Auto TransformersAnkit D Mahapatra School of Electronics Engineering, KIIT University Bhubaneswar - 751024, Odisha India [email protected][email protected] Abstract: This paper presents details about autotransformer, its operation, limitations and applications. Keywords: Transformer,winding,power applications,transmission,electrical isolation The winding has at least three taps where electrical connections are made. One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to I. INTRODUCTION An autotransformer (sometimes called autoformer) is an electrical transformer with only one winding. different voltages, measured from the common end. In a step-down transformer the source is usually connected across the entire winding while the load is connected by a tap across only a portion of the winding. In a step-up transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a portion of the winding. II. OPERATION As in an ordinary transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will halve the voltage. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected. The auto- prefix refers to the single coil rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary winding. Fig 1.Single-phase tapped autotransformer with output voltage range of 40%–115% of input for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130 and 200250). APPLICATIONS Autotransformers in power applications are to frequently interconnect used systems operating at different voltage classes. To increase the distance between electricity supply Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire (opposite phase) out of reach of the train's overhead collector pantograph. it is common to power the trains at 25 kV AC. On long rural power distribution lines. This is an important safety consideration when deciding to use an autotransformer in a given application. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. if the "neutral" side of the input is not at ground voltage. LIMITATIONS An autotransformer does not provide electrical isolation between its windings as an ordinary transfomer does. The variable ratio of the autotransformer compensates for the voltage drop along the line. up to a voltage ratio of about 3:1 . the "neutral" side of the output will not be either. IV. Because it requires both fewer windings and a smaller core. Like multiple-winding transformers. of not equipment are inserted as voltage regulators. In UK railway applications. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. In three phase power have transmission the applications. At autotransformers operate on time-varying magnetic fields and so cannot be used directly on DC. between microphone and a high-impedance amplifier input. .III. A large three-phase autotransformer may have a "buried" delta winding. The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end. to absorb some harmonic currents. In audio applications. an autotransformer for power applications is typically lighter and less costly than a two-winding transformer. A zig-zag transformer provides a path for current that is common to all three phases (socalled "zero sequence" current). as and a for impedance low-impedance autotransformers limitations suppressing harmonic currents and as acting as another source of ground fault currents. so that customers at the far end of the line receive the same average voltage as those closer to the source. special autotransformers with automatic tap-changing Furthermore. A special form of autotransformer called a "zig zag" is used to provide grounding (earthing) on threephase systems that otherwise have no connection to ground (earth).beyond that range a two-winding transformer is usually more economical. tapped autotransformers are used to adapt speakers to constant-voltage audio distribution matching such systems. not connected to the outside of the tank. etc.frequent (about 10 km) intervals. limitations and applications of autotransformer has been presented in this paper. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.org/wiki/File:Tapped_autotransformer. ACKNOWLEDGEMENT The author is grateful to Mr. V. Autotransformer has many limitations and advantages over ordinary transformer. regulators. an autotransformer links the contact wire to rail and to the second (antiphase) supply conductor. It is the Perfect way to safely troubleshoot power supplies.ac.svg [3] Limitations & Advantages – NPTEL site http://nptel.wikipedia. Debani Prasad Mishra.pdf [4] IEEE Paper Template in A4(V1) www.iitm.wikipedia. KIIT University. CONCLUSION Operations. Professor at the School of Electrical Engineering.doc NIT Hamirpur .in/courses/IITMADRAS/Electrical_Machines_I/pdfs/1_11.org/wiki/Autotransformer [2] Image Courtesy – Wiki File http://en. Asst.in/icacc10/ieee_formatting. This system increases usable transmission distance.ac.nith. reduces induced interference into external equipment and reduces cost. REFERENCES [1] Autotransformer – Wikipedia http://en. output sections. Bhubaneswar for providing a free hand in choosing the topic for our choice and allowing us to work as per our will for the completion of this document.