Solve problems in single and three phase low voltage machines

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Engineering

Description

unit covers ascertaining correct operation of single and three phase machines and solving machine problems as they apply to servicing

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Written Assessment Task 2 TAFE Student Name: E С ΤΙ flu dia Section A (Total Marks 20) INSTRUCTIONS: Select the best response for the following questions and place an 'X' in the appropriate box on the answer sheet. 1.. The speed at which the RMF rotates is called the: A slip speed B synchronous speed rotor speed D stator speed 2. The rotating magnetic field of an induction motor: A Loses speed and strength as the load is increased. B Has constant speed but the strength falls as load increases. C Has a constant speed and strength. D Has constant strength but the speed decreases as load increases. 3.. The speed of the rotating magnetic field in an induction motor is proportional to: А supply voltage B supply current stator frequency D developed torque 4.. The main advantage of a slip ring motor over a squirrel cage rotor machine is: A lower initial cost С Sinc of pl The curre 20tc dis B maximum torque can occur at starting C a wide range of speed control is available D lower maintenance and higher reliability 5.. Torque developed by a squirrel cage motor is maximum when: A the rotor is at standstill thi 16 B the rotor speed equals slip speed C С synchronous speed equals rated speed D rotor reactance equals rotor resistance 6.. If a rewound motor when first started produces low torque and a rough magnetic sound the most likely problem is: A one supply line is disconnected B one phase winding is reversed C the star connected motor had been started in delta D a high resistance joint has developed in one phase winding Written Assessment Task vi Last Updated: 22 January 2018 Page 3 of 9 Written Assessment Task 2 TAFE Student Name: 7. The power factor of an induction motor will: A depend on the rotor frequency and resistance. B decrease with an increase in load. С increase with an increase in load. Compe D not change with a change of load 8.. Rotor cores are laminated to A increase starting torque Unit N B increase run torque C reduce the core reluctance Date: f 1 d 2 D reduce eddy current losses 9.. A dual cage rotor motor would be used where A a high torque is required across the speed range B a high start torque and low run torque is required С a low start torque and high run torque is required D slow run-up is required with high run torque 10.As the speed of an induction reduces, the rotor emf: A increases. 3 4 5 B 6 decreases. n С reaches a minimum value. 7 D remains constant. 8 11. With regard to the diagram, which curve best represents the torque characteristic of a wound rotor motor with its brushes short-circuited? S A A torque (%) A B B B с C С D D 12.Increasing the load on a three phase induction motor will cause the: A torque to decrease. B slip speed to increase. C С rotor reactance to decrease. D slip to increase. Written Assessment Task v1 Last Updated: 22 January 2018 Page 4 of 9 Written Assessment Task 2 TAFE Student Name: NEW SOUT Competency Number: Unit Name: Date: 7/5/1 First Name 1 f d 1 13. The current in the rotor conductors of a squirrel cage induction motor are produced by: A self-induced emf. B mutual induction. С rotor connected to supply by slip rings. D rotor inductance equals rotor resistance. 14. To improve the starting torque of a split phase motor a capacitor is connected: A in parallel with the start winding. B in parallel with the run winding. C in series with the start winding. D in series with the run winding 15. The run and start winding of a split phase induction motor together produce a: А constant strength pulsating magnetic field. B constant strength rotating magnetic field. С variable strength non-rotating magnetic field. D variable strength rotating magnetic field. 16. The direction of rotation of a single phase induction motor con be reversed by: A reversing the supply leads. B reversing either the start winding or the run winding. С reversing the start winding and the run winding. D changing two supply lines over. 17. The power factor of an induction motor : A is 0.8 if a higher rated motor is used. B decreases with an increase in load. 2 3 Jona Arnaint Ronal Wilbert Moha 4 5 6 7 Sin of p 00 The 9 curri moto s dis 10 f the 11 C increases with an increase in load. ne v 12 the 13 14 D is unaffected by the load. 18.A split phase induction motor fails to start when energised, but will run when the shaft is manually turned. Which one of the following is most likely cause of the fault: A low supply voltage. B welded centrifugal switch contacts. С open-circuit in start winding. D faulty capacitor. 15 16 17 1 18 19 20 Written Assessment Task vi TES Last Updated: 22 January 2018 Class Roll ICT & Desig Disclaimer: Written Assessment Task 2 TAFE Student Name: NEW SOUTH V 19. One possible cause of the induction motor bearings becoming overheated after the motor has been serviced is: A the air gap between stator and rotor has changed. B one phase being reversed. С too much grease being forced into bearings. D the cooling fan has become dislodged. 20. A single phase motor is connected to the supply through a flexible cord and a three-pin plug. When tested with an ohmmeter the following results were recorded: Competency Number: Unit Name: L Date: 7/5/18 First Name T f1 di 1 А Active pin to Earth pin in finite ohms. Neutral pin to Earth pin = infinite ohms Active pin to Neutral pin = 3 ohms Earth pin to motor frame = 0.25 ohms Safe to connect to the supply. Unsafe to connect due to the low resistance between Active and Neutral pins. Unsafe to connect due to the high resistance between frame and Earth pin. Undetermined because the wrong meter was used to make the measurements. 2 Jonath Arnarins B C 3 D 4 Ronal Wilbert Moha 5 6 7 Sino 00 of p The curri 9 10 moto is dis of the 11 The v 12 to the . 13 14 15 16 17 18 trical Mac Frical Tra aimer: P 19 20 Written Assessment Task v1 IEST Last Updated: 22 January 2018 Pag Class Roll Form ICT & Design - sclaimer: Prin
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Explanation & Answer

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Last Name 1

Name
Instructor's name
Course
Date
Electric Machines
Section A
Question 1: The answer is B.
Question 2: The answer is C
Question 3: The answer is C.
Question 4: The answer is B.
Question 5: The answer is D.
Question 6: The answer is A.
Question 7: The answer is C.
Question 8: The answer is D.
Question 9: The answer is B.
Question 10: The answer is B.
Question 11: The answer is C.
Question 12: The answer is A.
Question 13: The answer is B.
Question 14: The answer is C.
Question 15: The answer is A.
Question 16: The answer is A.
Question 17: The answe...


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