Learner Workbook
Version 1
Training and Education SupportIndustry Skills Unit
Meadowbank
Product Code: 5633
Alternating Current Machines
SAMPLE
Alternating Current Machines
© TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012
ContentsIntroduction ................................................................................... 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
SAMPLE
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Section 1: Operating Principles of Three-Phase Induction Motors PurposeIn this section you will learn about the fundamental principles of operation of three-phase induction motors.
ObjectivesAt 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
• listthecharacteristicsofthemagneticfieldproducedbyasingle,twoandthree-phase winding
• calculatethespeedofrotationofarotatingmagneticfield
• 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, 6th Ed., McGraw-Hill, Sydney.
• Hampson, J., 2011, Electrical Trade Principles - A Practical Approach, 2nd Ed Pearson Education, Sydney.
• Phillips P., 2012, Electrical Principles, 2nd Ed, Cengage Learning, Melbourne.
• Wildi, T., 1991, Electrical Machines, Drives and Power Systems, 2nd Ed., Prentice Hall, USA.SAMPLE
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Learner ExercisesLearner Exercise 1.1
Usingtheright-handconductorrule,andtheconventionalsymbolsforcurrentflow,showthedirectionofcurrentflowandthemagneticfielddirection.
Figure 1.1
Learner Exercise 1.2
Usingtheright-handsolenoidrule,indicateonthediagramthemagneticfieldpolarity of the coils drawn below.
Figure 1.2
Learner Exercise 1.3
UseFleming’sleft-handruletodeterminedirectionofforce,fieldpolarityordirectionofcurrentflowinthediagramsbelow:
N
S
force force
S
N
direction offorce ?
fieldpolarity ?
direction ofcurrent flow ?
Figure 1.3
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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
S
S
N
direction ofinduced emf ?
direction of conductormovement ?
motionmotion
motion
N
S
direction ofinduced emf ?
Figure 1.4
Learner Exercise 1.5
Determinethesynchronousspeedoftherotatingmagneticfield(RMF)ofaneightpole three-phase winding if the supply frequency is 50 Hz.
Learner Exercise 1.6
Determine the number of poles per phase if the RMF rotates at 1500 rpm when the supply frequency is 25 Hz.
SAMPLE
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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.SAMPLE
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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
SAMPLE
Alternating Current Machines
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Practical Exercise 1.1: Three-Phase Induction Motor Reversal
TaskTo 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.
EquipmentYourteacherwillprovideyouwiththespecificationsoftheequipmenttobeused:
Specifications
• Three phase supply ___________________________________• Three-phase induction motor ___________________________________• Triple pole isolating switch Rating _____________________________• Phase sequence indicator Type ______________________________• Connection leads ___________________________________
Safety
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.
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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
Procedure1. 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:
L1
L2
L3
three phaseinductionmotor
motor isolatingswitch
supplyterminal
ACBABC
phasesequenceindicator
Figure 1.7
Record supply phase sequence = ________________________________.
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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:
L1
L2
L3
three phaseinductionmotor
motor isolatingswitch
supplyterminal
ACBABC
phasesequenceindicator
Figure 1.8
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.
FeedbackHave your teacher check your results
Teacher Initials/Date
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Observations1. Explain the relationship between phase sequence and direction of rotation.
___________________________________________________________
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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?
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
FeedbackHave your teacher check your results
Teacher Initials/Date
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Review QuestionsThese questions will help you revise what you have learnt in Section 1.
1. On the diagrams below draw the symbol to represent the conventional directionofcurrentflowtoestablishthemagneticfieldsshown.
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 fieldpolarity.
Figure 1.11
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4. ApplyFleming’sruletoeachofthefiguresbelowtodeterminetherotation direction of each machine.
Figure 1.12
5. What is the minimum number of phase windings required to produce a rotatingmagneticfield?
___________________________________________________________
6. Whattypeofmagneticfieldwillbeproducedbyasinglephasesupply?
___________________________________________________________
7. At what speed does the RMF produced by the stator winding rotate?
___________________________________________________________
8. Whatdeterminesthedirectioninwhichathree-phasemagneticfieldwill 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 three- phasemotorstatorcomparedtothefluxproducedbyonephasealone?
___________________________________________________________
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11. What is developed in the rotor of an induction motor by the interaction of the rotorandstatorfields?
___________________________________________________________
12. Howistherotorfieldofaninductionmotorproduced?
___________________________________________________________
13. What determines the speed of rotation of the RMF produced by a three- phase induction motor?
___________________________________________________________
14. Brieflyexplainwhytherotorspeedofaninductionmotorisalwayslessthan 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 currentshownanddrawintheresultantflux.
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.
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18. Explain the effect of operating a motor designed to operate on 60 Hz if it is connected to a 50 Hz supply.
___________________________________________________________
___________________________________________________________
19. Explainhowarotorfieldisestablishedeventhoughthereisnoelectrical connection between the rotor and the a.c. supply.
___________________________________________________________
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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.SAMPLE