Relay Circuit Assignment - Job #3
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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momentary contact
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holding circuit
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holding contacts
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line voltage
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low voltage
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three wire control
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multiple start buttons
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multiple stop buttons
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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
The most common relay circuit wiring method is called three wire control.
This is a circuit which has a stop and start button plush a holding circuit.
The design of this circuit provides a method to start a motor by hand. Then
when ever it is desirable, the stop button will stop it.
An electrical circuit parallel to the start button is completed by a set of
normally open contacts on the magnetic relay. When energized, the no contacts go
closed and the circuit will stay on.
This wiring diagram relay circuit can be used to control equipment which is
different than control voltage. For example, with a coil on the contactor at 120
volts, contacts on the control relay can be used to turn on and off even a three
phase large motor. The control voltage does not need to match the load voltage.
It can, but it need not.
SOURCE
In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is
almost always drawn at the right-hand side of the rung. While it doesn't matter
electrically where the relay coil is located within the rung, it does
matter which end of the ladder's power supply is grounded, for reliable
operation.
Take for instance this circuit:
Here, the lamp (load) is located on the right-hand side of the rung, and so
is the ground connection for the power source. This is no accident or
coincidence; rather, it is a purposeful element of good design practice. Suppose
that wire #1 were to accidentally come in contact with ground, the insulation of
that wire having been rubbed off so that the bare conductor came in contact with
grounded, metal conduit. Our circuit would now function like this:
With both sides of the lamp connected to ground, the lamp will be
"shorted out" and unable to receive power to light up. If the switch
were to close, there would be a short-circuit, immediately blowing the fuse.
However, consider what would happen to the circuit with the same fault (wire
#1 coming in contact with ground), except this time we'll swap the positions of
switch and fuse (L2 is still grounded):
This time the accidental grounding of wire #1 will force power to the lamp
while the switch will have no effect. It is much safer to have a system that
blows a fuse in the event of a ground fault than to have a system that
uncontrollably energizes lamps, relays, or solenoids in the event of the same
fault. For this reason, the load(s) must always be located nearest the grounded
power conductor in the ladder diagram.
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REVIEW:
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Ladder diagrams (sometimes called "ladder logic") are a type
of electrical notation and symbology frequently used to illustrate how
electromechanical switches and relays are interconnected.
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The two vertical lines are called "rails" and attach to
opposite poles of a power supply, usually 120 volts AC. L1
designates the "hot" AC wire and L2 the
"neutral" (grounded) conductor.
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Horizontal lines in a ladder diagram are called "rungs," each
one representing a unique parallel circuit branch between the poles of the
power supply.
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Typically, wires in control systems are marked with numbers and/or
letters for identification. The rule is, all permanently connected
(electrically common) points must bear the same label.
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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.
Job
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Assignment
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Diagram
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Lecture
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Picture
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Questions
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Answers
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