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Electrical Sensing Devices.Montana State Univ., Bozeman. Dept. of Agriculturaland Industrial Education.Montana State Office of Public Instruction, Helena.Dept. of Vocational Education Services.Jul 8653p.Guides - Classroom Use - Guides (For Teachers) (052)

MF01/PC03 Plus Postage.Agribusiness; *Agrir-ltural Education; *AgriculturalProduction; Behavioral Objectives; Electric Circuits;*Electricity; Electric Motors; *Electronic Control;*Electronic Equipment; Higher Education; LearningActivities; Teaching Methods; Units of Study;*Vocational Education*Electrical Sensing Devices

ABSTRACTThis unit of instruction on electrical sensing

devices is designed especially for use with freshman vocationalagriculture students. A unit plan discusses the general aims andgoals, lesson titles, student and teacher activities, and references.The unit consists of four lessons. A lesson plan for each lessonprovides these components: need; references; objectives; interestapproach; an outline of key questions, problems, and concerns withappropriate teaching techniques and information; application andfollowup; and transparency masters, exercises, handouts, and/orworksheets. Lesson topics are: electric controls and controlcircuits, using switnhes to control electrical equipment, using relaydevices in the electrical circuit, using magnetic or automaticelectric motor control devices, and using automatic sensing controldevices. (YLB)

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a Educed** Research ard ImprovementUCATIONAL. RESOURCES INFORMATION

CENTER (ERIC)

Rut document has been reproduced asreceived trom the Person or onianiatton

r originating it0 Minor changes have been made to improve

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"PERMISSION TO REPRODUCE THISMATERIAL HAS BEEN GRANTED BY

q0 THE EDUCATIONAL RESOURCESINFORMATION CENTER (ERIC) "

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ELECTRICAL SENSING DEVICES

The work presented here was supported by theMontana Office of Public InstructionDepartment of Vocational Education

Services

o

Montana State UniversityDepartment of Agricultural & Industrial Education

Room 126, Cheever Hall, Bozeman, MontanaJuly, 1986

'0

I

I

a

FOREWORD

This unit of instruction has been designed especially for

use with freshmen vocational agriculture students. For your

convenience, the material has been prepared to fit into a three-ring, loose-leaf notebook. Other material that is prepared toaccompany this unit of instruction will be prepared in a similar

manner.

The instructor should study the entire unit carefullybefore attempting to teach any of the lessons. The key conceptsthat should be presented to meet the objectives of the core

curriculum are included; however, all material that would beapplicable may not be provided. Each instructor should look forways to include additional material where appropriate.

Some handouts and visual materials are included with eachlesson. Here again, each teacher may have additionalillustrative material that would be appropriate. It is alsoimportant to have available all references listed in the unit.

The lesson content is based on the references listed; however,there are many more references that could be listed witn thiaunit.

i.4

vlb

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TABLE OF CONTENT3

Electric Controls and Control Circuits

Using Switches to Control Electrical Equipment

Using Relay Devices in the Electrical Circuit

Using Magnetic or Automatic Electric Motor ControlDevices

Using Automatic Sensing Control Devices

Page

1

7

16

25

34

tAl

UNIT PLAN

Unit: Electrical Sensing Devices

Situation:

Electricity has had a tremendous impact on mechanization andautomation in agriculture production and agribusiness. Theuse of electrical sensing devices has resulted in many laborsaving arrangements. In many courses the use of electricalsensing devices has resulted in greater accuracy andreliability and ultimately greater productivity.

The demand for and use of electricity will continue to grow.As its use increases, more and more producers and businesspersons will use electrical sensing equipment to gain bettercontrol over their production inputs. More use of suchequipment means more opportunity to be faced with caring forand repairing electrical sensing units. The ability toexamine, repair and connect this equipment will becomeincreasingly important.

General Aims and Goals:

1. To acquaint the students with the various types ofelectrical control devices.

2. To illustrate to the students how electrical controldevices can modify or change the way we perform commontasks.

3. To help the students see how the proper use ofelectrical control devices can provide both comfort andconvenience, as well as some time and money.

4. To provide training needed to help the students developthose skills required to understand and use electricalsensing devices.

