Troubleshooting is a field of control work which generally sepa rates the men from the boys. Many a man who can do a beautiful job of wiring a new control circuit from a circuit diagram is lost if the circuit fails to function as expected. Your chief asset in this field is an analytical mind trained in all of the aspects of control functions, components, circuits, and circuit analysis. The secret to efficient and accurate troubleshooting lies in deter- · mining the section of the control circuit that contains the trouble component and then selecting the proper component to be checked. This can only be accomplished by efficient and accurate circuit analysis, not by trial and error, long, extended wire trac ing, or indiscriminate checking of components at random.
8 · 1 GENERAL PROCEDURE
First let us consider a new circuit which has just been wired but fails to function as expected. Here there is a possibility that the wiring has been misconnected or even that the circuit was not properly designed. If we check all of the connections in all of the wiring, however, it becomes a trial-and-error process and generally involves a considerable waste of time.
The first procedure should be to analyze the circuit to deter mine that it has been properly designed and should work as expected if the wiring was done properly. The next step is to follow the operation of the equipment through the expected se quence until we find the section of the circuit which is not prop erly operating. When you have located the section of the circuit which is giving trouble, it should be simple procedure to check the wiring and operation of the components involved in this section of the circuit and clear whatever the trouble might be.
In this process you have already made use of your knowledge of analysis of circuits and your knowledge of components and their proper functions to determine whether or not they are op erating as they should. Any lack of knowledge on your part of control functions, control components, control circuits, or circuit analysis will cause undue delay and wasted time in this process. When you have located the trouble in this section of the control circuit, the sequence should be started over and run through until either it has operated successfully or another sec tion of the control circuit has been determined to be malfunctioning.
When considering troubleshooting an existing circuit, we can generally eliminate the possibility of improper connections. If the circuit had been improperly wired, it would not have oper ated originally. It is surprising, however, how many men will begin their troubleshooting procedure by checking out the wir ing, connection by connection, to determine if it was properly made. This procedure is an injustice to the plant owner and the operator of the machine, who are interested in speedy and efficient repair rather than time-consuming experimentation.
The first step in troubleshooting an existing circuit which has developed trouble is to understand that circuit and to understand the operation of the machine it controls. On complex circuits
time generally does not allow the service man or troubleshooter to digest the complete circuit. With the help of the operator, however, you can determine how much of that circuit is operat ing. Follow the machine through its cycles until it reaches the point where it does not function properly. Having determined this point, you can analyze the circuit, starting with the section that does not operate. A careful check of this circuit and a location of the components involved in this section of the circuit will generally lead you to the source of the trouble you are seeking. The malfunction of some control component must be the cause of the circuit failure.
In the rare case where insulation breakdown is the cause of the trouble, it should be evident from a visual inspection of the components and the wiring. Quite frequently, however, a grounding of a wire in the control circuit may escape detection in a visual inspection, and if it is suspected that a ground is the cause of the trouble, careful checks should be made with the power off. With an ohmmeter determine the resistance to ground of the wires in this particular section of the control circuit.
Let us assume that you have now located the section of the control circuit which seems to be causing the trouble. The first step is to locate the components involved in this part of the circuit. There must be a coil of a relay, a contactor, or some other device which is energized by this section of the control, and the machine should be run through its sequence to determine if this coil does receive energy.
If the contactor or relay does not close as it should, the circuit should be disconnected and the wires removed from the coil of the relay or contactor so that a voltage check can be taken. Apply a voltmeter across the wires which were connected to the coil and again energize the circuit operating the control se quence up to this point. If the voltmeter indicates a proper volt age applied, then the trouble most likely is in the windings of the coil itself. Do not attempt to check the voltage or resistance of the coil while it is connected in the circuit, since false readings
are likely to result from feedback and parallel paths in the control circuit.
If it is suspected that the coil is at fault, disconnect the power from the circuit and with an ohmmeter check the resistance of the coil, which should be very low on a d-e resistance check. If the coil is burnt out, you will receive a high resistance reading or a reading of infinity on the ohmmeter, indicating that the coil needs to be replaced. Do not depend on the coil smelling burnt or showing any visible evidence of being burnt out, since this is not always the case.
Suppose that our voltage check showed that the voltage did not reach the coil when it should have in the sequence of opera tion of the control circuit. This indicates that some contact is not closing when it should, thus deenergizing the circuit to the coil. A careful study of this section of the control circuit follow ing the principles outlined in Chap. 7 should easily show what contacts should close in order to energize this coil. You must now locate the components which contain these contacts and again operate the machine through its sequence, observing the operation of the relay, limit switch, float switch, pressure switch, or other device that contains these contacts, to determine whether it operates mechanica11y as it should. If this component does operate mechanically, it indicates two possibilities. The first and most likely is that the contacts involved are not properly closing or are coated with copper oxide or other insulating ma terial which prevents them passing current to the coil as they should. The other possibility is an open circuit due to a broken or burnt wire. This generally, however, is the least likely cause of trouble. Having checked the contacts and eliminated the trouble, which probably will be found there, again operate the control circuit with all coils connected, and if it does complete its sequence, then proceed to apply the above procedure to the next section of the control which does not function.
