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Chapter 12

Chapter 12. Transformers. Objectives. Explain the operation of a transformer. Discuss the relationship between mutual induction and transformers. Describe the effect of self induction. Calculate the various values of currents and voltages in transformer circuits. Objectives.

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Chapter 12

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  1. Chapter 12 Transformers

  2. Objectives • Explain the operation of a transformer. • Discuss the relationship between mutual induction and transformers. • Describe the effect of self induction. • Calculate the various values of currents and voltages in transformer circuits. Permission granted to reproduce for educational use only.

  3. Objectives • List three types of transformer losses. • Identify delta and wye transformer connections. • Discuss grounding an electrical circuit. • Explain troubleshooting procedures for transformers. • Describe several special transformer applications. Permission granted to reproduce for educational use only.

  4. Transformer Theory • Induction • Turns ratio principle • Transformer power • Transformer losses • Impedance • Phase relationship • Taps Permission granted to reproduce for educational use only.

  5. Transformer Theory (Cont.) • Primary and secondary windings Permission granted to reproduce for educational use only.

  6. Transformer Theory (Cont.) • Core- and shell-type construction Permission granted to reproduce for educational use only.

  7. Induction • Voltage can be induced on a conductor with motion and a magnetic field • With a coil, motion, and a magnetic field, an emf is produced Permission granted to reproduce for educational use only.

  8. Induction (Cont.) • Mutual induction • No physical motion in transformer • Rising and collapsing magnetic field produces motion Permission granted to reproduce for educational use only.

  9. Induction (Cont.) • Self induction • Separate fields combine to form an overall magnetic field • EMF produced in direction opposite to force generating magnetic field Permission granted to reproduce for educational use only.

  10. Turns Ratio Principle • To increase or decrease a voltage, increase or decrease number of turns on the secondary winding Permission granted to reproduce for educational use only.

  11. Turns Ratio Principle (Cont.) • Primary of a transformer connects to the source • Secondary of a transformer connects to the load Permission granted to reproduce for educational use only.

  12. Turns Ratio Principle (Cont.) • Relationship between primary and secondary voltages and current values Permission granted to reproduce for educational use only.

  13. Turns Ratio Principle (Cont.) • Voltage ratio • Current ratio Permission granted to reproduce for educational use only.

  14. Turns Ratio Principle (Cont.) • Step-up transformers • Step-down transformers Permission granted to reproduce for educational use only.

  15. Transformer Power • Power on primary side equals power on secondary side • PP = PS • Expressed in VA (volt-amps) or kVA (kilovolt-amps) Permission granted to reproduce for educational use only.

  16. Transformer Power (Cont.) • Find power in primary • Find power in secondary • PS = IS × ES • IS × ES = IP × EP = 900 VA Permission granted to reproduce for educational use only.

  17. Transformer Power (Cont.) • Power companies raise voltage to supply cities with electricity • Wires of least resistance capable of carrying strong current Permission granted to reproduce for educational use only.

  18. Transformer Losses • Copper losses • Eddy current losses • Hysteresis losses Permission granted to reproduce for educational use only.

  19. Impedance • Combination of conductor resistance and the self induction • Total resistance value Permission granted to reproduce for educational use only.

  20. Phase Relationship in Transformers • Output voltage should be in phase or 180 degrees out of phase with primary voltage • Polarity of induced voltage is opposite to voltage producing it Permission granted to reproduce for educational use only.

  21. Taps • In secondary winding • Allow a transformer to provide different voltages Permission granted to reproduce for educational use only.

  22. Review What is a transformer? Device used to transfer energy from one circuit to another using electromagnetic induction Permission granted to reproduce for educational use only.

  23. Review What is the ability to produce electrical energy in a conductor without making physical contact with the conductor? Induction Permission granted to reproduce for educational use only.

  24. Review How is voltage induced on a conductor? With motion and a magnetic field Permission granted to reproduce for educational use only.

  25. Review What produces an emf? A coil, motion, and a magnetic field Permission granted to reproduce for educational use only.

