In many ways the problems of installing and troubleshooting static systems are easier and less complicated than those of relay systems. The logic part of the circuit is generally packaged in a panel or panels where it offers ready access to most of the circuit. The modular construction makes unit replacement rela tively simple, and trouble location is greatly simplified over a comparable relay system.
14·1 INSTALLATION OF STATIC SYSTEMS
There is one idea which cannot be overstressed in this area of static control: Follow the manufacturer’s instructions. Each of the currently available component lines offers reliable, efficient, long-life control, but only when installed as recommended. Each company has engineered its components to work together as a system. They have run exhaustive tests on all phases of the system, including installation . The man in the field can hardly improve upon the proven methods developed by the company.
Some general statements can be made which will apply regard less of which system is being installed. First select good sensing devices. Limit switches and other devices which employ contacts should be selected to provide good contact pressure and good wiping action. Wire each sensing device to an individual signal converter. Never run signal wiring in the same enclosure with power leads. Generally speaking, iron conduit should be used as a raceway for signal wires, but the manufacturer’s instructions should be consulted on this. Some systems employ shielded cable under specific grounding requirements.
Should the power supplies be grounded? Which wires should be or can be grounded? Consult the manufacturer’s instructions, as there is no general approach to these questions.
If you are building up a panel from components, there are some general principles which should be followed. Group the signal converters so that the wiring from the sensing device does not mix with logic wiring. Place the signal converters so that the wire from the logic side of the logic element is as short as is practical. It is bad practice to feed a logic element in one panel directly from a logic output in another panel.
Group the logic elements within the panel so that a lead be tween elements can be as short as is practical. Use the size of wire recommended and terminate it in the specified manner.
Amplifiers, relays, and other a-c devices should be mounted to give good isolation from the logic elements and logic wiring. · A great deal of electric noise can be generated in these devices, particularly in the relays. Shielding helps prevent interference from electric noise, but isolation is much more effective in most cases.
Good electrical connections are a must in static control. When relays are used for control, the voltage and current in the circuit will allow a great deal of poor workmanship in the joints and connections. Static circuits operate at low voltage levels and at practically no current; therefore they cannot tolerate loose or dirty connections. Bad connections are very hard to locate after installation, and therefore it behooves the installer to be sure each and every connection is made properly when he makes it.
The logic element, whether it be an AND, OR, or NOT , is a very high-speed switch which will react to signals of such short duration that they are merely a flash on the screen of an oscillo scope. Any loose contact or electric noise which provides even a very short pulse to the logic input can cause a malfuncti on of the system. This type of trouble is very hard to locate unless the signal is applied to a MEMORY element, which will indicate that it has had a momentary input applied.
Any journeyman electrician who has studied the previous chapters of this book and who will read and follow the manufa c turers’ recommendations should have no trouble installing a static control system properly.
14·2 TROUBLESH OOTING STATIC SYSTEMS
Trouble in any control circuit is first indicated by some part of the machine or process not functioning properly. The cause of the malfunction could be in any part of the system from the sensing devices which initiate the control through the logic elements and amplifier which drive the final action device.
The function of the person who is servicing the equipment is to locate the cause of the trouble and repair it in an efficient manner so as to limit the down time of the system. The service man must follow an efficient system if he is to locate trouble and eliminate it with a minimum of wasted effort.
Failure to operate on initial start-up might be caused by errors in wiring. Any system which has been in operation must be wired properly. Therefore, there is no need to check it out wire by wire. Many servicemen with years of experience seem never to learn this fact and waste many hours in circuit tracing.
Consider the circuit of Fig. 14·1. The solenoid does not oper ate when it should. We will assume that all logic elements are equipped with state-indication lights, as are the signal converters.
The first step would be to make a voltage check at the input of the solenoid or the output of the amplifier, whichever location is accessible without opening connections or disturbing the wir ing. The presence of proper voltage at the output of the amplifier would indicate a bad solenoid or broken connections between the solenoid and amplifier. A further check must then be made at the solenoid.
Assume that there was no output from the amplifier and that all logic elements are equipped with state-indication lights. In this case it would not have been necessary to make any voltage
checks unless the MEMORY element indicated an output. With an output from the MEMORY, the trouble must be in the amplifier if it has no output. A check of the amplifier’s input voltage
will eliminate any possibility of trouble in the logic wiring between the output of the MEMORY and the input of the amplifier.
The above process can be quickly followed through the logic section by merely checking the state lights on each element. In this case the AND should be on, the OR should be on, and the NOT should be on.
Most cases of trouble will be found in the final action device or the sensing devices. When the signal converters are equipped with state lights, a visual check of these will indicate which sensing device is responsible for the trouble. In the circuit of
Fig. 14·1, the lights associated with LS 1 or LS2 or both should be on. The light for PSl should be on, and the one for PS2 should be off. Any light which is not giving the proper state indication will lead you to the actual cause of the trouble .
Suppose the system is not equipped with state lights. This in itself will make servicing the system a slower process but does not materially alter the steps to be used. The panel should be equipped with a tester designed by the manufacturer to be used to test for ON and OFF signals at the input and output of a logic element. It becomes necessary to use the test lead and check the input and output of each logic element from the amplifier to the signal converter until the faulty one is located.
There is no need to check the logic elements until all original inputs are determined to be correct and present at the output of the signal converters. A check of the input to the amplifier will then tell you whether there is any trouble in the logic sec tion. Whenever the input to the amplifier responds properly to changes in inputs to the logic section, the logic elements are operating properly.
Many times it is necessary to check part of the system under input conditions not represented by the condition of the sensing device contacts . For instance, the NOT in Fig. 14·1 can only be checked when PS2 is closed. This condition can be simulated by applying the proper ON voltage to the input of the NOT. This is best done by using the testing device available from the manu facturer as part of the component line for the system.
