Electrical
Industrial Troubleshooting
By Larry Bush
Troubleshooting In The Field - Motor
Testing - Motor Controller - Programmable
Logic Controllers (PLC)
A laptop computer with PLC programming, communication, and operating programs
are 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 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
problems. Many times, the inexperienced troubleshooter will mistake one or more
of the symptoms for the problems. Solving the symptom(s) will normally just
postpone the problems to a later date. By which time, the problems 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 fuse holder. 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.
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.
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 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
* 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
over-current 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 key switch; ensure that
turning the PLC off won't interrupt other running sub-set programs, turn keys
witch 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 key switch 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.
About the Author: Larry Bush has
been an electrician for 47 years, and in maintenance management for 22 years. Download
his e-Books "Maintenance
Policy and Procedures Manual" or "Emergency
Diesel Electric Generators Ebook"
!
Related Products:
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