EET272 Worksheet Week 8

Transcription

1 EET272 Worksheet Week 8 answer questions 1-5 in preparation for discussion for the quiz on Monday. Finish the rest of the questions for discussion in class on Wednesday. Question 1 Questions We will now look at a variety of circuits to control s. This is not an exclusive list. The goal is to learn to read ladder diagrams and diagnose what the function of the various parts of the circuit and the complete function is. In our text read and outline the section on Motor Control Circuits, Mutliple /Stop Controls, Multiple Motor er Control, Sequential ing Control, Various starting methods, and Braking in Ch 20 of our text (pgs ) file q0060 Question 2 A very common form of latch circuit is the simple start-stop relay circuit used for controls, whereby a pair of momentary-contact pushbutton switches control the operation of an electric. In this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage : F1 To 3-phase power source 480/120 V F2 Stop Explain the operation of this circuit, from the time the switch is actuated to the time the Stop switch is actuated. The normally-open contact shown in the low-voltage control circuit is commonly called a seal-in contact. Explain what this contact does, and why it might be called a seal-in contact. Kuphaldt file i

2 Question 3 Examine this control circuit for a start/stop/jog control: L1 L2 Stop CR1 CR1 Jog CR1 Explain in your own words what distinguishes the function from the Jog function, and think of a practical application where this might be useful. Kuphaldt file i

3 Question 4 The most common method of starting up a three-phase induction is to simply apply full power all at once by closing the three contacts of a large contactor relay. This is called across-the-line starting: "Across the line" starter 3-θ power Across-the-line starting is simple, but results in huge inrush currents at the moment of contactor closure, and also places a lot of mechanical and thermal stress on the as it rushes to attain full speed. A gentler method of starting an induction is to place impedances in series with the three-phase power, using two contactors (one start and one run ) to sequence the from start-up to full-speed run. The impedances ideally take the form of inductors ( reactors ): Run 3-θ power Explain how and why this method of starting is gentler than across-the-line starting. Suggestions for Socratic discussion Would large (high-power) resistors work instead of inductors? Would large capacitors work instead of inductors? Kuphaldt file i

4 Question 5 Examine this control circuit diagram for an air compressor, where a pair of pressure switches controls the starting and stopping of the electric turning the air compressor: Control circuit schematic L 1 L 2 Cut-out switch Hand M Off Auto Control switch Pictorial diagram of system PS Cut-out switch PS Control switch Receiver tank Intake filter Compressed air Compressor Condensate drain valve Explain what the Hand-Off-Auto switch does in this circuit, and also describe the functions of each pressure switch. Suggestions for Socratic discussion Which of these two pressure switches should have the greater trip setting, and why? Why do you think operations personnel might find it useful to have a Hand position as well as an Auto position on the switch in this air compressor system? Some Hand-Off-Auto switches place the Auto position in the middle, between the Hand and the Off settings explain why this might be a better way to arrange the three-position switch. Kuphaldt file i

5 Question 6 Suppose we wish to have three separate pushbutton start/stop stations for operators to use in controlling a single three-phase electric. The control circuit wiring schematic shows how this will work: Fuse L2 L3 Stop Stop Stop Sketch the necessary connecting wires to build this control circuit: L1 L2 L3 Transformer H1 H3 H2 H4 X1 X2 Stop Stop Stop Contactor Fuse Motor T1 T2 T3 Suggestions for Socratic discussion An overload contact has been omitted from this control system for simplicity s sake. Identify where one would be properly inserted into the schematic diagram, and also in the pictorial diagram. Kuphaldt file i

6 Question 7 This control circuit commands three s to start and stop together: Power circuit 1 Control circuit L 1 L 2 To 3-phase power source Stop 1 2 M3 2 M3 3 M3 3 Examine the control circuit and then explain how starting one starts up the others. Also, determine what will happen if #3 suffers an overload (i.e. 3 warms up enough to trip). Suggestions for Socratic discussion Explain why inrush current could be a problem in this three- control system, and identify at least one practical solution for it. Kuphaldt file i

7 Question 8 The direction of rotation for a three-phase AC electric may be reverse by reversing any two of the three power conductor connections. With this in mind, explain how this reversing control circuit works: L 1 L 2 Forward Reverse To 3-phase power source In particular, what it the function of the two normally-closed M contacts (called interlock contacts) in the control circuit? What do you think might happen if those contacts were not there? Suggestions for Socratic discussion Explain why reversing any two phase conductors supplying AC power to an induction will cause it to reverse direction. Explain what arc flash is, and how to protect yourself from it while working on high-voltage control circuits such as this one. Kuphaldt file i

