Transformers are said to have "additive" or "subtractive" polarity based on their physical arrangement of terminals and the polarity of windings connected to the terminals. The vector group in the nameplate of the transformer gives information about such phase shift. Unlike single-phase transformers, three-phase transformers may have a phase shift due to different winding configurations (for example, a wye connected primary and a delta connected secondary), resulting in a multiple of 30 degree phase shift between H1 and X1 bushing designations. Alphanumeric designations are typically in the form H 1 for primaries, and for secondaries, X 1, (and Y 1, Z 1, if more windings present). A dot may be used, or an alphanumeric designation. Two methods are commonly used to denote which terminals present the same relative polarity. Both leads behave like a continuous circuit, one current entering into the first lead and another current leaving the second lead. The second coil will, therefore, show an induced current opposite in direction to the inducing current in the first coil. In the case of two windings wound around the same core in parallel, for example, the polarity will be the same on the same ends: A sudden (instantaneous) current in the first coil will induce a voltage opposing the sudden increase ( Lenz's law) in the first and also in the second coil, because the magnetic field produced by the current in the first coil traverses the two coils in the same manner. Leads of primary and secondary windings are said to be of the same polarity when instantaneous current entering the primary winding lead results in instantaneous current leaving the secondary winding lead as though the two leads were a continuous circuit. In signal circuits, reversed connections of transformer windings can result in incorrect operation of amplifiers and speaker systems, or cancellation of signals that are meant to add. Reversed connections of paralleled transformer windings will cause circulating currents or an effective short circuit. A reversed instrument transformer winding may defeat protective relays, give inaccurate power and energy measurements, or result in display of negative power factor. Maintaining proper polarity is important in power system protection, measurement and control systems. The convention is that current entering a transformer at the end of a winding marked with a dot, will tend to produce current exiting other windings at their dotted ends. These markings may be found on transformer cases beside terminals, winding leads, nameplates, schematic and wiring diagrams. In electrical engineering, dot marking convention, or alphanumeric marking convention, or both, can be used to denote the same relative instantaneous polarity of two mutually inductive components such as between transformer windings. The low voltage side of the instrument transformer, with dot and X1 marking. For similar terms, see Dot notation (disambiguation).Īn instrument transformer, looking at the high voltage side with dot convention and H1 marking.
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