Relay Circuit Assignment - Job #5
Here are the tasks to complete for this assignment. You may want to print
this page. Check off each task when done. Then move on to the next task.
Definitions Before You Begin
Be sure you have an understanding of these terms before beginning this exercise.
You may need to use a dictionary, a code book, a theory book, or even ask other
electricians you may know for help. This is part of the learning activity. Begin
now.
terminology
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definition
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pilot light
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stop pushbutton
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start pushbutton
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dependent operation
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independent operation
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jog
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Read this
As background, here are some details which will help you understand this
assignment. If you encounter words, terms, phrases or abbreviations which you do
not understand, then stop. Research the meaning before going on. This is very
important to your learning activity. The essence of any technical education
activity is comprehensive understanding of all topics which are encountered.
Background Information
SOURCE
Contactors
When a relay is used to switch a large amount of electrical power through
its contacts, it is designated by a special name: contactor. Contactors
typically have multiple contacts, and those contacts are usually (but not
always) normally-open, so that power to the load is shut off when the coil is
de-energized. Perhaps the most common industrial use for contactors is the
control of electric motors.
The top three contacts switch the respective phases of the incoming 3-phase
AC power, typically at least 480 Volts for motors 1 horsepower or greater. The
lowest contact is an "auxiliary" contact which has a current rating
much lower than that of the large motor power contacts, but is actuated by the
same armature as the power contacts. The auxiliary contact is often used in a
relay logic circuit, or for some other part of the motor control scheme,
typically switching 120 Volt AC power instead of the motor voltage. One
contactor may have several auxiliary contacts, either normally-open or
normally-closed, if required.
The three "opposed-question-mark" shaped devices in series with
each phase going to the motor are called overload heaters. Each
"heater" element is a low-resistance strip of metal intended to heat
up as the motor draws current. If the temperature of any of these heater
elements reaches a critical point (equivalent to a moderate overloading of the
motor), a normally-closed switch contact (not shown in the diagram) will spring
open. This normally-closed contact is usually connected in series with the relay
coil, so that when it opens the relay will automatically de-energize, thereby
shutting off power to the motor. We will see more of this overload protection
wiring in the next chapter. Overload heaters are intended to provide overcurrent
protection for large electric motors, unlike circuit breakers and fuses which
serve the primary purpose of providing overcurrent protection for power
conductors.
Overload heater function is often misunderstood. They are not fuses; that
is, it is not their function to burn open and directly break the circuit as a
fuse is designed to do. Rather, overload heaters are designed to thermally mimic
the heating characteristic of the particular electric motor to be protected. All
motors have thermal characteristics, including the amount of heat energy
generated by resistive dissipation (I2R), the thermal transfer
characteristics of heat "conducted" to the cooling medium through the
metal frame of the motor, the physical mass and specific heat of the materials
constituting the motor, etc. These characteristics are mimicked by the overload
heater on a miniature scale: when the motor heats up toward its critical
temperature, so will the heater toward its critical temperature, ideally
at the same rate and approach curve. Thus, the overload contact, in sensing
heater temperature with a thermo-mechanical mechanism, will sense an analogue of
the real motor. If the overload contact trips due to excessive heater
temperature, it will be an indication that the real motor has reached its
critical temperature (or, would have done so in a short while). After tripping,
the heaters are supposed to cool down at the same rate and approach curve as the
real motor, so that they indicate an accurate proportion of the motor's thermal
condition, and will not allow power to be re-applied until the motor is truly
ready for start-up again.
Shown here is a contactor for a three-phase electric motor, installed on a
panel as part of an electrical control system at a municipal water treatment
plant:
Three-phase, 480 volt AC power comes in to the three normally-open contacts
at the top of the contactor via screw terminals labeled "L1,"
"L2," and "L3" (The "L2" terminal is hidden behind
a square-shaped "snubber" circuit connected across the contactor's
coil terminals). Power to the motor exits the overload heater assembly at the
bottom of this device via screw terminals labeled "T1,"
"T2," and "T3."
The overload heater units themselves are black, square-shaped blocks with
the label "W34," indicating a particular thermal response for a
certain horsepower and temperature rating of electric motor. If an electric
motor of differing power and/or temperature ratings were to be substituted for
the one presently in service, the overload heater units would have to be
replaced with units having a thermal response suitable for the new motor. The
motor manufacturer can provide information on the appropriate heater units to
use.
