As an example, reversing the polarity in an instrument transformer winding, for instance, may defeat protective relays, lead to inaccurate power and energy measurements, or result in the display of negative power factor during measurements. This phase loss, and inverse polarity, however trivial it sounds, creates serious problems in power system protection, measurement, and control systems. However, this assumption is not always correct, as the direction of the windings could be opposite (as shown in the image above), which will mean if connection is to the same terminals, then the voltage in the secondary winding (VS) will be out-of-phase and the direction of the current (Is) is in opposite direction to the primary current. The phase relation between primary and secondary currents and voltages depends on how each winding is wrapped around the core, hence if the winding is wrapped around the core in the same direction as shown below then the voltage and current on both sides should be in phase. This assumption is usually based on the belief that the secondary winding and the primary winding of the transformer are done in the same direction. In the study of transformers, it is generally assumed (at least for resistive loads) that the Voltage and Current are in phase for the secondary and primary windings. For instance, for the out-of-phase transformer example above, by switching how the terminals are connected like the image below, the secondary side is made in-phase with the primary. With the knowledge of this convention and the polarity of the transformer, the engineers now have their destiny in their hands, and can decide to reverse the phase relationships in whichever way they want by changing which end of their circuit is connected to the transformer’s terminals.
up on primary, down on secondary or vice versa), like the image below, It indicates that the primary and secondary current and voltages are 180° out of phase and the primary and secondary currents (IP and IS) will be in opposite in direction to one another.
However, if the dots are placed in reversed positions (e.g. Meaning the phase shift between the primary and secondary winding will be zero (in- Phase), and the direction of the Secondary current (Is) and Primary current (Ip) will be the same. When the dots are placed next to the top ends of the primary and secondary windings as shown below, it indicates that the polarity of the instantaneous voltage across the primary winding will be the same as that across the secondary winding.
0 kommentar(er)