  26. Review If two coils are coupled together by a rising and collapsing field, what kind of induction are they using? Mutual induction Permission granted to reproduce for educational use only.

  27. Review What does increasing or decreasing the number of turns on the secondary winding affect? It increases or decreases the voltage Permission granted to reproduce for educational use only.

  28. Review What is the current ratio? Ratio between currents in secondary and primary, inversely proportional to turns ratio Permission granted to reproduce for educational use only.

  29. Review What unit is used to express power in a transformer? Volt-amps (VA) or kilovolt-amps (kVA) Permission granted to reproduce for educational use only.

  30. Review What are three types of transformer losses? Copper losses, eddy current losses, and hysteresis losses Permission granted to reproduce for educational use only.

  31. Review What is impedance? Total opposition to current in an ac circuit, a combination of the conductor resistance and self induction Permission granted to reproduce for educational use only.

  32. Review What is a tap? Fixed electrical connection made to a winding at a point other than its terminals Permission granted to reproduce for educational use only.

  33. Typical Distribution Systems • Power generated at plant, voltage stepped up • Energy is routed onto transformers, voltage stepped down • Residential areas have lower voltage distribution systems Permission granted to reproduce for educational use only.

  34. Typical Distribution Systems (Cont.) • High voltage results in lower current values • Less current means less power lost Permission granted to reproduce for educational use only.

  35. Delta and Wye, Three-Phase Power Systems • Similar to generators • Windings connected in series are delta connected • Windings connected in parallel are wye connected Permission granted to reproduce for educational use only.

  36. Delta and Wye, Three-Phase Power Systems (Cont.) • Only two lines of delta connection are equal to 120 V, the other is equal to 200 V • High leg Permission granted to reproduce for educational use only.

  37. Grounding • Provides safe path for fault current • If one conductor is not connected to earth, a conductor-to-ground voltage reading will behave similarly to an open circuit Permission granted to reproduce for educational use only.

  38. Troubleshooting Transformers • Megohmmeter • Disconnect load, insulate secondary leads, fuse primary with small value fuse Permission granted to reproduce for educational use only.

  39. Review How are delta and wye connections made in transformers? Delta connections are in series, and wye connections are in parallel Permission granted to reproduce for educational use only.

  40. Review What is the function of a ground? Provides a safe path for fault current Permission granted to reproduce for educational use only.

  41. Review What is a megohmmeter? An ohmmeter that uses higher than normal voltages and accurately reads resistance into the megohm values Permission granted to reproduce for educational use only.

  42. Special Transformer Applications • Autotransformers • Induction circuit breakers • Lighting ballasts • Coupling transformers • Isolation transformers • Automobile ignition coils Permission granted to reproduce for educational use only.

  43. Autotransformers • Isolation for transformers • One winding without isolation • Used in low-voltage applications Permission granted to reproduce for educational use only.

  44. Induction Circuit Breakers • Inductor triggers a switch to shut off current • Strength of magnetic field breaks circuit • Used to protect equipment or prevent fires Permission granted to reproduce for educational use only.

  45. Lighting Ballasts • Start lamp by producing voltage • Limit current through lamp • Use capacitors to reduce power loss Permission granted to reproduce for educational use only.

  46. Coupling Transformers • Separate sections of an amplifier system • Impedance matching • Make sections compatible with one another Permission granted to reproduce for educational use only.

  47. Isolation Transformers • Used to separate commercial power systems from sensitive equipment power supply systems • Transient voltages Permission granted to reproduce for educational use only.

  48. Automobile Ignition Coils • Type of autotransformer • High turns ratio develops 30,000 V or more Permission granted to reproduce for educational use only.

  49. Electric Resistance Welding • When electrical current passes through metal, friction caused by resistance heats the metal • Includes spot welding, projection welding, seam welding, and upset welding • Welding current can be ac or dc, but resistance spot welding typically uses ac Permission granted to reproduce for educational use only.

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