power distribution - Siemens

7TRANSFORMERSWarehouse Stock TransformersWarehouse StockApplicationThe Buck-Boost Transformer has four separatewindings, two-windings in the primaryand two-windings in the secondary. The unitis designed for use as an insulating transformeror as an autotransformer. As an autotransformerthe unit can be connected toBuck (decrease) or Boost (increase) a supplyvoltage. When connected in either the Buckor Boost mode, the unit is no longer aninsulating transformer but is an autotransformer.Units are designed for 60Hz applications.Autotransformers are more economicaland physically smaller than equivalent twowindingtransformers and are designed tocarry the same function as two-windingtransformers, with the exception of isolatingtwo circuits. Since autotransformers maytransmit line disturbances directly, they maybe prohibited in some areas by local buildingcodes. Before applying them, care shouldbe taken to assure that they are acceptableaccording to local code.NOTE: Autotransformers are not used inclosed delta connections as they introduceinto the circuit a phase shift which makesthem uneconomical.As insulating transformers these units canaccommodate a high voltage of 120, 240or 480 volts. For units with two 12 voltsecondaries, two 16 volt secondaries, ortwo 24 volt secondaries, the output can bewired for either secondary voltage, or for3-wire secondary. The unit is rated (KVA) asany conventional transformer.OperationElectrical and electronic equipment isdesigned to operate on a standard supplyvoltage. When the supply voltage is constantlytoo high or too low, (usually greaterthan ± 5%), the equipment fails to operateat maximum efficiency. A Buck-Boosttransformer is a simple and economicalmeans of correcting this off-standardvoltage up to ± 20%. A Buck-Boosttransformer will NOT, however, stabilizea fluctuating voltage. Buck-Boosttransformers are suitable for use in a threephase autotransformer bank in eitherdirection to supply 3-wire loads. They arealso suitable for use in a three phaseautotransformer bank which provides aneutral return for unbalanced current. Theyare NOT suitable for use in a three phaseautotransformer bank to supply a 4-wireunbalanced load when the source is a3-wire circuit.ConstructionBuck-Boost Transformers are containedwithin a NEMA 3R, non-ventilated weatherproofenclosure. Wiring compartmentsare located at the bottom. Core and coilassemblies are encapsulated. Insulationsystem temperature is 180˚ C and thewinding temperature rise is 115˚ C. Coilsare copper wound through 3 kva.How To Select The ProperTransformerTo select the proper Transformer for Buck-Boost applications, determine:1. Input line voltage — The voltage that youwant to buck (decrease) or boost(increase). This can be found bymeasuring the supply line voltage with avoltmeter.2. Load voltage — The voltage at whichyour equipment is designed to operate.This is listed on the nameplate of the loadequipment.3. Load KVA or Load Amps — You do notneed to know both — one or the other issufficient for selection purposes. Thisinformation usually can be found on thenameplate of the equipment that youwant to operate.Selection4. Number of phases — Single or threephase line and load should matchbecause a transformer is not capable ofconverting single to three phase. It ishowever a common application to makea single phase transformer connectionfrom a three phase supply by use of oneleg of the three phase supply circuit. Caremust always be taken not to overload theleg of the three phase supply. This is particularlytrue in a Buck-Boost applicationbecause the supply must provide for theload KVA, not just the nameplate rating ofthe Buck-Boost transformer.5. Frequency — The supply line frequencymust be the same as the frequency ofthe equipment to be operated — either50 or 60 cycles.How To Use Selection Charts1. Choose the selection table with thecorrect number of phases for single orthree phase applications.2. Line/Load voltage combinations are listedacross the top of the selection table.Select a line/load voltage combinationwhich comes closest to matching yourapplications.3. Follow the selected column down untilyou find either the KVA or load amps ofyour application. If you do not find theexact value, go on the next highestrating.4. Now follow across the table to the farleft-hand side to find the catalog numberand the KVA of the transformer you need.5. Follow the column of your line/load voltageto the bottom to find the connectiondiagram for this application.NOTE: Connection diagrams show lowvoltage and high voltage connection terminals.Either can be input or outputdepending on Buck or Boost application.6. In the case of three phase loads eitherwo or three single phase transformersare required as indicated in the “quantityrequired” line at the bottom of the table.The selection is dependent on whether aWye connected bank of three transformerswith a neutral is required or whetheran open Delta connected bank of tworansformers for a Delta connected loadwill be suitable.7. For line/load voltage not listed on theselection tables, use the pair listed on thetable that is slightly above your applicationfor reference. Then apply the firstformula at the bottom of the table todetermine “new” output voltage. Thenew KVA rating can be found using thesecond formula.7-20Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog