Electrical Machinery, electrical machines, transformers
When a transformer is energized, it presents an initial impedance (resistance or load) to the source supply. This starting impedance is different from the running impedance. This impedance depends on a lot of variables like: Input supply voltage Input supply frequency Inductance of transformer (coil and core) Resistance of windings Load connected on the secondary The initial impedance is quite low at the time of energizing and hence there is a heavy inrush current. The inrush current last only for a few cycles of the supply voltage. This info is important as the source should have the capacity to supply the inrush current - other wise the source will trip or fuses in the source will blow off.
A transformer steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. When a transformer is livened, the magnetizing inrush currents are estimated as multiples of the transformer's nameplate kVA. Time Current (sec) - (amps) 0.01 -- 25x 0.10 -- 12x In addition, during the normal life of a transformer you can expect hot-load and cold-load pick-up events. These are estimated as Time Current 1.00 -- 6x 10.0 -- 3x 100. -- 2x You must select transformer fusing that will not be damaged by these events. Drop me an e-mail for a more detailed explanation.
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