The above procedure is based on years of experience and an understanding of the fact that control circuits are made up basically of only two things: contacts, which make and break the circuit, and coils, which operate these contacts. If the con tacts close and open as they should, then the proper voltages will be applied or disconnected from the coils as they should. If this is true, then the malfunction must lie in the coil itself. If the contacts do not operate properly, however, then the trou ble must be in the contacts or in the associated wire which carries this current from the contact to the coil.
The most important rule in troubleshooting is to change only one thing at a time. If you find a set of contacts that you suspect is not properly functioning, correct this trouble and try the cir cuit again before changing anything else. If you find a coil you suspect to be burnt or otherwise causing trouble, repair or re place it and try the circuit again before attempting any other changes. One of the most common mistakes of troubleshooters is to change or correct several supposed troubles at one time before trying the circuit for operation. Quite frequently several changes made at one time may introduce more trouble than you had originally. This should be made a cardinal law in your work as a troubleshooter and will put you far ahead of the field in efficiency of your work. It is very seldom that several parts of a machine would wear out at the same instant. Therefore, even though the overall condition of the control components may be poor, it still remains probable that only one component has failed completely.
If the machine that you are troubleshooting is not thoroughly familiar to you, do not underestimate the value of the operator in your process of determining the cause of trouble. His knowl- · edge of the normal operation of this piece of equipment can be put to work to eliminate a lot of wasted time on your part in determining how the machine should operate . Depend on him to help you locate components which may be hidden by parts of the machine, since he probably knows where they are. In short, make use of every available source of information to shorten the time necessary for you to arrive at the spot of trouble.
All failures of electric control circuits are not necessarily caused by electrical troubles. Quite frequently, the mechanical malfunction of some component may be the sole source of trou ble, so remember to examine suspected components not only for electrical trouble but also for mechanical trouble.
It must also be pointed out that a man who is attempting control troubleshooting who is not equipped with a voltmeter, an ammeter, and an ohmmeter is wasting valuable time and money. He must also be trained and competent in the proper use of these instruments and the proper interpretation of the readings that they give him. Even though you may know many men who do not make use of all these instruments in their trou bleshooting work, it is an indisputable fact that their efficiency could be greatly increased by a proper understanding and appli cation of these instruments to their work.
9 ·2 TROUBLESHOOTING CONTROL COMPONENTS
All that need be said of the individual problems involved in the various components of control has been covered in Chap.8. The trouble spots recommended in this chapter for checking under maintenance procedure are identical with those trouble spots which will have to be detected in the process of trouble shooting and repairing the circuit after it has failed to perform as it should.
Again, the best equipment for efficient and proper trouble shooting of individual components is a complete knowledge of their proper operation and a familiarity with as many manufac turers’ versions of each component as possible . Much of this knowledge will of necessity have to be gained through experi ence. The student may obtain a sizable portion of this required knowledge by a study of manufacturers’ literature and by making a concerted effort to familiarize himself with the various compo nents he comes in contact with in his daily work.
9 ·3 STEP-BY-STEP PROCEDURE
In order to make the procedure outlined in Sec. 9 ·1 clearer to you, we shall now consider a circuit and determine the probable cause of some troubles which we shall assume to have oc curred in this circuit.
The circuit of Fig. 9 ·1 is that of a chilled-water air-condition ing compressor. The components as shown on the diagram are as follows: The coil CR is a control relay. The coil Ml is the starter for the chilled-water pump. The coil M2 is the starter for the condenser-water pump. The coil M3 is the starter for the oil pump on the compressor itself. The coil M4 is the com pressor-motor starter. The contact identified as T is a thermostat
which senses the temperature of the chilled-water return. Its function is to start the condenser-water pump when this tempera ture reaches a predetermined high level. The contact identified as PSl is an oil-pressure switch whose function it is to stop the compressor should the oil pump fail and also to prevent its starting before the proper oil pressure has been obtained. The contact identified as FSl is a flow switch in the chilled-water piping system. Its function is to prevent the compressor from running unless there is sufficient flow of chilled water. The con tact identified as FS2 is a flow switch in the condenser water piping system.