Lesson Titles:

1. Electric Controls & Control Circuits2. Using Switches to Control Electrical Equipment3. Using Relay Devices in the Electrical Circuit4. Using Magnetic or Automatic Electric Motor Control

Devices5. Using Automatic Sensing Control Devices

6

Student Activities:

1. Each student will disassemble and reassemble electricaldevices.

2. The students will complete worksheets on variouselectrical devices.

3. The students will wire circuits which includeelectrical control devices.

Teacher Activities:

"overlays" for instructional1. Prepare a series ofpurposes.

2. Purchase multiples ofincluded in the lesson.

the various control devices

3. Gather an assortmentclassroom use.

of reference material for

4. Prepare a series of demonstrationscontrol devices.

with electrical

References:

Gustafson, Robert J. Fundamentals of Electricity forAgriculture, AVI Publishing Co., Inc., Westport,Connecticut, 1980.

Bear, Hoerner, Electric Motors, Principles, ControlsService and Maintenance, Hobar Publications, St. Paul,MN, 1983.

Suggestions for Teaching Electrical andBasic Controls Used in Agricultural Production, EdisonElectric Institute, New York, New York.

iv

UNIT: 'Electrical Sensing Devices

Lesson 1: Electric Controls and Control Circuits

Need:

The use of electrical controls has had a tremendous effecton mechanization and automotion in production agricultureand agribusiness. In many cases, the use of automaticcontrols has resulted in greater accuracy and morereliability. Because of the increased use of electricalsensing devices, we must have a general understanding of howthey work.

References:

Electrical and Basic Controls Used in AgricultureProduction, Hobson.

Fundamentals of Electricity for Agriculture, Gustafson.

Ob ectives:

Given examples of non-automatic and automatic controldevices, the students will be able to:

a. Describe why and where they are used.b. Identify both types.

Given examples of the different types of switches, thestudents will be able to identify them without usingreferences.

Interest Approach:

Prepare an electrical circuit in advance of class thatincludes an automatic timer. Set the timer so it willactivate the device while you are introducing the lesson.Then begin to talk about the advantages of using such a

device.

Key Questions, Problems Teaching Techniques andand Concerns Information

1. Why is agriculture so a. There are many good reasonsdependent on electrical why electrical controls are socontrols? popular.

1. Save time.2. Save labor.3. They (controls) don't

forget (accurate andreliable).

1

8

2. What are the basicfunctions of electricalcontrol?

3. What axe some examples oz.some non-automatic typecontrols? Note: showthese to students as youdiscuss them.

4. Show the students a groupof automatic controldevices.

4. Makes equipment moreadaptable.

5. Saves money (labor).

a. Using the various types ofelectrical controls allows usto:

1. Build in many safety.features. Example:breaker, etc.

2. Make our work moreconvenient. Example:time clock, etc.

3. Create a more comfortableenvironment. Example:thermostat, etc.

4. Provide desired typemotion. Example: relay,etc.

a. These are the devices thatrequire the help of anoperator. However, they arevery simple.

1. The attachment plug on theend of an appliance cord.

2. Snap action switch.a. Review the operation of

the switch.

3. Magnetic switch.a. Hand activated.b. Pushing the start button

supplies a small amountof current to theelectromagnet.

c. The start button may beaway from the actualdevice.

a. There are many automaticdevices that make our workeasier. These devices havetwo basic parts; a sensingelement and a switch. Thesensing element reacts auto-matically.

2

9

5. Review some common termsassociated with controldevices.

1. A humidistat.2. Magnetic relay.3. Thermostatic switch.

a. Become familiar with thelanguage we use when discuss-ing control devices.

1. Switches (OH 1)a. Single pole.

1. They break only oneline of wire.

2. Generally used in 120volt circuits.

b. Double pole (OH 2)1. Break two line wires.2. Used on 240 volt

systems.

c. Three pole. (OH 3)1. Used in 3-phase

circuits.2. The switch will con-

trol more than onecircuit.

d. Normally Open - NO;Normally Closed - NC.1. A NO switch would

be like a doorbellswitch.

2. A NC switch would belike a stop buttonused in magneticstarter circuits.

Application and Followup:

Have the class make diagrams of the following types ofswitches; SPST, SDDT, DPST, DPDT, 3PST, and 3PDT.

