Методическое пособие для студентов iv курса очного и заочного обучения специальности 26. 05. 07 "Эксплуатация судового электрооборудования и средств автоматики"
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UNIT V. TRANSFORMERS 1. Read and remember the following words and word combinations.
2. Read and translate the text. PURPOSE, CONSTRUCTION, TYPES A transformer is an electromagnetic device converting an alternating current of one voltage into an alternating current of another voltage but of the same frequency. Although there are many types of transformers and a great variety of different applications, the operation principle is the same in each case. A transformer is composed of a laminated iron core and two separate windings. The core is made of electrical sheet steel. The winding, to which the voltage U1 is led, is called the primary winding W1. The one from which the voltage U2 is collected, is called the secondary winding W2. The operating principle of a transformer is based upon the phenomenon of mutual induction. The currents in the transformer windings are inversely proportional to their voltages. In construction, transformers are divided into two types - a bar- type transformer and an ironclad transformer. Their cooling may be either airy or oily. On board ships, in accordance with the work safety rules, only air- cooled transformers are used. In the number of phases, transformers are one-phased, two-phased, or multiphased; in the number of windings they are either two-circuit or multicircuit, there are also transformers which have windings with special tappings. In application, transformers are classified as power transformers, special purpose transformers and autotransformers. 3. Match the following English and Russian word combinations. a) bar-type transformer, ironclad transformer, air-cooled transformer multiphased transformer, mutual induction, power transformer, special purpose transformer, work safety rules b) стержневой трансформатор, трансформатор спец. назначения, силовой трансформатор, многофазный трансформатор, трансформатор с воздушным охлаждением, взаимоиндукция 4. Answer the questions. 1. What kind of electromagnetic device is a transformer? 2. What elements is a transformer composed of? 3. What is the transformer core made of? 4. What sort of windings does a transformer have? 5. What is the operating principle of a transformer based upon? 6. In what proportion are the transformer currents to the transformer voltages? 7. What types of transformers do you know according to their 1)construction, 2)number of phases, 3)number of windings, 4)application? 8. What kind of cooling is used in shipboard transformers? 5. Read and translate the text. SPECIAL TRANSFORMERS Special transformers are available in three types: 1) current measuring transformers, 2) voltage measuring transformers, 3) welding transformers. Transformers measuring current and voltage are designed to widen measurement limits of ammetres, voltmetres, wattmetres etc. in a.c. circuits. Above all, they provide operational safety of measuring instruments in a high voltage network. The rated secondary voltage of voltage transformers is equal to 100V, the one of shipboard voltage transformers is 127V, therefore measuring instruments should be rated in accordance with this voltage. The rated secondary current value of current transformers is equal to 5 A, the one of shipboard transformers should be 1A. Fuses are not mounted in current transformers' secondary circuits since in case they burn off, the transformer magnetic flux and the emf of the secondary open winding will go up to dangerous values: winding insulation breakdown, overheating of the core and electric shock hazard of an operating personnel become possible. If measuring transformers are connected to a high voltage network their secondary windings and their bodies are grounded. Welding transformers are applied for arc welding by using an alternating one-phase current. Their external characteristic is steeply drooping. The voltage in the secondary winding ranges between 30- 70V. They supply the arc across the reactive resistance, i.e. the reactor. The current value may be adjusted by varying the air-gap of the reactor magnetic conductor the current value of which may rise to 2,500A high. On board ships only air-cooled transformers are in use. They differ in the following types: - one-phase dry open transformer, - one-phase dry waterproof transformer, - one-phase dry splashproof transformer, - three-phase dry waterproof transformer, - three-phase dry splashproof transformer, - one-phase dry ironclad waterproof type. 6. Answer the questions. 1. What types are special transformers available in? 2. What are current measuring transformers and voltage measuring transformers designed for? 3. In accordance with what should measuring transformers be rated? 4. Why are fuses not mounted in current transformers' secondary circuits? 5. In what case are the measuring transformers' secondary windings and bodies grounded? 6. How is the value of the primary voltage obtained? 7. What are welding transformers applied for? 8. What range does the voltage in the welding transformer's secondary winding vary in? 9. How can the welding transformer's current value be adjusted? 10. What types of air-cooled transformers are in use on board ships? 7. Read and translate the text. AUTOTRANSFORMERS The primary and the secondary windings of ordinary transformers interact electromagnetically. Autotransformers only have one winding which is primary and secondary at the same time. This winding is arranged on a closed magnetic conductor; therefore, the conventional primary and secondary windings of an autotransformer are not only connected electromagnetically, but also electrically. The common part of an autotransformer winding is wound by the wire of smaller section, which makes it possible to save the mass and the volume of the copper expended and, therefore, to reduce the steel mass. When the steel mass is decreased, the magnetic losses are decreased too; this being so, autotransformers are cheaper, but their efficiency level is higher in comparison with that of ordinary transformers of the same power. The ratio between voltages and currents in an autotransformer is identical to that of an ordinary transformer: Autotransformers are used for starting a.c. electric motors, regulating light, etc. Their use is prohibited, however, in low voltage portable lighting because of the electrical type of connection between the windings of lower and upper voltage, which can lead to the penetration of high voltage in a low voltage network and cause injury of attending personnel. Autotransformers may be of a step-down and step-up type, one- phased and multiphased. If the autotransformer winding possesses a large number of tappings, the secondary voltage may be adjusted in the range between zero and the mains voltage. The disadvantage of autotransformers is the electrical type of connection between the secondary and primary circuits, which makes their field of application narrower. 8. Answer the questions. 1. How many windings does an autotransformer have? 2. Is the autotransformer winding primary and secondary at the same time? 3. Where is the autotransformer winding arranged? 4. In what way are the conventional autotransformer windings connected to each other? 5. What makes it possible to save the mass and the volume of the copper expended in autotransformers? 6. Why are autotransformers cheaper but their efficiency level is higher than that of ordinary transformers of the same power? 7. Where are autotransformers used? 8. What can lead to the penetration of high voltage in a low voltage network and injure the attending personnel? 9. What types of autotransformers do you know? 10. What is the disadvantage of autotransformers? 9. Complete the sentences with the correct form of Present Simple Active. Choose from the verbs in the box. Use each verb only once. become, show, provide, supply, have, range, interact, be, cause 1) Figure 12 in the last column . . . that the shift is equal to 360. 2) Current and voltage measuring transformers . . . operational safety of ammetres, voltmetres, wattmetres etc. 3) The winding insulation breakdown and the overheating of the сonductor . . . possible. 4) The voltage in the secondary windings . . . between 30 70 V. 5) Welding transformers ... the arc across the reactor. 6) Autotransformers only . . . one winding. 7) The transformer primary and secondary windings . . . electromagnetically. 8) The penetration of high voltage in a low voltage network … injury of an attending personnel. 9) The welding transformers' external characteristic ... steeply drooping. 10. Make one sentence from two using the Participle. 1) A transformer is an electromagnetic device. It converts an alternating current of one voltage into an alternating current of another voltage. 2) The current produces the alternating magnetic flux in the core. It passes through the primary winding. 3) We obtain the transformer ratio. We divide one equation by another. 4) The alternating magnetic flux in the core induces the emf of self-induction. It cuts the turns of the primary winding. 5) We obtain the value of the primary voltage U1 in a measuring circuit. We multiply U2 of the secondary winding by the transformer ratio k. 6) An autotransformer is a device. It has one winding - the primary and the secondary at the same time. 7) The transformer magnetic flux and the emf of the secondary open winding go up to dangerous values. They cause the insulation breakdown of the winding and the overheating of the core. 11. Speak about transformers. |