Learner Workbook Version 1 Training and Education Support Industry Skills Unit Meadowbank Product Code: 5633
Contents Introduction... 5 Section 1: Operating Principles of Three-Phase Induction Motors.7 Section 2: Three-Phase Induction Motor Construction... 23 Section 3: Three-Phase Induction Motor Characteristics... 37 Section 4: Single Phase Motors Split Phase... 67 Section 5: Single Phase Motors - Capacitor & Shaded Pole Type.. 81 Section 6: Revision, Consolidation and Assessment 1... 95 Section 7: Single Phase Motors - Series Universal... 105 Section 8: Three Phase Induction Motor Starters... 117 Section 9: Reduced Voltage Three-Phase Induction Motor Starters.....135 Section 10: Braking and Rotation Reversal of Three-Phase Induction Motors... 163 Section 11: Motor Speed Control... 181 Section 12: Revision, Consolidation and Assessment 2... 195 Section 13: Motor Protection... 205 Section 14: Three-Phase Synchronous Machines - Operating Principles and Construction... 227 Section 15: Alternators and Generators... 237 Section 16: Three-Phase Synchronous Motors... 255 Section 17: Single Phase Synchronous Motors... 269 Section 18: Revision, Consolidation and Assessment 3... 275 Review Questions - Answers... 290 Sample Theory Test 1 - Answers... 306 Sample Theory Test 2 - Answers... 308 Sample Theory Test 3 - Answers... 310 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
Section 1: Operating Principles of Three-Phase Induction Motors Purpose In this section you will learn about the fundamental principles of operation of threephase induction motors. Objectives At the end of this section you should be able to: apply the right hand (grip) rule for conductors and solenoids and Fleming s left and right hand rules to determine circuit operating characteristics USAṠAMPLE list the characteristics of the magnetic field produced by a single, two and three-phase winding calculate the speed of rotation of a rotating magnetic field calculate the rotor speed, slip and rotor frequency describe the basic principle of operation of an induction motor reverse the direction of rotation of a three-phase induction motor. References Jenneson J.R., 2010, Electrical Principles for the Electrical Trades, 6 th Ed., McGraw-Hill, Sydney. Hampson, J., 2011, Electrical Trade Principles - A Practical Approach, 2 nd Ed Pearson Education, Sydney. Phillips P., 2012, Electrical Principles, 2 nd Ed, Cengage Learning, Melbourne. Wildi, T., 1991, Electrical Machines, Drives and Power Systems, 2 nd Ed., Prentice Hall, TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 7 of 316
Learner Exercises Learner Exercise 1.1 Using the right-hand conductor rule, and the conventional symbols for current flow, show the direction of current flow and the magnetic field direction. Figure 1.1 Learner Exercise 1.2 Using the right-hand solenoid rule, indicate on the diagram the magnetic field polarity of the coils drawn below. Learner Exercise 1.3 Figure 1.2 Use Fleming s left-hand rule to determine direction of force, field polarity or direction of current flow in the diagrams below: N S force force S N direction of force? field polarity? direction of current flow? Figure 1.3 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 9 of 316
Learner Exercise 1.4 Use Fleming s right hand rule to determine direction of induced emf, or direction of conductor movement in the diagrams below: N motion N motion S S S motion N Hz. direction of direction of direction of conductor induced emf? induced emf? movement? Figure 1.4 Learner Exercise 1.5 Determine the synchronous speed of the rotating magnetic field (RMF) of an eight pole three-phase winding if the supply frequency is 50 Hz. Learner Exercise 1.6 supply frequency is 25 Determine the number of poles per phase if the RMF rotates at 1500 rpm when the Page 10 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
Learner Exercise 1.7 Determine the supply frequency required to cause the RMF of a four pole winding to rotate at 1200 rpm. Learner Exercise 1.8 A four pole 50 Hz induction motor operates with a rotor speed of 1440 rpm. Determine the slip speed. Learner Exercise 1.9 Determine the slip % for the motor described in Learner Exercise 1.8. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 11 of 316
Learner Exercise 1.10 An induction motor operates with a 2% slip. If the synchronous speed of this motor is 1500 rpm, determine the actual rotor speed. Learner Exercise 1.11 Determine the rotor frequency of the motor in Learner Exercise 1.10, if the supply frequency is 50 Hz. Learner Exercise 1.12 Connect the motor terminal block 2 to reverse the direction of rotation. Figure 1.5 Three Phase Induction Motor Reversal Page 12 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
Practical Exercise 1.1: Three-Phase Induction Motor Reversal Task To reverse the direction of rotation of a three phase induction motor. Objectives At the completion of the exercise you should be able to: measure the supply phase sequence to determine the direction of rotation of a three phase induction motor reverse the direction of rotation of a three-phase induction motor by interchanging any two supply leads. Equipment Your teacher will provide you with the specifications of the equipment to be used: Safety Specifications Three phase supply Three-phase induction motor Triple pole isolating switch Rating Phase sequence indicator Type Connection leads Remember: Work safely at all times! Isolate supply before connecting or altering circuits Always select the correct test equipment Be careful when working near rotating machines. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 13 of 316