3

10

POWER

SWITCH

SINGLE POLE SINGLE THROW SWITCH

(SPST)

111111

THIS SWITCH HAS ONLY TWO POSITIONS =

at AND 4.E.E. (SPST)II

SPST SWTTCH

4

OH 1

DOUBLE POLE SINGLE THROW SWITCH

(PST)

SINGLE POLE DOUBLE THROW SWITCH

(SPOT)

OH

o

I

o

THREE POLE SINGLE THROW SWITCH

(3PST)

THREE POLE MILE THROW SWITCH

OPDT)

1163

OH 3

UNIT: Electrical Sensing Devices

Lesson 11. Using Switches to Control Electrical Equipment

Magi

Switches, although not thought of by most people as asensing device, are extremely important in controllingcircuits. Each different type serves a specific functionunder specified conditions. Our system will workefficiently only if we use the right switch at the righttime.

References:

Fundamentals of Electricity for Agriculture, GustafsonSuggest ons ni-7ERFETEririarical and Basic Controls

Used in Adinultriar-RaffEETEE---

Ob actives:

1. Given the actual switches, the students will be able toidentify and describe the most common characteristicsof each switch.

2. Given the classroom instruction, demonstration andstudent practice, the students will be able to:a. connect switches,into a circuit.b. connect a limit switch into a circuit.c. demonstrate their ability to wire a 3-way and 4-

way switch.

Interest Approach,:

Before teaching the lesson, lay put a group of switchesof different types for the students to attempt toidentify. Follow this with a preliminary discussion onwhere they might be used.

Rey Questions, Problems Teaching Techniques andand Concerns Information

1. How are electric controlswitches classified?

a. Switches and complex controlsare classified according totheir actions.

1. The number of poles perswitch.a. Each movable contact

point is called a pole.b. A single-pole (SP)

switch has one movablecontact.

2. Describe thecharacteristics of thedifferent types ofswitches.

3. How does a mercuryswitch operate?

2. The number of throws theswitch has.a. Each stationary contact

position is called athrow.

b. A single-throw switchhas only one positionin which the polesmake contact.

3. Whether the switch isnormally open or normallyclosed.a. Used to designate the

position of the switch.b. If the switch is acti-

vated to complete thecircuit, it isdesignated normallyopen (NO).

c. If the switch isactivated to open thecircuit, it is desig-nated normally closed(NC).

a. Switches may be single orcomplex. Hive the studentscomplete the chart on handoutnumber one.Note: Provide each student

with appropriatereference material andthe blank worksheet.After they have beengiven time to completethe exercise, discusstheir findings.

a. The mercury switch is openedand closed by liquid mercuryin two closed tubes (OH 1).

1. The contact points areinside the self-containedtube.

2. By tilting the tube theswitch is activated.

3. A common application forthe mercury switch is thelight on an auto trunk.

5. What is an overloadprotection device ina switch?

4. A toggle may also be usedto tilt the switch.

a. This type of switch protects.It is for small single-phasemotor on heaters, strokers,refrigeration compressors,etc. (OH 2).

1. A snap action switch with athermal overload device.

2. These types of switchesoften have a small lighton the front to indicatethe motor is running.

b. Several factors determine thetype and size of overloadprotection.?, Motor characteristics.2. Type of starter.3. Starter enclosure.

Application and Followup:

Use the two exercises taken from the Edison ElectiveInstitute Manual and included as Exercise A and B of lesson.

9

16

Exercise A

Using the tumbler or toggle switches:

1. Connect the switch so that you can control one motor (or asocket and light bulb) connected to 120 volts. Do youconnect the grounded (neutral) wire into the switch or doyou connect the ungrounded (hot) wire? Why?

2. Connect the switch and start and stop the motor severaltimes.

a. Does this switch provide short circuit and overloadprotection for the motor?

b. How would you protect the motor against damage fromoverloads and short circuits when using this typeswitch as a motor control?

10

17

Exercise B

Using the tumbler switch with built-in overload protector:

1. Remove the cover plate and than take out the overloadprotector, which also is called a "heater". What is theampere rating of the heater?

a. What would happen if more than the rated number ofamperes went through the switch?

b. What is the maximum size 120 volt motor for which thiscontrol is adequate?

c. How do you select the proper size overload protectorelement?

2. Connect the switch for controlling one motor. The wiringdiagram is shown above. You will only need to connect thetwo line wires and the two motor leads at the ends of theswitch.

3. Start and stop the motor several times.

4. Lift out the overload protector element. Will the motor runnow that you have removed the element? If you answer is no,explain why.

5. To demonstrate operation of overload protection, connect aheavy load or several motors (3) in parallel to this switch.

a. How many amperes will flow through the switch when all3 motors are running?

b. What should happen to the circuit and flow of current?

c. Did the overload protector open up and the motors stop?

d. Snap the switch to "off" and let the heater elementcool.

e. Snap the switch to "on" and time the period necessaryto cause the heater to open the circuit. What was thetime?

