24 volts (0.25 amps current-limited)

Transcription

1 Question 1 Questions Suppose the lamp refuses to light up. A voltmeter registers 24 volts between test points C and D: A C E + 24 volts (0.25 amps current-limited) B D F First, list all the possible (single) faults that could account for all measurements and symptoms in this circuit, including failed wires as well as failed components: Now, determine the diagnostic value of each of the following tests, based on the faults you listed above. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure V CF Measure V ED Measure V AB Measure V AD Measure V CB Measure V EF Measure current through wire connecting A and C Jumper A and C together Jumper B and D together Jumper A and B together Finally, develop a rule you may use when assessing the value of each proposed test, based on a comprehensive list of possible faults. Suggestions for Socratic discussion Identify which fundamental principles of electric circuits apply to each step of your analysis of this circuit. In other words, be prepared to explain the reason(s) why for every step of your analysis, rather than merely describing those steps. Suppose the fault were intermittent: sometimes the lamp lights up, and other times it goes out. Explain how you could use a digital multimeter (DMM) set to record voltage as a troubleshooting tool to determine where the fault is located in the circuit over a span of time too long for you to personally observe the circuit. file i

2 Question 2 Suppose you are asked to check the integrity of a multi-conductor signal cable run between two locations. The cable has six signal conductors in it plus a shield, each one terminated at a terminal block at each end: a long distance! 1 2 Pair A Pair A Pair B Pair B Pair C Multi-conductor cable Pair C Shield wire Shield wire V A V OFF A A COM Faults you are looking for include open conductors, as well as shorts between conductors and/or shorts to ground. Devise a series of tests you could perform with nothing but a multimeter to comprehensively check the electrical integrity of this cable. file i

3 Question 3 Suppose a voltmeter registers 6 volts between test points C and D while the pushbutton is released (not pressed), and also 6 volts between the same test points while the pushbutton is pressed: A C 1 kω E R 1 1 kω R 2 1 kω R volts (0.25 amps current-limited) B D F Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure V AB with switch pressed Measure V AC with switch pressed Measure current through wire connecting B to D with switch pressed Measure V AB with switch unpressed Measure V AC with switch unpressed Measure R EF with switch pressed and source disconnected from E Measure R EF with switch unpressed and source disconnected from E file i

4 Question 4 The following electric heater seems to have a problem: it heats up slower than usual with all three switches turned on. 800 W Oven W Fuse 200 W volts With three differently-sized heating elements (200 watt, 400 watt, and 800 watt), the oven operator can set the power in seven discrete steps by turning on specific combinations of switches: 200 watts, 400 watts, 600 watts, 800 watts, 1000 watts, 1200 watts, and 1400 watts. You are summoned to diagnose this oven s problem without turning it off. You are allowed to turn off any single switch for a few seconds at most, but otherwise you need to leave all three heaters on because the oven needs to heat up as fast as it can! The idea is to figure out where the problem might be, then gather together any parts necessary for repairs while the oven is still being used, and fix the oven as fast as possible when you finally get the chance to turn it off completely. First, assess whether or not the following diagnostic test would provide any useful information about the fault: suppose a technician connects an AC voltmeter between terminals 4 and 8. Will this test provide information to help us diagnose the nature and/or location of the fault? Why or why not? Next, propose a diagnostic test that would definitely provide useful information about either the location or the nature of the fault in this system. Your proposal must identify the meaning of at least one possible result of the test (e.g. If I jumper terminals X and Y together and I measure a decrease in source voltage, it means the fault must be a short somewhere in branch A-B-C of the circuit ). Remember that the best diagnostic test is one that yields definitive answers no matter what its result might be. Directly checking a suspected component is not a good diagnostic test, unless there are simply no other options! file i

5 Question 5 A water-cooled generator at a power plant has two sources of cooling water flow, each source equipped with a flow switch that returns to its normally-open status if the water flow through the pipe stops. If either water source ceases supplying cooling water to the generator (for whatever reason), that flow switch will deactuate and turn the warning light on. This is all that is required, as the generator will still receive adequate cooling from only one source. If both water sources cease supplying water, however, a trip solenoid will energize to shut down the generator before it overheats: L 1 L 2 Cooling water flow (source A) TP1 TP2 CR1 Cooling water flow (source B) TP3 TP4 CR2 CR1 TP5 TP6 Warning light CR2 CR1 TP7 CR2 TP8 TP9 TP10 Shut-down solenoid One day the warning light comes on, but there is still cooling water flowing to the generator so it does not shut down. You are asked to determine what the problem is, while maintaining the system in an operating condition (i.e. you are not allowed to shut off control power or do anything else that might shut down the generator). First, assess whether or not the following diagnostic test would provide any useful information about the fault: suppose a technician connects an AC voltmeter between terminals TP6 and L2. Will this test provide information to help us diagnose the nature and/or location of the fault? Why or why not? Next, propose a diagnostic test that would definitely provide useful information about either the location or the nature of the fault in this system. Your proposal must identify the meaning of at least one possible result of the test (e.g. If I jumper terminals X and Y together and I measure a decrease in source voltage, it means the fault must be a short somewhere in branch A-B-C of the circuit ). Remember that the best diagnostic test is one that yields definitive answers no matter what its result might be. Directly checking a suspected component is not a good diagnostic test, unless there are simply no other options! file i

