Practical considerations

Chapter 9.8 Practical considerations

Lessons In Electric Circuits Volume II – AC Book
Pages 556
Views 6,295
Downloads : 13 times
PDF Size : 3.3 MiB

Summary of Contents

Lessons In Electric Circuits Volume II – AC Book

  • 266CHAPTER 9. TRANSFORMERS• A Scott-T transformer converts 3-φ power to 2-φ power and vice versa.• A linear variable differential transformer, also known as an LVDT, is a distance measuringdevice. It has a movable ferromagnetic core to vary the coupling between the excited primaryand a pair of secondaries.9.8Practical considerations9.8.1Power capacityAs has already been observed, transformers must be well designed in order to achieve acceptablepower coupling, tight voltage regulation, and low exciting current distortion. Also, transformersmust be designed to carry the expected values of primary and secondary winding current withoutany trouble. This means the winding conductors must be made of the proper gauge wire to avoidany heating problems. An ideal transformer would have perfect coupling (no leakage inductance),perfect voltage regulation, perfectly sinusoidal exciting current, no hysteresis or eddy current losses,and wire thick enough to handle any amount of current. Unfortunately, the ideal transformer wouldhave to be infinitely large and heavy to meet these design goals. Thus, in the business of practicaltransformer design, compromises must be made.Additionally, winding conductor insulation is a concern where high voltages are encountered, asthey often are in step-up and step-down power distribution transformers. Not only do the windingshave to be well insulated from the iron core, but each winding has to be sufficiently insulated fromthe other in order to maintain electrical isolation between windings.Respecting these limitations, transformers are rated for certain levels of primary and secondarywinding voltage and current, though the current rating is usually derived from a volt-amp (VA)rating assigned to the transformer. For example, take a step-down transformer with a primaryvoltage rating of 120 volts, a secondary voltage rating of 48 volts, and a VA rating of 1 kVA (1000VA). The maximum winding currents can be determined as such:1000 VA120 V= 8.333 A (maximum primary winding current)1000 VA48 V= 20.833 A (maximum secondary winding current)Sometimes windings will bear current ratings in amps, but this is typically seen on small trans-formers. Large transformers are almost always rated in terms of winding voltage and VA or kVA.9.8.2Energy lossesWhen transformers transfer power, they do so with a minimum of loss. As it was stated earlier,modern power transformer designs typically exceed 95% efficiency. It is good to know where someof this lost power goes, however, and what causes it to be lost.There is, of course, power lost due to resistance of the wire windings. Unless superconductingwires are used, there will always be power dissipated in the form of heat through the resistance ofcurrent-carrying conductors. Because transformers require such long lengths of wire, this loss can