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12 - 1 Electricity Principles & Applications Eighth Edition Richard J. Fowler Chapter 12 Transformers (student version) McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 2 INTRODUCTION • • • • • McGraw-Hill Transformer Fundamentals Transformer Efficiency Loaded Transformers Three-phase Transformers Impedance Matching © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 3 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. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 4 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. McGraw-Hill (Page 312) © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 5 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 (I2R) 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© 2013 inTheseries or parallel. McGraw-Hill Companies Inc. All rights reserved. McGraw-Hill 12 - 6 Transformer Fundamentals (Page 309) Primary - Secondary + + Load - 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. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 7 Transformer Fundamentals Primary + Secondary - - Load + 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. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 8 Primary-Secondary Terminology (Page 310) Transformers are bidirectional devices. 120 V Source 120 V Primary 90 V Load Secondary This transformer was designed to step 120 V down to 90 V. Load 120 V Secondary 90 V 90 V Source Primary However, either winding can be used as the primary. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 9 Turns-Per-Volt Ratio (Page 306) _ + 0.25 V + _ 1.0 V _ 0.75 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. © 2013 The McGraw-Hill Companies Inc. All rights reserved. McGraw-Hill 12 - 10 Concept Review • 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. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 11 Concept Preview • Core and copper losses produce heat and reduce transformer efficiency. (Page 316) • Hysteresis, eddy current, and copper(I2R) 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) McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 12 Transformer Losses (Page 315) Copper (I2R) loss B H Hysteresis loss (green area of the hysteresis loop) Eddy current loss (I2R in the core) These losses are minimized by a narrow hystersis loop, thin laminations, and large diameter wire. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 13 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 McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 14 Transformer-Action Quiz The ____ coil of a transformer is connected to the source. primary Flux builds and collapses in a core ____ time(s) each cycle. two A transformer provides ____ degrees of phase shift between its two windings. 180 Any winding of a transformer can be used as the ____ winding. primary (or secondary) Core losses can be reduced by using ____ laminations. thinner Transformer inefficiency is caused by core and ____ losses. McGraw-Hill copper © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 15 Concept Review • Core and copper losses produce heat and reduce transformer efficiency. • Hysteresis, eddy current, and copper(I2R) 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. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 16 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) McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 17 Primary Current (Page 320) Load Iref Vpri Ipri With no load, Ipri is the energizing current (Ien). The transformer acts like an inductor. Theta is large; power is low. McGraw-Hill Ien Vpri Ipri With a load, Ipri is composed of Ien and Iref . Iref is the resistive load current reflected from the secondary to the primary. © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 18 Flux In A Three-Phase Core Core Phase 1 coil Phase 2 coil Phase 3 (Page 331) Phase 1 Phase 2 Phase 3 coil Time 1 At time 1, flux created by phase 2 and phase 3 join together. There is no phase-1 flux because phase-1 current is zero. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 19 Flux In A Three-Phase Core ( Time 2) Core Phase 1 coil Phase 2 coil Phase 3 Phase 1 Phase 2 Phase 3 coil Time 2 At time 2, flux created by phase 2 and phase 3 still join together. Also, flux created by phase 1 and phase 2 join together. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 20 Flux In A Three-Phase Core (Time 3) Core Phase 1 coil Phase 2 coil Phase 3 Phase 1 Phase 2 Phase 3 coil Time 3 At time 3, flux created by phase 1 and phase 2 still join together. Also, flux created by phase 1 and phase 3 join together. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 21 Flux In A Three-Phase Core (Time 4) Core Phase 1 coil Phase 2 coil Phase 3 Phase 1 Phase 2 Phase 3 coil Time 4 At time 4, flux created by phase 1 and phase 3 still join together. Also, flux created by phase 2 and phase 3 join together. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 22 Primary-Current Quiz The no-load primary current is called the ____ current. energizing The additional primary current caused by loading reflected the transformer is called the ____ current. (or resistive) Angle theta ____ when a load is connected to a transformer. decreases A transformer with no load acts like a(n) ____. inductor In a three-phase transformer, at least ____ primary coils are creating flux at any instant. two McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 23 Concept Review • 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. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 24 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) McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 25 Series-Opposing Windings (Page 328) _ 8V 2A + + _ 120 V _ + 6V 3A + 2V 2A _ Connect two terminals with the same instantaneous polarities. Take the output from the other two terminals. The output voltage equals the difference between the two voltages. The current is limited to the lesser of the two winding currents. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 26 Series-Aiding Windings (Page 328) _ _ 8V 2A + + _ 120 V _ 14 V 2A 6V 3A + + Connect two terminals that have opposite instantaneous polarities. Take the output from the other two terminals. The output voltage equals the sum of the two winding voltages. The current is limited to the lesser of the two winding currents. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 27 Parallel Windings _ _ 9V 4A + + _ 120 V (Page 327) _ 9V 8A 9V 4A + + The two windings must have equal voltage ratings. The two windings should have equal current ratings. Connect the negative terminals and the positive terminals. Take the output from the negative and positive terminals. Voutput = Vwinding and Ioutput = 2 x Iwinding McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 28 Impedance Matching 100 W 10 V 0.1 A 10:1 ratio 0.1 A 10 V 1W 1V 20 V Notice the source in both circuits provides 0.1 A at 10 V. Thus, the transformer makes the 1-W resistor act like a 100-W resistor in terms of the load on the source. McGraw-Hill (Page 324) 1W 1V 1A 1W 100 W 10 V 0.1 A 100 W 10 V 0.1 A 1W 20 V © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 29 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. difference series- 18, 2 For parallel windings, ____ instantaneous polarities should be connected together. like Parallel windings must have equal ____ ratings. voltage A transformer used to make a load appear to be other impedancethan its true value is called a(n) ____ ____ transformer. matching McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 30 Concept Review • 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. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 12 - 31 REVIEW • • • • • McGraw-Hill Transformer Fundamentals Transformer Efficiency Loaded Transformers Three-phase Transformers Impedance Matching © 2013 The McGraw-Hill Companies Inc. All rights reserved.