6 Question 6 Suppose a voltmeter registers 0 volts between test points G and C, and 24 volts between test points C and F in this circuit: Current mirror 8 ma A B C R 1 G 250 Ω Q volts (0.5 amps current-limited) R 2 (Q 1 serves as a current regulator) D E F 250 Ω H Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure V BE with power applied Measure V GH with power applied Measure V FH with power applied Measure V AD with power applied Measure I R1 with power applied Measure R AD with wire disconnected between B and C Measure R GH with source disconnected from H file i

7 Question 7 This single-loop flow control system has a problem: the flow rate indicated on the controller s faceplate shows it to be precisely at setpoint (250 SCFM), yet the Annubar flow gauge does not agree. The two indications used to agree with one another just fine, until some time recently. Annubar 0 to 100 "WC Gauge reads 207 SCFM 250 Ω TB Single-loop controller H Input N G Output E.S. Black White Green V 1 L H V 2 10 I/P transducer (coil resistance unknown) H N G Power supply V 4 V 3 FC A.S. 24 VDC Air from blower Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure AC line voltage Measure DC power supply output voltage Inspect PID tuning parameters in controller Check pressure transmitter calibration Measure transmitter current signal Put controller into manual mode and move valve Measure DC voltage between TB1-3 and TB1-4 Measure DC voltage between TB1-7 and TB1-8 file i

8 Question 8 Examine the state of this fluid-heating system: Cold fluid in Position = 90% open Air-to-open valve (3 PSI = shut) (15 PSI = full open) Steam in Transducer Heat exchanger I/P Controller PV SP Temp = 160 o F Air supply Out PV = 167 o F SP = 250 o F Out = 7% Steam out TT Thermocouple Warm fluid out Temperature transmitter The temperature of the exiting fluid is well below setpoint, so we know there is a problem somewhere in this system. Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure millivolt signal output by thermocouple Measure 4-20 ma signal output by TT Measure 4-20 ma signal output by controller Measure instrument air supply pressure to I/P Measure 3-15 PSI signal output by I/P Measure temperature of incoming steam and compare with normal Also, explain the rationale of assuming only one fault when initially diagnosing a system problem. Why not keep an open mind to include multiple faults when first assessing possibilities? Does the prior history of the system matter (i.e. is it relevant whether or not it functioned properly in the past)? file i

9 Question 9 The flow computer connected to this turbine flowmeter (with electronic pick-up) does not register any flow, even though we know there to be fluid flowing through the pipe. A voltmeter connected between terminals TB1-1 and TB1-3 registers approximately 11.0 volts DC, and 10.8 volts AC at a frequency of 86 Hz: TB-1 1 TB amp fuse Cable 2 Cable 6 R VDC 4 Turbine flowmeter with transistor pulse output 8 Optocoupler R 2 To flow computer discrete input (sinking) TB ma fuse + 5 VDC Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure DC voltage between terminals TB2-5 and TB2-8 Measure resistance between TB2-7 and TB2-8 with the 1 amp fuse pulled Measure DC voltage across 100 ma fuse Measure DC voltage across 1 amp fuse Measure AC voltage between terminals TB3-9 and TB3-11 Measure continuity of conductor connecting terminals TB1-4 and TB2-8 file i

10 Question 10 Something is wrong with this valve control circuit. When the operator presses the open pushbutton, the valve position indicating lights still show it to be closed (green light on, red light off): E F G H Open Close To 480 VAC power source J K F1 F2 N Relay P L M Vent cap Silencer A B C D H4 H2 X2 H3 X1 H1 Control power transformer 480x120 VAC C NO NC Control valve C P V1 C P E E Q R Solenoid valve Green F3 Pressure gauge V2 Red F4 100 PSI air supply S T U V Determine the diagnostic value of each of the following tests. Assume only one fault in the system, including any single component or any single wire/cable/tube connecting components together. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark yes. Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark no. Measure AC voltage across Fuse F1 with Close button pressed Measure AC voltage across Fuse F3 with Close button pressed Measure AC voltage across solenoid coil with Open button pressed Measure AC voltage across solenoid coil with Closed button pressed Measure AC voltage between terminals X2 and T with Open button pressed Measure AC voltage between terminals L and D with Open button pressed Check air supply pressure (look at pressure gauge) file i

11 Answer 1 Answers Here is a comprehensive list of faults, each one individually capable of accounting for the symptom (no light) and the measurement of 24 volts between C and D: Lamp burned out (failed open) Wire failed open between A and C Wire failed open between B and D Based on this short list of possible faults assuming only one of them is actually true the value of each proposed test is as follows: Measure V CF Measure V ED Measure V AB Measure V AD Measure V CB Measure V EF Measure current through wire connecting A and C Jumper A and C together Jumper B and D together Jumper A and B together A good rule to apply when evaluating proposed tests is to ask the question: Will this test give me the exact same result no matter which one of the possible faults is true? If so, the test is useless. If not (i.e. the results would differ depending on which of the possible faults was true), then the test has value because it will help narrow the field of possibilities. Answer 2 Answer 3 Answer 4 Answer 5 Answer 6 Answer 7 Answer 8 Answer 9 11

12 Answer 10 12