6. How do you reset the overload protector?

In actual practice, the connection of 3 motors through thisone switch would not be done. The major advantage of aswitch with overload protection is that the heater can besized to protect one motor. If 3 motors were connected, theheater would have to be so large that none of the motorswould receive adequate overload protection. Three motorsare connected in this exercise only to show that the totalamperes will cause the switch to open.

Name

Toggle switch. 1.

2.

3.

Push Button(non-toggle)

1.

Time Delay 1.

2.

Dimmer Switch 1.

Knife switch 1.

Mercury Switch 1.

ELECTRICAL SWITCHES

Advantage Disadvantage

Simple design.Inexpensive.Readily available.

Helpful whencurrent must beturned on or offmomentarily.

Light will remainon a short timeafter the switchis turned off.Good.

Turns the lamp onat any level ofintensity.

Appropriate forheavy currentloads.

Very quiet.

1219

1. May not be attrac-tive.

2. Many are noisy.3. There are mechani-

cal parts to wareout.

1. Switch must be heldin position.

1. More expensive

1. Cost is greater.

1. More expensive.

_EorrJus.ply.

120V

Motor

OVERLOAD PROTECTION

SPST Switch

WITHOUT OVERLOAD PROTECTION

To LoadHeater

I .11

L9 cul.

OH 2

Power Supply

-J

Switch

WITH OVERLOAD PROTECTION

1320

T.120V

o

I

HOW A MERCURY SWITCH OPERATES

OFF POSITION

ON POSITION

II .

o

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I

ELECTRIC SWITCHES

15

22

UNIT: Electrical Sensing Devices

Lesson 3: Using Relay Devices in the Electrical Circuit

Need:

Relay became useful because a small current in the controlcircuit can control a relatively large current in the loadcircuit. This has a distinct advantage when working withlarge pieces of electrical equipment. Understanding the useof relay devices will mean money in the business person'spocket.

References:

Fundamentals of Electricitv for Agriculture, GustafsonSuggestions for Teaching Electrical and Basic Controls Used

in Agricultural Production

Ob ectives:

Given a time delay relay and the necessary equipment, thestudents will be able to properly connect the relay into thelight and motor control circuit.

Interest Approach:

Tour the shop or school facility with the students andidentify the relay devices that are being used. Discussbriefly why they are being used.

Key Questions, Problems Teaching Techniques andand Concerns Information

1. What is a relay device?

2. What are the advantagesof using a relay device?

a. An electrical device to con-trol a circuit carrying alarge current by means of alow-current circuit. Example:photoelectric cell.

a. The relay has some distinctadvantages over the mechanicalswitch.

1. Wires on the control circuit can be smaller.

2. Therefore, it is lessexpensive.

3. Switches can be placedgreat distances from theload.

4. Heavy currents are notpassed through the controlcircuits which improvesthe life of the switchesand safety of operation.

5. Will reduce the number oflarge loads startingsimultaneously.

6. They can be used tosequence certain loads.

3. Study the components of a. The basic parts of a relaya relay switch. are:

4. How does the relay work?Use both the overlay andan actual relay.

17

1. Solenoid or electromagnet.a. Made up of a coil and a

movable armature or ironcore, and,

b. a set of contact points.

2. The relay has twoelectronic circuits.a. The control circuit.

(Usually the low voltagecircuit.)

b. The load circuit. (Hasthe contact pointsattached.)

Note: Use OH 1 to discussthe parts of the relay.

a. Trace the flow of currentthrough the relay. OH 2

1. Observe the small coil withmany turns.

24

2. As a small amount of elec-trical current flows intothe coil, an electromagnetis produced.

3. This magnet pulls thepoints closed.

4. The main current thenbegins to flow within thecircuit to operate theelectrical equipment.

5. Study a relay.

Application and Pollownp:

a. Provide the students with oneor more examples of relays.Have them respond to the fol-lowing questions.

1. What is the ampere rating?

2. What is the voltage rating?a. Voltage of coil.b. Voltage of relay load

circuits.

3. What is the largest sizeload that can becontrolled?a. Load in watts or kilo-

watts.b. Size of motor.

4. Determine the number ofpoles or contacts on therelay.a. Is it a normally open or

normally closed relay?