8 Question 9 Large electric s are often equipped with some form of soft-start control, which applies power gradually instead of all at once (as in across the line starting). Here is an example of a simple reduced voltage start control system using a time-delay relay (TD1): L 1 L 2 Stop CR1 CR1 TD1 TD1 R To 3-phase power source Analyze this ladder logic diagram, and explain the function of the time-delay relay, particularly how to interpret its switch symbol (with arrowhead). How does the time-delay relay cause a soft start? Kuphaldt file i

9 Question 10 The following ladder logic diagram is for a reversing control circuit: L 1 L 2 Stop Forward Reverse To 3-phase power source Study this diagram, then explain how reversal is accomplished. Also, identify the function of each M contact in the control circuit, especially those normally-closed contacts in series with the starter coils. 9

10 Now consider the following modification made to the reversing control circuit ( and power contacts not shown here): L 1 L 2 Stop Forward TD2 TD1 Reverse TD1 TD2 What extra functionality do the time-delay relays contribute to this control circuit? Kuphaldt file i

11 Question 11 This -control bucket has a problem: the refuses to start when the pushbutton is pressed. A voltmeter connected to test points C and E indicates 118 volts AC with no pushbuttons pressed: To 3-phase power source F1 B 480/120 V F F2 A C Stop D E Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. no multiple faults), determining whether or not each fault could independently account for all measurements and symptoms in this circuit. Fault Possible Impossible Fuse F1 blown Fuse F2 blown switch failed open Stop switch failed open coil failed open auxiliary contact failed open power contact(s) failed open contact failed open switch failed shorted Stop switch failed shorted Transformer secondary failed shorted Finally, identify the next diagnostic test or measurement you would make on this system. Explain how the result(s) of this next test or measurement help further identify the location and/or nature of the fault. Kuphaldt file i

12 Question 12 An interesting way to achieve reduced-voltage starting for a three-phase is to use a 6-lead where the three stator winding sets are individually wired so as to allow either wye (start) or delta configurations: 6-lead, 3-phase Wye connection Delta connection A contactor sends power to the stator windings in a wye configuration for a short start-up time (perhaps 10 seconds), then that starter disengages and a Run starter energizes to send power to the stator windings in a delta configuration. In the wye configuration, each winding receives 1 3 of the line voltage. In the delta configuration, each winding receives the full line voltage. Sketch the proper wire connections to create just such a wye-delta starter. Hint: terminals 1, 2, and 3 of the always connect to the three-phase power lines! To three-phase power source Fuses Motor Run 3 6 Suggestions for Socratic discussion Explain the purpose of using reduced-voltage starting for a large electric. Kuphaldt file i

13 Question 13 Synchronous AC s by their nature rotate at precisely the same speed as the rotating magnetic field produced by the stator windings. The practical problem with this is how to get a synchronous started, since it is physically impossible for the rotor to jump from a stand-still to 100% speed in zero time. Therefore, synchronous s are usually started as regular induction s at first, and then they are switched to synchronous mode when their speed is very near 100%. The following control circuit shows one scheme for this dual-mode start-up. The rotor on this synchronous has its own winding: Three-phase synchronous M Line power 125 VDC Run Stator windings + Rotor winding Control circuit wiring L 1 L 2 Stop M M F Run Explain how this start-up circuit functions, and what goes on with the switching of the rotor winding to make the start up and then run in two different modes. Suggestions for Socratic discussion What practical applications might warrant the use of a synchronous AC instead of an induction AC? Kuphaldt file i

14 Question 14 There are three basic types of braking control circuits. They are coasting, plugging, and dynamic braking. Explain each one in your own words. What are some advantages and disadvantages of each type of braking circuit? Suggestions for Socratic discussion Many electric cars are able to recharge their batteries when the brakes are applied. They call this regenerative braking. Explain how this works. file q

15 Question 15 Expalin how this control circuit works. What is the function of TD1? L 1 L 2 Stop TD1 TD1 To 3-phase power source Suggestions for Socratic discussion If you are unsure how long the delay of TD1 should be. Decide if you would rather error on the side of too short or too long. Explain why you chose the way you did. file q

16 Question 16 Examine this three-phase control circuit(sometimes referred to as a bucket ), where fuses protect against overcurrent faults, a three-pole relay (called a contactor) turns power on and off to the, and a set of overload heaters detect mild overcurrent conditions. Control circuit wiring has been omitted for simplicity s sake. Only the power wiring is shown: Line 1 Line 2 Line 3 Fuse Fuse Fuse Schematic diagram Fuses s Motor Shaft Contactor Overload block Reset Afteryearsoffaithfulservice, onedaythisrefusestostart. Itmakesa humming soundwhenthe contactor is energized (relay contacts close), but it does not turn. A mechanic checks it out and determines that the shaft is not seized, but is free to turn. The problem must be electrical in nature! 16

