Transformer cores

Chapter Transformer cores

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Epri/Esec Tpri/Tsec117/Esec9/191/Esec9/117Esec117/913 VProblem 18-2A transformer has a primary-to-secondary turns ratio of exactly 1:9. The voltage at theprimary is 117 V. What is the voltage at the secondary?This is a step-up transformer. Plug in numbers again:117/Esec1/9Esec/1179/19Esec91171053 VThis can be rounded off to 1050 V.A step-down transformer always has a primary-to-secondary turns ratio greaterthan 1, and a step-up transformer has a primary-to-secondary turns ratio less than 1.Sometimes the secondary-to-primary turns ratio is given. This is the reciprocal ofthe primary-to-secondary turns ratio, written Tsec/Tpri. In a step-down unit, Tsec/Tpri <1; in a step-up unit, Tsec/Tpri > 1.When you hear someone say that such-and-such a transformer has a certain turnsratio, say 10:1, you need to be sure of which ratio is meant, Tpri/Tsec or Tsec/Tpri! If youget it wrong, you’ll have the secondary voltage off by a factor of the square of the turnsratio. You might be thinking of 12 V when the engineer is talking about 1200 V. One wayto get rid of doubt is to ask, “Step-up or step-down?”Transformer coresIf a ferromagnetic substance such as iron, powdered iron, or ferrite is placed within thepair of coils, the extent of coupling is increased far above that possible with an air core.But this improvement in coupling takes place with a price; some energy is invariably lostas heat in the core. Also, ferromagnetic transformer cores limit the frequency at whichthe transformer will work well.The schematic symbol for an air-core transformer consists of two inductor symbolsback-to-back (Fig. 18-3A). If a ferromagnetic core is used, two parallel lines are addedto the schematic symbol (Fig. 18-3B).Transformer cores32918-3Schematic symbols forair-core (A) andferromagnetic-core (B)transformers.