Have th..: students complete the time delay relay exercise.The exercise is taken from the material entitled"Suggestions for Teaching Electrical and Basic Controls Usedin Agricultural Production".

WORKSHEET NO. 1

Time Delay

Examine the 115 N/O 10 time delay relay. This type used in thisexercise has the physical appearance of a radio or TV tub. The115 in the type designation 115 N/O 10 means that the heaterelement in the relay is designated for 115 volts. The N/0 meansthat the relay contacts are normally open. The 10 means that thecontact points will close 10 seconds after the heater isenergized. When the heater is energized with a flow current, a10 second period is required before the heat will cause the metalplate to bend and thus close the contacts. This closes theswitch. Locate the two contact points. Are they together or arethey apart?

1. Application in control circuits:

a. Examine the 115 N/0 10 time delay relay. The type usedin this exercise has the physical appearance of a radioor TV tube. The 115 in the type designation 115 N/0means that the heater element in the relay is designedfor 115 volts. The N/0 means that the relay contactsare normally open. The 10 means that the contactpoints will close 10 seconds after the heater isenergized. When the heater is energized with a flow ofcurrent, a 10 second period is required before the heatwill cause the metal plate to bend and thus close thecontacts. This closes the switch. Locate the twocontact points. Are they together or are they apart?

b. Perform the following work:

1. Connect the 115 N/O 10 time delay relay byfollowing the diagram on the next page. Note thatthe switch within the relay is in series with thelight bulb, therefore the full ampere flow goesthrough the switch. The contact points are ratedat 3 amps. What is the maximum size bulb that canbe used directly with this relay?

2. Energize the circuit. Does the bulb come onimmediately? Wait a few seconds. Does the bulbgo on or off?

3. De-energize the circuit. Does the bulb go on oroff immediately? Why? Wait 10 seconds or so andrepeat step 2.

4. The contact points are rated at 3 amperes. Canyou connect this time delay relay directly to a1/4 HP motor?

NW Switch

120 V

T

N/O Time DelayRaley

Light Bulb

(6),7"

0,... Lood

o

I

p

WORKSHEET NO. 2

Time Delay

Read the following explanation and perform the work described:

1. Because a 1/4 HP or larger motor requires a greater current

than the 3 ampere rating of the contact points of the time

delay relay, an additional relay (magnetic starter) must beused with such a motor. The time delay relay then connectsonly to the coil of the additional relay and the motorconnection is made through the additional relay contacts.

2. Connect the motor to the 115 N/0 10 time delay relay byfollowing the diagram below.

N/0 Time DelayRelay DPST Relay

rPower _1Supply 1

I.

120VTo Load

3. Energize the circuit. Does the motor come on immediately?Wait about 10 seconds, does the motor then come on? Explainthe action that occurs. De-energize the circuit. Time thedelay in motor action.

2821

0

Power Circuit

0

1:52

Control Circuit

29

A,4s

Spring..

Switch.217

SOP NMI 411 am OM =lb NO em

30

OH 1

Movable ArmCoil

N/C ContactN/O Contact

Insulating Material

. ,. ,. ,,i . .

41,iN/C Circuit

Pc

004\ %

0/ °41

SCHEMATIC SKETCH OF SPOT RELAY

31

23

1

OH 3

CircuitTo Load

240V

SCHEMATIC SKETCH OF DPST RELAY

Control Circuit

UNIT: Electrical Sensing Devices

Lesson 4: Using Magnetic or Automatic Electric Motor ControlDevices

Need:

A person need not be at the motor location in order to turnon the machine. The irrigation motor in the south 80 acrescan be turned on from the farmstead. On the other hand, wemay want to turn on a fan in the farrowing shed when thehumidity gets too high or we may want to turn on the stockwater pump at certain times during the day. If so, we needto understand how to include an automatic control deviceinto the system.

References:

Fundamentals of Electricity for Agriculture, Gustafsonlectric Motors, Principles, Controls, Service andMa ntenance, Bear & Horner

Suggestions for Teaching Electrical and Basic Controls Usedin Agriculture, Edison

Ob ectives:

Given class discussion, individual study, teacherdemonstration and student application, the students will beable to:

a. Describe where and when the electric motor controls canbe utilized.

b. Describe the purpose and components of the variouselectric motor control devices.

c. Wire sensing control devices into motor controlcircuits.

Interest Approach:

Construct a single electronic magnet using electrical wireand some type of soft metal core. Compare the way itfunctions with the old style permanent magnet.

