Single & Three Phase Transformers SAMPLE. Learner Workbook. Version 1. Training and Education Support Industry Skills Unit Meadowbank

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1 Single & Three Phase Transformers Learner Workbook Version 1 Training and Education Support Industry Skills Unit Meadowbank Product Code: 5634

2 Table of Contents Introduction... 5 Section 1. Transformer Construction... 7 Section 2. Transformer Operation Section 3. Transformer Losses, Efficiency and Cooling Section 4. Transformer Voltage Regulation and Impedance Section 5. Transformer Auxiliary Equipment and Tap-Changers Section 6. Auto-Transformer and Instrument Transformers Section 7. High Voltage Safety Procedures Section 8. Transformer Connections & Harmonics Section 9. Parallel Operation of Transformers Review Questions - Answers Sample Theory Test Sample Theory Test Sample Practical Test Sample Theory Test 1 - Answers Sample Theory Test 2 - Answers A Glossary of Transformer Terms Appendix Resource Evaluation Form TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

3 Section 1. Transformer Construction Purpose In this topic you will develop knowledge of the construction of single and three phase transformers. Objectives At the end of this topic you should be able to: Describe the principle of mutual induction of a transformer Describe the construction of the various types of lamination style and core construction used in single phase, three phase, auto and instrument transformers Identify the different winding styles used in transformers Describe the construction of transformer tanks for distribution transformers List the types of information stated on transformer nameplates Describe the methods used to insulate low and high voltage transformers Perform basic insulation resistance, continuity and winding identification tests. References Jenneson, J.R., 2010, Electrical Principles for the Electrical Trades, 6th Edition, McGraw-Hill, Sydney Hampson, J., 2011, Electrical Trade Principles - A Practical Approach, 2nd Edition, Pearson Education, Sydney Phillips, P., 2012, Electrical Principles, 2nd Edition, Cengage Learning, Melbourne. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 7 of 156

4 Student exercise 1 Calculate the induced voltage in a coil of 433 turns which is connected to a 50 Hz supply, when the maximum flux is 2.5 milliwebers. Student exercise 2 A single phase transformer is wound with 200 turns on the primary and 60 turns on the secondary. The maximum value of core flux is 12.5 mwb and the frequency is 50 Hz. Determine: (a) (b) The primary voltage The secondary voltage. Page 8 of 156 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

5 Student exercise 3 How many turns are required on each winding of a :415 volt 50 Hz single phase transformer if the core flux is 0.03 weber. Student exercise 4 A 300/32 volt, 50 Hz single phase transformer has 600 turns on the primary winding. Determine the maximum value of core flux. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 9 of 156

6 Practical exercise: Transformer identification and testing Task Given a double wound single phase transformer conduct a continuity test to identify the separate windings Test the insulation resistance of a double wound single phase transformer Identify the windings of different types of double wound transformers. Equipment 1 Insulation resistance tester Type 1 Voltmeter Type Range Unmarked double wound transformer Type Ratio Three types of double wound step-down transformers, each randomly labeled as X, Y and Z Connecting leads Risk Assessment Identify any hazards, list the supervision level (D, G or B), list the risk class (A, B or C) and list control measures required in the table below: Hazard Identification Supervision Level Risk Class Control Measures Procedure 1. Select and name an appropriate instrument to conduct the continuity test: 2. Mark each terminal of the transformer A, B, C and D. 3. Using the winding markings below, conduct the continuity test and indicate each continuity readings as either low resistance (LR) or open circuit (OC). Page 10 of 156 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

7 TABLE 1.1 Winding marking A-B A-C A-D B-C B-D C-D Test results 4. From the above test results, which of the winding markings have connections as a coil? 5. From their conductor cross sectional area, identify the low tension and high tension windings, assuming the transformer is used to step down the voltage. To answer, choose from the winding markings in Step 4. Low tension winding: High tension winding: 6. Select and name an appropriate instrument to conduct an insulation resistance test on the transformer. 7. Conduct an insulation resistance test: Between the transformer windings Between both windings and the core (earth). Record your test results as ohmic values in Table 1.2 below. TABLE 1.2 Winding marking Coil to coil Primary coil to core Secondary coil to core Test results 8. From the test results, state whether this transformer complies with AS/NZS 3000 for connection to the supply. Circle: Yes or No TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 11 of 156

8 9. Place the three different transformers side by side, inspect the leads, identify each transformer by the letter X, Y, or Z using tags, then write X, Y and Z opposite the corresponding transformer type below: Double wound without tapping: Double wound center tapped: Double wound dual secondaries: 10. For each transformer X, Y and Z, use a continuity tester and the diagrams to: Identify each coil Tag and mark each coil end as indicated on the diagrams A, B and C. Figure Visually inspect the coil leads of the three transformers. On the transformers place a tag on each winding to identify the primary and secondary coils. 12. Briefly explain why resistance measurement using an ohmmeter to identify primary and secondary voltage windings would be likely to give incorrect results on large and well designed transformers. 13. Ask your teacher to check the completed practical exercise. Page 12 of 156 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

9 Review questions These questions will help you revise what you have learnt in this topic. 1. Transformer laminations are manufactured from: (A) Copper (B) Corrosion resistant steel (C) Silicon steel (D) Iron. 2. Transformer laminations are prevented from shorting out by: (A) An oxide layer on each lamination (B) Using spacing blocks to provide air gaps (C) Sandwiching PVC sheets between laminations (D) Painting the sides of the laminations. 3. The material most commonly used for transformer windings is: (A) Aluminium (B) Copper (C) Silicon steel (D) Iron. 4. The cross-sectional shape most commonly used for high power transformer winding conductors is: (A) Circular (B) Flat strip (C) Hexagonal (D) Square. 5. The lid of a transformer tank is secured to the main tank by: (A) Welding (B) Bolting (C) Hinging (D) Clamping. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 13 of 156

10 6. Most high-voltage transformers operate in a medium of: (A) (B) (C) (D) Synthetic oil Air Mineral oil Solid insulant. 7. The most common insulation used for the conductors of oil-immersed transformer is: (A) Glass fibre (B) Cotton (C) Mica used? (D) Asbestos. 8. What are the main parts that make up a high-voltage transformer? 9. Does hot-rolled or cold-rolled silicon sheet provide a better material for transformer laminations? Why? 10. What is the ideal cross-sectional shape for a transformer core? 11. How does the practical cross-sectional shape of laminations differ from the ideal one and why is it Page 14 of 156 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

Alternating Current Circuits SAMPLE. Learner Workbook. Version 1. Training and Education Support Industry Skills Unit Meadowbank. Product Code: 5632

Alternating Current Circuits SAMPLE. Learner Workbook. Version 1. Training and Education Support Industry Skills Unit Meadowbank. Product Code: 5632 Learner Workbook Version 1 Training and Education Support Industry Skills Unit Meadowbank Product Code: 5632 First published in August 2012 by Training and Education Support, Industry Skills Unit, Meadowbank