Download Electrical Industrial Troubleshooting

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.
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 faultfinding 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
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.
Motor Controller:
* 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):
* 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.