage above ground, and it may not be insulated for this voltage. In general, such connections will probably be safe for transformers rated to 600-V class, but the potential problems should be kept in mind.
Figure 7.14: Autotransformer connections.
Transformers are often equipped with primary taps for adjustment of the output voltage. It is generally easier and safer to bring out low- voltage than high-voltage taps on step-up transformers. When the primary turns are lowered by connecting to the lower-voltage tap, the secondary voltage increases over the nominal tap. Figure 7.15 shows the relationships with a constant primary voltage of 480 V. If the primary voltage is 5% low, connecting to the 5% tap will restore the secondary voltage to 4160 V.
Figure 7.15: Transformer primary taps.
Paralleling transformers is a common way of increasing the power capability over that of a single transformer. Some care should be exercised in paralleling to be sure one of them will not be overloaded.
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