Suppose now that you are called in to troubleshoot this circuit. The first step should be to determine from the owner or operator what trouble he is having with his circuit. Suppose that he tells you that the condenser-water pump does not start as it should. Then from a study of a diagram we can assume that the section of the circuit for the control relay is functioning properly, that contact CR2 closes, and that the chilled-water pump runs as it should. Something must be wrong in the third line of our schematic diagram.
The first procedure is probably to check the overload relays and determine that they were not tripped. Having done this, we next check the thermostat to see that its contact is closed as it should be. Here it must be pointed out that determining the setting of this thermostat and the actual water temperature will indicate whether it should be open or closed. We are assum ing that through the shutdown of the machine the water tempera ture has increased to a point that demands that these contacts be closed. Let us assume that the thermostat contacts are closed; then inspection of the starter for the chilled-water pump is indi cated to determine if contact Ml is closing when this contactor is energized.
If our inspection of this starter shows that this contact seems to be closing properly, then the next procedure is to disconnect the wires from coil M2 and apply an ohmmeter to the coil to determine whether it was open or not. From the preceding analy sis, it is almost certain that this coil will be found open, and for the sake of this illustration we will assume that it is. Before you replace this coil, the starter should be examined for proper mechanical operation. Determine that the contact arm which raises and lowers or swings to move the contacts is free from bind and that the spring tension is not excessive. Also examine the faces of the magnetic pole pieces to see that they have not been abused and possibly damaged by someone forcing them or even through the many operations of closing of the contactor. When all mechanical problems have been eliminated, install a new coil in the starter.
It would be good practice to check the voltage at the ends of the wires which feed this coil before putting it back into service. This can be done by connecting a voltmeter between the ends of these wires and operating the control circuit up to this point. If the voltage is excessively low or excessively high, then the cause of this trouble must be determined and eliminated. Otherwise, the new coil will also burn out.
Suppose that this circuit did not malfunction in this way , but instead the report was that everything seemed to work except the compressor itself. Then our operation would be to energize the circuit and watch its sequence to determine for ourselves where it failed. We would see that the control relay operates, the chilled-water pump starts, the condenser-water pump starts, and then that the oil pump on the compressor starts.
Here we shall assume that our sequence stopped and the com pressor did not come on the line as it should. Again examining our circuit, we find that we have a contact on the oil-pump starter which could cause trouble. We have a pressure switch and two flow switches which might be the source of trouble . So again we must determine which of these components is not properly functioning. If these components are readily accessible, a physical examination of each of them may immediately dis close the trouble. If they are inaccessible, however , a good pro cedure to follow is to disconnect the wires from the starter coil and operate the control circuit to determine if voltage is reaching the coil, thus eliminating the possibility of trouble being in the coil itself.
Let us assume that the contact M3 is properly functioning and we have checked it. The two flow switches have been deter mined to be properly functioning and their contacts closed. Then the examination of the pressure switch is the only remaining possibility. It may even be necessary in some cases to recalibrate pressure switches with known pressures to see that they are op erating at the settings which show on their indicating dials. Again, however, the procedure is to inspect physically and deter mine the actual cause for the part not functioning properly.
Summary
While this procedure may seem oversimplified, as you are guided through the diagram on a supposed troubleshooting job, it is the basis upon which good troubleshooting practice is laid .
No matter how complex the control circuit is, it can be separated into simple branches such as we have illustrated here and in other sections of this book. The efficient troubleshooter will nar row his trouble down to one of these simple branches of even a very complex circuit, so that the actual process of locating the troublesome component will be as simple as outlined here.
Review Questions
1. When is it necessary to check completely the connections of a whole control circuit?
2. Why must the wires be disconnected from a coil in order to determine accurately whether the coil winding is damaged or not?
3. Is all control-circuit trouble necessarily electrical trouble?
4. Does the fact that contacts appear to be touching indicate that the electric circuit is complete through them?
5. Why should the troubleshooter operate a machine through part of its sequence before starting to look for the trouble?
6. What are the two possible causes for repeated tripping of overload relays?
7. Should the troubleshooter try his circuit after repairing one fault, or should he attempt to fix everything that seems as if it might be causing trouble before trying the circuit?
8. What are the most frequent sources of trouble in motor starters?
9. When troubleshooting a circuit which has been operating, is it wise first to check to see if the wiring was done properly?
10. What is the chief source of failure of pilot devices such as float switches or limit switches?
11. In Fig. 9 ·1 what would be the most likely cause of the circuit operating only as long as the START button were held down?
12. What is the most likely source of trouble if , when we press the START button, the control relay remains energized but coil Ml does not pull in?
13. What would be the results if the overload relays on the circuit for coil M2 were to open while the compressor was runni!lg?
14. Which is generally the most difficult, finding the source of trouble or repairing the trouble after it is located?
15. Which of the above requires the most skill?