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Unformatted text preview: 9 ‐ 1 Experiment 9 The Autotransformer OBJECTIVE To study the voltage and current relationship of an autotransformer. To learn how to connect a standard transformer as an autotransformer. DISCUSSION There is a special type of transformer which physically has only one winding. Functionally, though, the one winding serves as both the primary and secondary. This type of transformer is called an autotransformer. When an autotransformer is used to step up the voltage, part of the single winding acts as the primary and the entire winding acts as the secondary. When an autotransformer is used to step down the voltage, the entire winding acts as the primary and part of the winding acts as the secondary. Figure 9-1 (a) and Figure 9-1 (b) show autotransformers connected for both step-up and step- down operation. (a) Step-up (b) Step-down Figure 9-1 The action of the autotransformer is basically the same as the standard two-winding transformer. Power is transferred from the primary to the secondary by the changing magnetic field. The amount of step-up or step-down in voltage depends on the turns ratio between the primary and 9 ‐ 2 secondary, with each winding considered as separate, even though some turns are common to both the primary and secondary. Voltages and currents in the various windings can be found by two simple rules: a) Primary apparent power (VA) equals Secondary apparent power (VA)....
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- Spring '08