17 You are called to investigate. Using a clamp-on ammeter, you measure the current through each of the lines (immediately after each fuse) as another start is once again attempted. You then record the three current measurements: Line Current amps amps 3 0 amps Determine at least two possible faults, either one fully capable of causing the s refusal to start and the three current measurements taken. Then, decide what your next measurement(s) will be to isolate the exact location and nature of the fault. Suggestions for Socratic discussion Is there a way we could have determined a lack of current in line 3 without the use of a clamp-on ammeter, using a multimeter incapable of directly measuring current over 10 amps? Kuphaldt file i

18 Question 17 This lift station pump control circuit has a problem. The sump pump is supposed to come on when the high level is reached, and turn off when the water pumps down to the low level point. Instead, however, the cycles on and off at the low-level point. Using an AC voltmeter, you measure a voltage from point B to point D that switches back and forth between 120 volts and 0 volts: To 3-phase AC power (480 V) F1 H1 H2 H3 F2 H4 F3 120 VAC C A LSH B LSL D E F G Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. no multiple faults), determining whether or not each fault could independently account for all measurements and symptoms in this circuit. Fault Possible Impossible High level switch failed open Low level switch failed open Broken wire between D and coil Contactor auxiliary contact failed open Contactor auxiliary contact failed shorted Contactor main contact(s) failed open Broken wire between B and G Thermal overload unit tripped High level switch failed shorted Transformer secondary winding failed open Finally, identify the next diagnostic test or measurement you would make on this system. Explain how the result(s) of this next test or measurement help further identify the location and/or nature of the fault. Kuphaldt file i

19 Answer 1 Answers Answer 2 Even though the and Stop switches are momentary, the seal-in contact makes the circuit latch in one of two states: either energized or de-energized. Answer 3 The function is latching, whereas the Jog function is not. A common application of this concept is in a kitchen blender, where one button starts (and latches) the blender, while another simply pulses the blender. Answer 4 The contactor must be energized first, then at a later time is de-energized as the Run contactor is simultaneously energized. Either timing relays or a PLC handles this sequencing of contactors. Answer 5 Both pressure switches are normally closed and open when a specified pressure is reached. Answer 6 L1 L2 L3 Transformer H1 H3 H2 H4 X1 X2 Stop Stop Stop Fuse Motor T1 T2 T3 Answer 7 19

20 Answer 8 The normally-closed contacts are referred to as interlock contacts, and they prevent simultaneous forward and reverse actuation of the. Answer 9 For starting should be closed and the resistors limit the current. During normal running is closed and is open. This applies full voltage to the. Remember that time-delay relay symbols always use an arrowhead at the switch contact to denote the direction of timing. With this switch, the arrowhead points in the closed direction, which means the relay takes time to close. Being normally-open, this means the delay happens upon energization of the relay coil, the implication being that the relay will return to its normal (open) state immediately upon de-energization. Answer 10 The normally-open and normally-closed M contacts provide seal-in and interlock functions, respectively. The time-delay relays prevent the from being immediately reversed. Answer 11 Fault Possible Impossible Fuse F1 blown Fuse F2 blown switch failed open Stop switch failed open coil failed open auxiliary contact failed open power contact(s) failed open contact failed open switch failed shorted Stop switch failed shorted Transformer secondary failed shorted 20

21 Answer 12 To three-phase power source Fuses Motor Run 3 6 Answer 13 In the start-up mode, the s rotor winding is short-circuited by the contact. This makes the behave like a normal squirrel-cage induction with its rotor bars and shorting rings. As soon as the speed switch detects adequate rotor speed, the coil de-energizes and the Run coil energizes, connecting the rotor winding directly to a DC power source to magnetize it and lock it into synchronous mode. Answer 14 Coasting takes the longest to stop a. Plugging is faster but can be hard on the. Sometimes plugging is pulsed on an off if you have digital control. Dynamic Braking is the fastest but also the most complictated. Answer 15 TD1 is involved in a braking of the. Be sure to explain the type of braking Answer 16 Here are some possibilities: Fuse #3 blown open Third relay contact damaged (failed open) inside the contactor Overload heater #3 failed open One winding failed open inside the (assuming a Y winding configuration) There are several valid next steps you could take from this point. Discuss alternatives with your classmates. 21

22 Answer 17 Partial answer: Fault Possible Impossible High level switch failed open Low level switch failed open Broken wire between D and coil Contactor auxiliary contact failed open Contactor auxiliary contact failed shorted Contactor main contact(s) failed open Broken wire between B and G Thermal overload unit tripped High level switch failed shorted Transformer secondary winding failed open 22