Troubleshooting or fault finding in control circuits can take a few minutes, hours or days depending upon the complexity of the problem and the expertise of the troubleshooter.
If the troubleshooter does not understand the control functions and is not familiar with control components and troubleshooting procedures, then, even a small fault in the electrical control circuit can baffle him. This may lead to unnecessary production loss in the plant.
A technician engineer who may be skilled in wiring a control circuit from the given diagram may not necessarily be able to troubleshoot Motor control circuits efficiently. For troubleshooting of control circuits the main asset is the possession of analytical mind trained in all aspects of control functions, knowledge of components and circuits.
The secret to efficient and accurate troubleshooting lies in determining the section of the control circuit that contains the trouble component and then selecting the proper component to be checked.
This can however be accomplished by efficient and accurate circuit analysis and not by trial and error or by checking of components at random.
Troubleshooting starts with the analysis of the problem.
To analyze any problem it is helpful to first identify the type of problem. This limits the area in which troubleshooting is to be done.
In identifying the type of fault the following areas might be considered:
Electrical
Mechanical
Hydraulic power system
Pneumatic system
Trouble spots of Motor control Circuit
As it is impractical, if not impossible to list all the trouble spots, only those trouble spots have been discussed in which the troubles are quite frequent. The possible trouble spots are as follows.
Fuses
Loose Connections
Faulty Contacts
Incorrect Wire Marking
Combination Problems
Low Voltage
Grounding
Momentary Faults
Poor Maintenance
Fuses
One of the trouble at this spot could be looseness of fuse carrier or its falling out due to looseness. Fuse carrier can become loose and come out due to vibrations of the machine.
In such case the permanent solution is to insulate the control panel from vibration. Temporary solution is to adjust the male contacts of the fuse carrier so as to fit tightly into female contacts of fuse base.
Alternatively the fuse carrier can be replaced with a new one.
Another trouble at this spot could be the blowing of the fuse. As it is known, the fuse will blow only when there is some short circuit or grounding in the control circuits.
One can try once or twice by replacing the fuse, as some times the fault is of transitory nature and may get cleared by itself. If replaced, the fuse should be exactly of the same voltage and current rating. If the fuse continues to blow out then further checking should be done to detect the fault.
If the control circuit is very large then it is better to have sub control fuses for different sections of the control circuit.
Loose Connections
The problem of loose connection becomes serious in case of power circuits as the current handled is of higher magnitude. A loose connection in a power circuit generates local heat which spreads to other parts of the same component and also to adjoining components.
Loose power connection finally damages the component due to insulation failure. Loose connection in thermally sensitive components may result in malfunctioning. For example, a thermal sensitive overload relay may trip due to conduction of heat to it.
A regular check for loose connection is the best remedy for avoiding such troubles.
Faulty Contacts
This trouble occurs in such components as motor starters, contactors, relays, push buttons, and various types of switches. The most common problem appears with normally closed contacts.
Although visible observation shows that such contacts are closed but in actual practice they may not be conducting current. Contacts may not be conducting due to dirt or formation of copper oxide films which is an insulator. Another reason could be improper spring pressure.
Oxide film from the contact can be cleaned by drawing a piece of rough paper between the contacts. Care should however be taken to use only a fine abrasive to clean contacts.
Contacts should never be filed. Most of the contacts being used today have silver coating, the silver coating will get destroyed if filing is done.
If the contacts are badly worn out and heavily pitted these should be replaced.
Incorrect Wire Marking
This problem will usually appear at the initial stage of control panel wiring or at the stage of assembly and erection of the machine in the plant.
The error can also creep in during preventive maintenance or during any alteration done in the control circuit.
This error becomes difficult to locate as control cables running from a control panel to the machine may have as large as 20 conductors.
One common problem is the transposition of numbers. Another problem that may occur with numbering ferrules is during connecting conductors into a terminal block.
With a long block and many conductors, it is a common error to connect a conductor either one block above or one block below the proper position.
Combination Problems
As described earlier some of the problems in machines cannot be referred to as faults exclusively due to defective electrical or mechanical or hydraulic or pneumatic system.
The problem may be in one area or it may be simultaneously in two areas.
Whenever such a discrepancy arises, it is better to first check the electrical circuit as it is faster to check an electrical circuit than a mechanical system.
To quote an example, one very common combination problem is overload tripping of a motor. The motor may be tripping due to actual mechanical overload or due to malfunction of overload relay or due to defect in the motor itself.
A motor may take more current due to damaged be a ring.
An electrical cum temperature combination problem can occur in ovens and furnaces.
A methodical approach should be made to analyze such problems instead of attempting a trial and error method.
Low Voltage
If the system voltage becomes low, relays, contactors, timers etc. start dropping (getting deenergized) and as a result control circuit starts malfunctioning. Motors would trip at low voltage as they would draw more current from the supply.
The system voltage may become low because of heavy loading or inadequate conductor size. A common source of such problem is the addition of new machines without properly checking the capacities of power supply line and the transformer. Low voltage will result in generation of inadequate heat in ovens.
For example, if the voltage is dropped to one half the heating element’s rated voltage, the heat output will be reduced to one fourth.
