2. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Electricity Principles & Applications Eighth Edition Chapter 12 Transformers (student version) Richard J. Fowler McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 1

3. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. INTRODUCTION Transformer Fundamentals Transformer Efficiency Loaded Transformers Three-phase Transformers Impedance Matching 12 - 2

4. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to. 12 - 3

5. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview The primary takes power from the source. (Page 310) The secondary provides power to the load. (Page 310) Primary and secondary voltages are 180  out-of-phase. (Page 319) Any winding can be used as the primary. (Page 310) With unity coupling, the primary T/V equals the secondary T/V. (Page 312) 12 - 4

6. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Facts About Transformers Transformers operate on mutual inductance. A transformer has a primary winding and a secondary winding. The coefficient of coupling is the portion of primary flux that links the secondary. With 100% coupling, the turns-per-volt ratio is the same for all windings. Transformers can have hysteresis, eddy current, and copper (I 2 R) losses. Transformer losses can be reduced by using silicon steel cores, laminated cores, and small gage wires. As the load on a transformer increases, angle theta decreases. Three-phase transformers use a three-legged core. Transformer windings can be connected in series or parallel. 12 - 5

7. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. + - Load PrimarySecondary Transformer Fundamentals (Page 309) A transformer has a primary coil and a secondary coil. The primary is connected to a source.The secondary is connected to a load. During the first half-cycle,the flux builds up and collapses. This creates a half-cycle of induced voltage in the secondary. - + 12 - 6

8. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Load PrimarySecondary Transformer Fundamentals During the next half-cycle, the flux again builds up and collapses. This creates another half-cycle of induced voltage in the secondary. Notice that the primary and secondary voltages are out-of-phase. + - - + 12 - 7

9. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Primary-Secondary Terminology (Page 310) 90 V Secondary Load Transformers are bidirectional devices. However, either winding can be used as the primary. 120 V Secondary Load This transformer was designed to step 120 V down to 90 V. 120 V Primary Source 120 V Source 90 V Primary 90 V 12 - 8

10. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Turns-Per-Volt Ratio (Page 306) + _ 0.25 V _ + 0.75 V 1.0 V + _ The 4-turn primary with a 1 volt source provides 4 turns-per- volt; therefore, each turn has 0.25 V across it. Each turn will produces a specific amount of flux in the core. This same flux will, in turn, produce 0.25 V in each secondary turn. Thus, a 1 turn secondary provides 0.25 V, and a 3 turn secondary provides 0.75 V. 12 - 9

11. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. The primary takes power from the source. The secondary provides power to the load. Primary and secondary voltages are 180  out-of-phase. Any winding can be used as the primary. With unity coupling, the primary T/V equals the secondary T/V. Concept Review Repeat Segment 12 - 10

12. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview Core and copper losses produce heat and reduce transformer efficiency. (Page 316) Hysteresis, eddy current, and copper(I 2 R) are the three losses in a transformer. (Page 315) Thinner laminations reduce eddy currents. (Page 316) Use laminations that have a narrower hysteresis loop to reduce hysteresis loss. (Page 315) Use larger conductors to reduce copper loss. (Page 317) 12 - 11

13. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Transformer Losses (Page 315) Copper (I 2 R) loss Eddy current loss (I 2 R in the core) These losses are minimized by a narrow hystersis loop, thin laminations, and large diameter wire. Hysteresis loss (green area of the hysteresis loop) B H 12 - 12

14. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Transformer Efficiency (Page 315) Source provides 1640 W Transformer 90 W loss (heat loss) Load 1550 W consumed The transformer core and copper losses cause the transformer to heat up as electric energy is converted to heat energy. Efficiency = 1550 W / 1640 W = 0.945 12 - 13

15. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Transformer-Action Quiz The ____ coil of a transformer is connected to the source. Flux builds and collapses in a core ____ time(s) each cycle. A transformer provides ____ degrees of phase shift between its two windings. Any winding of a transformer can be used as the ____ winding. Core losses can be reduced by using ____ laminations. Transformer inefficiency is caused by core and ____ losses. primary two 180 primary (or secondary) thinner copper 12 - 14

16. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Repeat Segment Core and copper losses produce heat and reduce transformer efficiency. Hysteresis, eddy current, and copper(I 2 R) are the three losses in a transformer. Thinner laminations reduce eddy currents. Use laminations that have a narrower hysteresis loop to reduce hysteresis loss. Use larger conductors to reduce copper loss. 12 - 15

17. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview Energizing current is the no-load primary current. (Page 318) Primary current is increased by the reflected current when a load is applied. (Page 318) Loading a transformer decreases theta. (Page 318) At any instant, at least two phases are creating flux in a 3-  transformer. (Page 331) Flux density shifts from leg to leg in a 3-  transformer. (Page 331) 12 - 16

18. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Load Primary Current (Page 320) V pri I pri With no load, I pri is the energizing current (I en ). The transformer acts like an inductor. Theta is large; power is low. I en I ref I pri With a load, I pri is composed of I en and I ref. I ref is the resistive load current reflected from the sec- ondary to the primary. 12 - 17

19. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Flux In A Three-Phase Core (Page 331) Phase 3Phase 1Phase 2 Phase 1 coil Phase 2 coil Phase 3 coil Core Time 1 At time 1, flux created by phase 2 There is no phase-1 flux because phase-1 current is zero. and phase 3join together. 12 - 18

20. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Flux In A Three-Phase Core Phase 3Phase 1Phase 2 Phase 1 coil Phase 2 coil Phase 3 coil Core Time 2 At time 2,flux created by phase 2 and phase 3 still join together. Also, flux created by phase 1and phase 2join together. ( Time 2) 12 - 19

21. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Flux In A Three-Phase Core Phase 3Phase 1Phase 2 Phase 1 coil Phase 2 coil Phase 3 coil Core Time 3 At time 3, (Time 3) flux created by phase 1 and phase 2 still join together. Also, flux created by phase 1 and phase 3 join together. 12 - 20

22. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Flux In A Three-Phase Core Phase 3Phase 1Phase 2 Phase 1 coil Phase 2 coil Phase 3 coil Core Time 4 At time 4, (Time 4) flux created by phase 1 and phase 3 still join together. Also, flux created by phase 2and phase 3join together. 12 - 21

23. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Primary-Current Quiz The no-load primary current is called the ____ current. The additional primary current caused by loading the transformer is called the ____ current. Angle theta ____ when a load is connected to a transformer. A transformer with no load acts like a(n) ____. In a three-phase transformer, at least ____ primary coils are creating flux at any instant. energizing reflected (or resistive) decreases inductor two 12 - 22

24. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Repeat Segment Energizing current is the no-load primary current. Primary current is increased by the reflected current when a load is applied. Loading a transformer decreases theta. At any instant, at least two phases are creating flux in a 3-  transformer. Flux density shifts from leg to leg in a 3-  transformer. 12 - 23

25. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview Windings can be connected series-aiding or series-opposing. (Page 328) For parallel connection, like instantaneous polarities are connected together. (Page 327) The series winding with the smallest I rating determines the available current. (Page 328) Transformers can provide impedance matching between load and source. (Page 324) 12 - 24

26. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Series-Opposing Windings (Page 328) 120 V + + + _ _ _ 8 V 2 A 6 V 3 A Connect two terminals with the same instantaneous polarities. 2 V _ + Take the output from the other two terminals. 2 A The output voltage equals the difference between the two voltages. The current is limited to the lesser of the two winding currents. 12 - 25

27. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Series-Aiding Windings (Page 328) 120 V + + + _ _ _ 8 V 2 A 6 V 3 A Connect two terminals that have opposite instantaneous polarities. 14 V _ + Take the output from the other two terminals. 2 A The output voltage equals the sum of the two winding voltages. The current is limited to the lesser of the two winding currents. 12 - 26

28. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. (Page 327) 120 V + + + _ _ _ 9 V 4 A 9 V 4 A Connect the negative terminals 9 V _ + Take the output from the negative and positive terminals. 8 A The two windings must have equal voltage ratings. The two windings should have equal current ratings. and the positive terminals. V output = V winding andI output = 2 x I winding Parallel Windings 12 - 27

29. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Impedance Matching (Page 324) 100  10 V 0.1 A 1 W 20 V 100  10 V 0.1 A 1 V 10 V 1  1 V 1 A 1 W 10:1 ratio 20 V 100  10 V 0.1 A Notice the source in both circuits provides 0.1 A at 10 V. Thus, the transformer makes the 1-  resistor act like a 100-  resistor in terms of the load on the source. 12 - 28

30. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Series and Parallel-Windings Quiz Series-opposing windings produce a voltage equal to the ____ of the voltages of the two windings. The current rating of ____ connected windings is that of the winding with the smallest current rating. A 12-V, 2-A secondary series aiding a 6-V, 3-A secondary will provide ____ V and ____ A. For parallel windings, ____ instantaneous polarities should be connected together. Parallel windings must have equal ____ ratings. A transformer used to make a load appear to be other than its true value is called a(n) ____ ____ transformer. difference series- 18, 2 like voltage impedance- matching 12 - 29

31. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Repeat Segment Windings can be connected series-aiding or series-opposing. For parallel connection, like instantaneous polarities are connected together. The series winding with the smallest I rating determines the available current. Transformers can provide impedance matching between load and source. 12 - 30

32. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. REVIEW Transformer Fundamentals Transformer Efficiency Loaded Transformers Three-phase Transformers Impedance Matching 12 - 31