Plant Maintenance Resource Center
A laptop computer with PLC programming, communication, and operating programs is a necessary tool in today’s modern plant. Engineers, production supervisors, maintenance supervisors, maintenance technicians, electricians, instrument technicians, and maintenance mechanics all need to have PLC and computer knowledge, training and skills in PLC troubleshooting.
On the job training on PLC’s is usually not very effective until the person being trained has reached a certain level of expertise in several areas. Knowledge and skills in electricity, troubleshooting, and computer operation are necessary prerequisites to effectively assimilate basic PLC training. The author found that long term retention of material studied was higher from a vocational course taken at a local junior college than from a fast-paced, cram-course through a manufacturer.
The manufacturer’s course covered essentially the same material as a course at the junior college (JC). The major differences were the amount of study time and shop time. The JC course was four hours of class time per week for 15 weeks. There were three hours of shop time doing actual hands on work of the problems and material covered in the first hour. Additional time was spent at home studying the manual and writing programs. Also, the JC was open at night for extra shop time on the PLC’s and computers.
In contrast, the manufacturer’s course was five, eight hour days. Class work was extremely fast and condensed in order to cover the amount of material involved. The instructor was very knowledgeable and covered the course material as we tried to input the programs into desktop training equipment in order to see how it worked. By the end of each day, our minds were jammed with information. By the end of the week, we all passed the course, but I had a hard time remembering what we had studied on the first day.
Basic troubleshooting techniques apply to every situation and occupation. Positive identification of the problem(s) is absolutely essential to solving the problem(s). Many times, the inexperienced troubleshooter will mistake one or more of the symptoms for the problem(s). Solving the symptom(s) will normally just postpone the problem(s) to a later date. By which time, the problem(s) may have grown to mountainous proportions.
An example is when a person experiences a headache and takes a mild pain reliever, such as aspirin. The actual problem might be any number of things: eyes need to be checked, medication or lack of medication, muscle strain, stress, tumor, blood vessel blockage, or old war injury. The same thing occurs in industry, a fuse in a circuit blows and the maintenance person gets the replacement fuse and inserts it into the fuseholder. There are many things that could have caused the fuse to blow, depending on the complexity of the circuit.
Excess current caused the fuse to open (blow). Excess current could have been caused by: overload on the load; short circuit between the wires, grounded wires, short circuit in the load, ground in the load, voltage spike, voltage droop, etc. If the maintenance person does not troubleshoot the circuit prior to replacing the fuse and restoring power, negative consequences could arise.
It is not uncommon for a process to develop a number of small problems and continue to function at a degraded level of operational capability. Then, one more small problem occurs and the whole process breaks down. Finding and correcting the last problem will not necessarily restore the operational capability of the process. The process continued operations with the small problems, but the small problems may not allow the process to restart from a dead stop. All the other small problems must be identified and corrected before the process is restored to full operational capability.
This situation arises in industry as well as a person. The person can continue to function with a number of small problems, such as fatigue, blood pressure problems, hardening of the arteries, artery blockage, but one more small blood clot in the wrong place could easily cause the death of the person. Clearing the blood clot does no good to the person. They will not be restored to full operational capability.
Troubleshooting In The Field
Unless prior experience dictates otherwise, always begin at the beginning.
Ask questions of the Operator of the faulty equipment:
- Was equipment running when problem occurred?
- Does the Operator know what caused the problem, and if so, what, in their opinion, caused the problem?
- Is the equipment out of sequence?
- check to ensure there is power
- turn on circuit breaker, ensure motor disconnect switch is on, and operate start button/switch
Use voltmeter to check the following at incoming and load side of circuit breaker(s) and/or fuses, ensure that voltages are normal on all legs and read voltage to ground from each leg:
- main power, usually 460 VAC between phases and 272 to ground
- control & power, 208/240 between phases and 120 to ground and 120 VAC to neutral on a grounded system
- low voltage control power, usually 24 to 30 VAC and/or VDC between phases and possibly to ground, usually negative is connected to ground
Check controlling sensors in area of problem, then make complete check of all sensors, limit switches and other switches to ensure they are in correct position, have power, are programmed, set, and are functioning correctly.
If and when a problem is found, whether electrical or mechanical, the problem should be corrected and the fault-finding begun anew, a seemingly unrelated fault or defect could be the cause of the problem.
When there is more than one fault, the troubleshooting is exponentially more difficult, do not assume that all problems are solved after completing one, always test the circuit and operation prior to returning the equipment to service.
If available, check wiring diagrams and PLC programs to isolate problem.
Variable Frequency Drive (VFD) can be reset by turning power off, wait till screen is blank and restore power; on some VFD’s, press Stop/Reset – then press Start.
Check that wiring is complete and that wires and connections are tight with no copper strands crossing from one terminal to another or to ground.
Ensure that the neutral reading is good and that the neutral is complete and not open.