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. Connect the equipment as shown below leaving line two (L2) disconnected. Figure 1.6 2. Energise the supply and close the motor isolating switch. Record the effect on the operation of the motor. Motor rotation =. 3. Open the motor isolating switch and isolate the supply. Connect line two to the motor isolating switch. 4. Energise the supply and monitor the phase sequence of the supply with the phase sequence indicator, as shown in the diagram below: supply terminal L1 L2 L3 ACB ABC phase sequence indicator motor isolating switch Figure 1.7 three phas induction motor Record supply phase sequence =. Page 14 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
5. Close the motor isolating switch and record the effect on the operation of the motor. Motor rotation =. 6. Open the motor isolating switch and isolate the supply. Interchange the supply connections between lines one and two as shown in the diagram below: supply terminal L1 L2 L3 ACB ABC phase sequence indicator motor isolating switch Figure 1.8 three phase induction motor 7. Energise the supply and monitor the phase sequence of the supply with the phase sequence indicator, as shown in the diagram in step 6. Supply phase sequence =. 8. Close the motor isolating switch and note the effect on the operation of the motor. Motor rotation =. Isolate the supply and return all equipment to its appropriate location. Feedback Have your teacher check your results Teacher Initials/Date TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 15 of 316
Observations 1. Explain the relationship between phase sequence and direction of rotation. 2. What would be the effect on the motor direction of rotation in step 6, if line three (L3) and line one (L1) of the motor were interchanged? Feedback Have your teacher check your results Teacher Initials/Date Page 16 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
Review Questions These questions will help you revise what you have learnt in Section 1. 1. On the diagrams below draw the symbol to represent the conventional direction of current flow to establish the magnetic fields shown. Figure 1.9 2. On the diagrams below show the direction in which the resulting force will act. Figure 1.10 3. On the diagram below show the connections required to produce the required field polarity. Figure 1.11 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 17 of 316
4. Apply Fleming s rule to each of the figures below to determine the rotation direction of each machine. Figure 1.12 5. What is the minimum number of phase windings required to produce a rotating magnetic field? 6. What type of magnetic field will be produced by a single phase supply? 7. At what speed does the RMF produced by the stator winding rotate? 8. What determines the direction in which a three-phase magnetic field will rotate around the stator? 9. What is the relationship between the direction of rotation of the RMF and the rotor? 10. What is the difference in magnitude between the resultant RMF of a threephase motor stator compared to the flux produced by one phase alone? Page 18 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
11. What is developed in the rotor of an induction motor by the interaction of the rotor and stator fields? 12. How is the rotor field of an induction motor produced? 13. What determines the speed of rotation of the RMF produced by a threephase induction motor? 14. Briefly explain why the rotor speed of an induction motor is always less than the speed of the stator RMF. 15. The diagram below shows the RMF and the induced rotor conductor current. On the diagram show in which direction the RMF must rotate to induce the current shown and draw in the resultant flux. Figure 1.13 16. Calculate the synchronous speed of an eight pole, 50 Hz induction motor. 17. If the synchronous speed of a 50 Hz induction motor is 3000 rpm, calculate the number of stator poles. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 19 of 316
18. Explain the effect of operating a motor designed to operate on 60 Hz if it is connected to a 50 Hz supply. 19. Explain how a rotor field is established even though there is no electrical connection between the rotor and the a.c. supply. 20. Figure A below represents the isolating switch of a three-phase induction motor. When energised the motor is found to rotate in the wrong direction. Show on Figure B the changes needed to reverse the direction of rotation. Figure 1.14 21. A six pole 50 Hz induction motor operates with a 4% slip. Determine the rotor speed. 22. A 50 Hz induction motor has rotor currents with a frequency of 2 Hz. Determine the slip % at which this motor operates. Page 20 of 316 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012