Grounding
There are many locations on a machine where a ground can occur. However, there are a few spots in which grounds occur more often. These spots are discussed below:
Often ground fault occur in limit switches, pressure switches, temperature switches, float switches etc. Most of the time this is due to the compact design of many components allowing very small space for making connections. Due to lack of space, bare conductors at terminals of a component may come in contact with the case of the component. Many times the cables/wires going to a component may get pulled and cause breakage or grounding of the wires. Control wires in control panel can also get grounded in the panel itself or with an adjacent ground wire.
A ground may occur due to damaging of insulation while pulling control wires through a conduit, if proper care is not taken. Insulation damage may also occur when there are several bends in the conductors.
A ground fault may also occur due to improper use of a stranded conductor. Care must be taken while placing a stranded conductor into a connector. All strands must be used. One or two strands which have not been used may touch the case or a normally grounded conductor. Even if this does not happen, current carrying capacity of the conductor gets reduced due to less number of strands in use.
Momentary Faults
Sometimes faults occur but do not persist for a long time. It becomes really difficult to locate the origin of such momentary faults.
The operator of the machine needs to keep a close observation to find out at which part of the cycle of control circuit operation the fault occurs.
Loose connection or a broken conductor inside the insulation can also be the cause of momentary fault.
Sometimes a control component may be malfunctioning. In case of doubt a component can be replaced by a new one.
If the machine is new, fault may already exist in the control circuit due to improper connection.
Poor Maintenance
If regular maintenance of a machine is not done the frequency of occurrence of faults will increase.
Dust, dirt and grease should be removed from electrical parts, otherwise short circuit may develop.
It is recommended that a regular maintenance schedule should be followed.
History sheet of each individual machine should be maintained.
General Procedure for troubleshooting Motor
The procedure for detecting fault in such a circuit is as follows:
(1) The first step should be to analyze the control circuit and ascertain that it has been properly designed as per the control function requirements;
(2) The next step is to run the machine and follow the operation through the expected sequence until one finds the section of control circuit which is not operating;
(3) After locating the faulty section, wiring should be checked. If wiring is as per drawing then control components of this section should be checked thoroughly;
(4) When trouble in the faulty section is located and removed, the machine should be started again to run successfully throughout the complete cycle. In case of fault existing in any other section of the control circuit, one should now try to locate the fault of that section.
Secondly, troubleshooting procedure for an already existing circuit which was working properly before the occurrence of a fault. The possibility of improper connection therefore is eliminated. The procedure for troubleshooting in such a circuit is as follows:
(1) The first step is to understand the operation and control circuit of the machine. If the control circuit is very large and complex, help of the operator should be taken to find out how much of the circuit is operating. In this way one can directly start with the section of the circuit that does not function.
(2) When the faulty circuit section has been identified, first a careful check of the circuit and components involved in that section should be done. A careful visual inspection may help to detect a faulty component or an open wiring. If nothing is found out in the visual inspection then the procedure may be as follows.
(3) Find out which operation is not taking place and identify the contactor/solenoid valve coil which is responsible for this operation. Operate the machine and check whether there is voltage across the coil of relay or the contactor/solenoid valve coil. If proper voltage is available across the coil then trouble, most likely, is in the winding of the coil itself. Now, to check the continuity of coil, power is switched off and continuity of the coil is checked with an ohm meter. If the coil is alright, a low resistance is indicated. Working of the contactor or solenoid valve should be checked before replacing the burnt coil. If it is suspected that contactor closing mechanism or solenoid valve is defective, a new contactor or a new solenoid valve should be installed.
(4) Suppose that in the step discussed above voltage is found to be not reaching the contactor coil. This indicates that some contact is not closing when it should, thus disconnecting supply to the coil. In such a case control circuit drawing should be referred, to find out components whose contacts should close to energize the coil. These contacts may be of different relays, limit switches, pressure switches, temperature and float switches etc. Now the task will be to determine which of the components connected in series is not making contact.
(5) To find out contact of which particular component is not making, supply should be checked at various points leading to the contactor coil. Here, note that difficulties may be faced in checking if limit switches, pressure switches etc. which are on the machine are wired directly in the control circuit. If, however, relays are energized through limit switches etc., and then contacts of these relays are used in control circuit, checking will become easier. This point has already been explained in Chapter 2 while discussing relays. Once the defective component is detected then the cause of malfunctioning can be found out. If the contact is not making due to a copper oxide film or dirt, cleaning should be done and if contact is not closing properly, adjustment can be done. If, contact is badly pitted it should be replaced. The other possibility during this checking can be detection of open circuit due to a broken or burnt wire.
(6) Having eliminated the fault, the machine should be started again and if the machine does not operate successfully throughout the complete cycle of operation, the above procedure should again be applied to the next section of the control circuit which is faulty.
(7) Quite frequently, grounding of a wire going from control panel to the machine may be the cause of trouble. A check should be made for detection of ground fault, by putting off the power supply. Resistance to ground of the wires should be checked with an ohm meter, or alternatively, a test lamp can be used to detect ground, where 230 V supply with neutral earth is available. To check ground, one side of test lamp is connected to phase wire while the other end is connected to the wire which is to be tested for ground. If the wire is grounded the lamp will glow, otherwise it will remain off.
One important rule to be observed while troubleshooting is to attend to only one thing at a time. If you suspect that a contact is malfunctioning, first correct this contact and then check machine operation. If fault persists then proceed to check other suspected possible trouble spots. It is very seldom that several parts of a machine would wear out at the same instant even though their condition may be poor. Tempering with more than one component at a time may introduce more trouble than original trouble.