Motor Testing In Shop
Prior to connecting a motor:
- move motor to electric shop motor test and repair station
- connect motor leads for 460 volt operation and wrap connections with black electrical tape
- check motor windings with an ohmmeter, each reading between phases should be within one or two ohms of each other; A to B, B to C, A to C
- use megohmmeter to check insulation resistance to ground of motor windings on 500 volt scale; minimum reading is 1000 ohms of resistance per volt of incoming power that motor will be connected to
- connect motor to power test leads and safety ground after checking that test lead power is shut off; secure motor to table to prevent motor from jumping when started; turn disconnect on; press start button; check “T” leads for motor amperage; check for abnormal sounds and heat in bearings or windings; clean motor shaft; shut down and disconnect
Motor Testing In Field
When a motor overload or circuit breaker trips and/or blows fuses, certain procedures and tests should be carried out:
- lockout and tagout main circuit breaker;
- test insulation resistance of motor wires and windings by using megohmmeter between T1, T2, & T3 leads and ground, then;
- test “T” leads to motor with ohmmeter for continuity and ohmage of windings between A to B, B to C, A to C; each resistance should be within 1 or 2 ohms of each other; if the ohms readings are significantly different, or, if there is no continuity; go to the motor disconnect box, turn it off, perform the continuity and resistance test on the “T” leads, again; if the readings are good, the problem is in the wires from the motor controller to the disconnect switch;
- check the three wires by disconnecting all three wires from switch and twist together; go to controller and check for ground with megger, check for continuity between A to C, B to C, A to C; one or more wires will be open or grounded;
- correct solution is to pull all new wires in from controller to motor disconnect switch, whatever caused the problem may have damaged the other wires, also; replace all wires
if problem is on motor side of disconnect switch, open motor connection box and disconnect motor;
- check motor for resistance to ground with megohmmeter, if reading is below 500,000 ohms, motor is grounded and must be replaced;
- test motor windings for ohms between phases with ohmmeter A to B, B to C, A to C, readings should be within 1 or 2 ohms of each other; if readings indicate open or a significant ohmage difference, replace motor;
- if motor test readings are good, test the motor leads between the disconnect switch and the motor connection box for continuity and ground resistance, if readings are not good, replace wires;
- if all readings are OK, reconnect motor, remove lockout, and restore to service; the problem could have been mechanical in nature; an overload on motor caused by the chain, belt, bad bearings, faulty gearbox, or power glitch.
- check motor Full Load Amps (FLA) at motor and check setting on controller overload (OL) device; most newer OL devices are adjustable between certain ranges, some older OL devices use heaters for a given amperage
- if circuit disconnecting means in controller is a circuit breaker, it should be sized correctly
- if the disconnecting means is a Motor Circuit Protector (MCP), the MCP must be correctly sized for the motor it is protecting and the MCP has a trip setting unit which has to be correctly set based on the Full Load Amperage of the motor; using a small screwdriver, push in on the screw head of the device and move to a multiple of thirteen of the FLA; example: a motor FLA of 10 amps would require that the MCP trip device be set to an instantaneous trip point of 130 amps
- fuses protecting the motor should be the dual element or current limiting type and based on the motor FLA
Programmable Logic Controllers (PLC Troubleshooting)
- check to ensure main power is on( 120 VAC
- check 24V power available
- identify problem area
- check sensor operation in problem area
- check sensor Inputs to PLC
- check on PLC that a change in sensor state causes the corresponding Input LED on the PLC to go on or off
- identify Output controlled by Input on PLC ladder diagram
- ensure that Output LED is cycling on/off with Input
- check that Output voltage is correct and cycling on/off with Input
- locate Output device and ensure that voltage is reaching device and cycling with Input
- ensure that Output device is working correctly (solenoid coil, relay coil, contactor coil, etc.)
- an Input or Output module can be defective in one area or circuit and work correctly in all other circuits
- if each field circuit is not fuse protected, the modular internal circuit becomes a fuse and can be destroyed by a field short circuit or any other overcurrent condition
- check modular circuit; if bad, module must be replaced after correcting field fault
- shut down PLC prior to changing any module -main power and 24V power
- locate fault in field circuit by disconnecting wires at module and field device, check between wires for short circuit and to ground for short circuit; replace wire is short circuit found
- check device for ground, short circuit, mechanical and electrical operation, even when problem found in wires, always also check device for another fault, problem in wires can cause problem in device or vice versa; if device defective, replace device and then check total circuit before placing in operation and after restoring circuit, check again to ensure circuit and module are operating correctly
- check power supply module; if no output, shut down power and replace supply module
back plane can go bad, some of the modules with power and others with no power, replace backplane
- sometimes, the PLC can be reset using the Reset keyswitch; ensure that turning the PLC off won’t interrupt other running sub-set programs, turn keyswitch to far right, after 15 seconds, turn to far left wait, then return to middle position; this operation should reset program and enable a restart
- the PLC program can have a latch relay with no reset under certain conditions, the keyswitch reset may have no affect on the latch, try turning the power to the PLC off and back on, this operation may reset the latch and allow the program to be restarted
the PLC is usually part of a control circuit supplied with 120VAC through a 460V/120V transformer as part of a system with motors, controllers, safety circuits, and other controls; occasionally, cycling the main 480V power off/on will be necessary to try to reset all the safety and control circuits
- possession and use of an up-to-date ladder diagram, elementary wiring diagram, manufacturer’s manuals & diagrams, troubleshooting skills, operator’s knowledge, and time are all required to solve issues involved in maintaining a modern manufacturing production line
Larry Bush finished basic electrical training at a US Navy school in 1957 and has worked continuously as an electrician, electrical supervisor, electrical field engineer, maintenance supervisor, maintenance foreman, maintenance manager, or plant engineer for 47 years. Mr. Bush reached the rating of Electrician’s Mate Second Class, submarine qualified; worked as a Journeyman Marine Electrician IBEW & MFU, and Journeyman Maintenance Electrician in IBEW, GBBA, USW, & Teamsters; Electrician card from US Coast Guard for Merchant Marine; and journeyman construction electrician (not necessarily in that order).