A white pushbutton located between the "T1" and "T2"
line heaters serves as a way to manually re-set the normally-closed switch
contact back to its normal state after having been tripped by excessive heater
temperature. Wire connections to the "overload" switch contact may be
seen at the lower-right of the photograph, near a label reading "NC"
(normally-closed). On this particular overload unit, a small "window"
with the label "Tripped" indicates a tripped condition by means of a
colored flag. In this photograph, there is no "tripped" condition, and
the indicator appears clear.
As a footnote, heater elements may be used as a crude current shunt resistor
for determining whether or not a motor is drawing current when the contactor is
closed. There may be times when you're working on a motor control circuit, where
the contactor is located far away from the motor itself. How do you know if the
motor is consuming power when the contactor coil is energized and the armature
has been pulled in? If the motor's windings are burnt open, you could be sending
voltage to the motor through the contactor contacts, but still have zero
current, and thus no motion from the motor shaft. If a clamp-on ammeter isn't
available to measure line current, you can take your multimeter and measure
millivoltage across each heater element: if the current is zero, the voltage
across the heater will be zero (unless the heater element itself is open, in
which case the voltage across it will be large); if there is current going to
the motor through that phase of the contactor, you will read a definite
millivoltage across that heater:
This is an especially useful trick to use for troubleshooting 3-phase AC
motors, to see if one phase winding is burnt open or disconnected, which will
result in a rapidly destructive condition known as "single-phasing."
If one of the lines carrying power to the motor is open, it will not have any
current through it (as indicated by a 0.00 mV reading across its heater),
although the other two lines will (as indicated by small amounts of voltage
dropped across the respective heaters).
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REVIEW:
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A contactor is a large relay, usually used to switch current to
an electric motor or other high-power load.
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Large electric motors can be protected from overcurrent damage through
the use of overload heaters and overload contacts. If the
series-connected heaters get too hot from excessive current, the
normally-closed overload contact will open, de-energizing the contactor
sending power to the motor.
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Go to the Diagram of this Job
Click HERE to go to the diagram, lecture, and picture for this exercise. Follow
the instructions found there. Then return to this page with your back button.
Connect the Components
Use the material list on the diagrams page to have ready all the components you
will need to complete this exercise. Connect all required components based on
the diagram.
Check your Connections
With the diagram as a guide, verify that all the connections have been made
properly.
Apply Power To Test
Wear safety glasses when testing this circuit. Locate the overcurrent protective
device for the circuit you will use. If possible, connect to a GFCI protected
circuit for the added safety. Be sure you have a full understanding of this
circuit before applying power. Then when you test the circuit, there should be
no problem with it working properly. Make corrections to the wiring as needed so
the circuit works properly. Memorize these connections.
Sketch the diagram from Memory
A sure way to determine if you have internalized this circuit is to draw a
sketch of the diagram from memory. You may choose to use a straight edge. But
that is not necessary. When you have completed the sketch, check it against the
diagram which is given for accuracy. Electricians must have the ability to
remember circuit connections without reference. Here is your chance to
demonstrate what you have learned. If you are unable to sketch the ladder
diagram from memory, refer back to the given ladder diagram. Then try again.
Repeat this activity until you feel confident that you can sketch this specific
diagram without reference.
Reconnect From Memory
Now on to the real test. Reconnect this diagramed circuit from memory. This will
ultimately tell you if you understand this circuit or not.
Review Before Answering Questions
Go back to review definitions, the diagram, the picture, your notes, your sketch
and any other details which will help you as you move on to the questions about
this diagram.
Answer the Questions
When you are satisfied that you can draw a sketch of the diagram, and wire it
all from memory, it is time to go on to answer questions about the diagram.
Click HERE to go to the questions.
Check Your Answers
When you have completed the questions, go on to check your answers. If you have
any which are different than the given answers, try to determine the reason. Go
back to the diagram. Research the question. Ask other electricians you may know.
But do all it takes to understand the correct answer to each question.
Click HERE to go to the answers.
Go On
When you can answer all questions without error, and strongly feel you
understand all answers, it is time to go on to the next assignment.
Click HERE to go on to the next assignment page.
If you would like to research any of these topics further, enter the search
term in this Google Search Box:
Use these Relay Circuit links to go to where you left off in
your online activities.
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Assignment
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Diagram
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Lecture
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