Electrical 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 new e-Book "Maintenance
Policy and Procedures Manual !!
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