One of the real advantages of a static control system is that in most cases the source of trouble can be determ ined while you stand in front of the logic control panel.
The approach up to this point has been to use circuit testers designed especially for the system to be serviced. Signal tracing and service can be performed by taking voltage readings at the input and output of a logic element if good voltmeters are used. The minimum acceptable voltmeter would have a 20,000 ohms per-volt sensitivity. Do not use neon testers or 1,000 ohms-per volt voltmeters for this purpose.
If voltage testing is to be used , the serviceman m ust know what voltages represent ON and OF F in the particular system in question. Of those we have studied, the General Electric Com pany system uses 0 volts for ON and -4 volts for OFF. The Cutler-Hammer company system uses +10 volts for an ON signal and 0 volts for OFF . The SquareD company system uses -10 to -20 volts as an ON input and 0 volts for OFF.
When a zero-volt signal is indicated for the OFF or ON condition, a good voltmeter may well indicate up to about one-half volt due to leakage current through the transistors. This is nor mal and should cause no concern.
When more than one part of the system is malfunctioning or when the output amplifier seems to be bad, check the power supply. Power-supply troubles account for a large percentage of trouble in most electronic devices and systems and therefore should always be suspected until checked out.
Intermittent troubles are always hard to find and can be caused by bad contacts or poor connections. One factor which must always be considered when one seeks the cause of intermit tent trouble is the presence of electric noise. When a system has been in operation for some time without noise problems and then develops trouble which is suspected to be noise, certain things should be checked. If any new wiring has been installed, check it out. New installations of other equipment or wiring near the panel or field wiring for the system should be checked as a possible source of noise. When no changes have been made, check out grounding connections and filters which may have gone bad.
The static elements are probably the most reliable part of the entire system . Be sure to check fuses, switches, and other common electric parts of the system. They probably are the source of the trouble.
14·3 ELECTRIC NOISE
The term “electric noise” has been used many times in previous discussions and deserves some clarification and explanation. This section will be devoted to the subject of noise and its elimination.
Electric noise signals can be generated by almost any power equipment. The opening or closing of contacts under load almost always produces electric noise. The transmission system which feeds power to the building carries electric noise, and every piece of operating equipment connected to the wiring system of the building is a potential source of noise. Electric noise, then, can not be prevented, but must be kept out of the signals fed into the logic control components.
Electric noise exists in many forms, but for the purpose of this discussion consider it to be essentially a sudden change in current flow or voltage in a conductor. When changes in value of current or voltage are uniform or regular, we refer to this current as alternating current or a-c. We then might describe electric noise as nonuniform alternating current.
An acceptance of the above oversimplified description of electric noise provides us with some rules for keeping it out of the control circuit. Signals of alternating current, and therefore elec tric noise, are transferred or coupled from one circuit to another by one or more of three ways.
The first way, and probably the most common, is through mutual electromagnetic coupling. The second is through the capacitance between conductors, or electrostatic coupling. The third means of coupling is by means of common impedances between the circuits.
Most cases of electric noise will involve a combination of the three forms of coupling, since every circuit has induc tance, capacitance, and impedance in varying proportions. We will consider each form of coupling separately for simpli fication.
Electromagnetic coupling occurs whenever the circuit contain ing the noise is run parallel and close enough to the signal circuit to provide coupling through their mutual electromagnetic fields. The most effective means of reducing this form of coupling is by separation. The degree of coupling decreases with the square of the distance between the conductors. Electromagnetic fields tend to cancel each other in circuits with twisted-pair wires; therefore if either or both the noise circuit and the signal circuit employ twisted-pair wiring, the coupling will be reduced. The third method of reducing electromagnetic coupling is by means of shielding. The noise which is the source of trouble is generally low frequency and therefore requires a ferrous material as an effective shield. Rigid conduit of the common electrical variety is ideal for this purpose, provided only that signal wires are run in the pipe. The amount of electromagnetic coupling between two circuits is proportional to the distance for which they are parallel; therefore reducing the distance will reduce the coupling and the noise problem. Each case of noise coupling is different and may require analysis if it is severe, but most can be elimi nated by using good installation practice.
Electrostatic coupling is caused by two conductors run paral lel and at different voltages. The conductors form the plates of a condenser. The air or other material between the conductors forms a dielectric. The most effective means of reducing this coupling is by shielding the signal wires. Separation of the con ductors and reduction of the distance where the two circuits are parallel will also help.
Common impedance coupling occurs only in circuits which share some impedance. The most common source of this type of circuit is the temptation to use a common wire to feed one side of many sensing devices. The most effective way to eliminate this form of coupling is to use two wires from the panel to each sensing device.
The use of common arc-suppressing circuits on relay contacts .
and inductive loads can prevent or reduce the generation of noise signals at their source.
A summary of noise suppression would indicate that static systems should be wired with care. Separate circuits should be used for each sensing device. The use of twisted-pair leads for sensing or signal circuits might be indicated in some cases. Ridged conduit for field wiring seems to provide the best likeli hood for trouble-free operation . Remember, consult the manu facturer of the system in question for specific details.
Summary
The installation of static control systems requires only normal electrician’s skills plus a little care. Follow these rules: Make every joint carefully. Never run signal wires in conduit with power wiring. Keep signal wires and logic wiring separated in the panel. Follow the manufactu rers’ recommendations. Avoid noise sources.
Whenever possible, use the testing devices available from the manufacturer of the system in question. If it is necessary to service with a voltmeter, use one with at least a 20,000 ohms per-volt rating.