END-OF-CHAPTER REVIEWS are structured to reinforce chapter objectives CHAPTER 6 REVIEW QUESTIONS 1. Explain the basic operating principle of an electromagnetic control relay. 2. What is the operating difference between a normally open and a normally closed relay contact? 3. In what ways are control relay coils and contacts rated? 4. How do contactors differ from relays? 5. What is the main difference between a contactor and a magnetic motor starter? 6. a. Draw the schematic for an across-the-line AC magnetic motor starter. b. With reference to this schematic, explain the function of each of the following parts: i. Main contact M ii. Control contact M iii. Starter coil M iv. OL relay coils v. OL relay contact 7. The current requirement for the control circuit of a magnetic starter is normally much smaller than that required by the power circuit. Why? 8. Compare the method of operation of each of the following types of switches: a. Manually operated switch b. Mechanically operated switch c. Proximity switch 9. What do the abbreviations NO and NC represent when used to describe switch contacts? CHAPTER 6 PROBLEMS 10. Draw the electrical symbol used to represent each of the following switches: a. NO pushbutton switch b. NC pushbutton switch c. Break-make pushbutton switch d. Three-position selector switch e. NO limit switch f. NC temperature switch g. NO pressure switch h. NC level switch i. NO proximity switch 1. Design and draw the schematic for a conventional hardwired relay circuit that will perform each of the following circuit functions when a normally closed pushbutton is pressed: • Switch a pilot light on • De-energize a solenoid • Start a motor running • Sound a horn 2. Design and draw the schematic for a conventional hardwired circuit that will perform the following circuit functions using two break-make pushbuttons: • Turn on light L1 when pushbutton PB1 is pressed. • Turn on light L2 when pushbutton PB2 is pressed. • Electrically interlock the pushbuttons so that L1 and L2 cannot both be turned on at the same time. 11. Outline the method used to actuate inductive and capacitive proximity sensors. 3. Study the ladder logic program in Figure 6-77, and 12. How are reed switch sensors actuated? 13. Compare the operation of a photovoltaic solar cell with that of a photoconductive cell. answer the questions that follow: a. Under what condition will the latch rung 1 be true? b. Under what conditions will the unlatch rung 2 be true? c. Under what condition will rung 3 be true? d. When PL1 is on, the relay is in what state (latched or unlatched)? e. When PL2 is on, the relay is in what state (latched or unlatched)? f. If AC power is removed and then restored to the circuit, what pilot light will automatically come on when the power is restored? g. Assume the relay is in its latched state and all three inputs are false. What input change(s) must occur for the relay to switch into its unlatched state? h. If the examine if closed instructions at addresses I/1, I/2, and I/3 are all true, what state will the relay remain in (latched or unlatched)? 14. What are the two basic components of a photoelectric sensor? 15. Compare the operation of the reflective-type and through-beam photoelectric sensors. 16. Give an explanation of how a scanner and a decoder act in conjunction with each other to read a bar code. 17. How does an ultrasonic sensor operate? 18. Explain the principle of operation of a strain gauge. 19. Explain the principle of operation of a thermocouple. 20. What is the most common approach taken with regard to the measurement of fluid flow? 21. Explain how a tachometer is used to measure rotational speed. 22. How does an optical encoder work? 23. Draw an electrical symbol used to represent each of the following PLC control devices: a. Pilot light b. Relay c. Motor starter coil d. OL relay contact e. Alarm 24. Explain the function of each of the following actuators: a. Solenoid b. Solenoid valve c. Stepper motor 25. Compare the operation of open-loop and closedloop control. 26. What is a seal-in circuit? 27. In what way is the construction and operation of an L L 1 2 electromechanical latching relay different from a standard relay? 28. Give a short description of each of the following control processes: a. Sequential b. Combination c. Automatic Figure 6-78 Hardwired control circuit for Problem 4. will correctly execute the hardwired control circuit in Figure 6-78. Assume: Stop pushbutton used is an NO type. 29. Compare the type of sensor signal obtained from a thermocouple with that from an RTD. Run pushbutton used is an NO type. Jog pushbutton used has one set of NO contacts. OL contact is hardwired. 30. Explain how a magnetic reed float switch works. 31. What is the function of an electrical interlocking circuit? 5. Design a PLC program and prepare a typical I/O connection diagram and ladder logic program that will correctly execute the hardwired control circuit in Figure 6-79. Assume: PB1 pushbutton used is an NO type. 32. What is the role of instrumentation in an industrial process? 33. You have been assigned the task of calibrating an instrument. How would you proceed? 128 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs f. Heater g. Solenoid h. Solenoid valve i. Motor j. Horn PB2 pushbutton used is an NC type. PS1 pressure switch used is an NO type. LS1 limit switch used has only one set of NC contacts. L L 1 2 EXAMPLE PROBLEMS help bring home the applicability of chapter concepts 4. Design a PLC program and prepare a typical I/O connection diagram and ladder logic program that L1 Inputs Ladder logic program Outputs L2 I/1 CR2-1 1 2 xv I/2 I/3 I/1 I/2 O/9 L I/3 O/9 U Rung 1 Rung 2 PL1 PL2 O/9 O/10 Start CR1 CR1-2 SOL CR1-1 PS1 LS1 SS1 CR2-2 CR2 SOL SOL 1 2 3 Stop PB PB PL1 Stop Run OL Jog M M
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