Key Questions, Problems Teaching Techniques andand Concerns Information

1. What is an electro-magnetic switch.

a. A switch in which an electro-magnetic is used to open andclose a circuit.

2533

2. What are the advantagesof an electromagneticswitch.

a. There are three (3) distinctadvantages when using thelarger horse dower motors.

1. The electromagneticcreates a fast "make" and"break" circuit.

a. This will reduce startup arcing problemscaused by high startingamperes.

b. A spring is used tocreate a rapid break.

2. The electromagnet switchlends itself to remotecontrol.

a. Its use can result inmore convenience andsafety.

b. Discuss some examples ofapplication that willapply to both con-venience and safety.

3. It provides an opportunityfor a user to obtainprecision overload andovercurrent protection.

a. This will help protectthe motors.

b. Using the proper heaterelement will save money.

3. How does an electro- a. The electromagnet is themagnetic switch work? heart of the switch. (OH 1)

1. Current flows through coilcausing the iron core tobecome magnetized.

2. Once magnetized, itattracts the contactpoint immediately under it.

2034

4. What features areincluded with a standardmagnetic motor controller.

5. Examine a magneticstarting device.

3. Once the electromagnet isde-energized, a springhelps separate the pointsquickly.

Note: A typical automotiveor tractor crankingmotor operates the sameway.

a. The major components include:

1. Magnetic contractor:Includes a coil, movablecore and control circuit.a. The contacts are rated

in volts, amperes, andhorsepower.

b. The control circuit maybe the same as a lowervoltage.

2. Overload device:a. Sized according to the

motor or load itcontrols.

b. If an overload occursit will open a contact.

c. The overload must bereset before the motorwill start again.

a. Have the students examine amagnetic device carefully andanswer the followingquestions.

'45

1. Identify the following:a. Contact terminalsb. Coil magnet assemblyc. Overload relayd. Relay heatere. Reset mechanism

2. Is it a single pole, doublepole or three-pole switch?

3. After studying the nameplate on the magnetassembly determine:a. What is the maximum size

motor when connected to120 volts?

6. Examine a pushbuttonmotor control.

b. What is the maximum sizemotor? When connectedto the 240 volts?

C. Does the switch have anoverload protector?Where is it? Is itadjustable?

d. How do you select theproper size heater unitfor use in the protec-tor?

a. This control device, when usedwith a magnetic controller, isthe most common type used forstarting and stoping electricmotors. The pushbutton con-trol cannot be used as aswitch by itself, but requiresother equipment such as amotor starter as a relaydevice. Examine a pushbuttonmotor control and answer thefollowing questions.

3628

1. What is the current ratingand why is it so low?

2. Push the start button. Dothe contact points open ordo they close? Is thestart button NO or is itNC?

3. Push the stop button. Dothe contact points open ordo they close? Is the stopbutton NO or is it NC?

4. What is the maximum voltageon which this pushbuttonmay operate?

5. Is it suitable for the vol-tages you have at home?

6. What voltages do you haveat home?

7. The start side of thestation has two terminalsand the stop side also hastwo terminals. Only threewires, however, are broughtoutside. Why aren't therefour wires brought outside?

Note: Discuss the student'sresponses.

Application and Followup:

The application exercise suggested for this lesson ic takenfrom page 37-38 of the reference entitled, "Suggestions forTeaching Electrical and Basic Controls Used in AgriculturalProduction". Study the exercise carefully before class tomake certain you have all of the equipment.

MOTOR CONTROL WORKSHEET

1. Study the diagram below and make the connections as shown.Use the magnetic starter and one start-stop station.

a. Energize the circuit and push the START button. Doesthe motor start and run? Did the magnet assembly move?Release the START button. Does the motor continuerunning?

Power Supply 120 V.

..Start-Stop PushbuttonStation

Heater

L2

Coll

e-=--6240 V

i20 V

Load

When the start button is pressed, current may flow from Lithrough the stop button which is N/C, on through the starEbutton, on through 3 to the magnetic coil. When the coil isenergized, the 3 switches within the magnetic starter close andthe current may flow from Li to T1 to the motor. The neutralgoes from L2 through the closed switch to T2, and then to themotor. To stop the motor, the coil needs Eo be de-energized.Pressing the stop button will open the circuit to the coil. The3 switches within the magnetic starter open and the motor willstop.

b. Push the STOP button. Did the magnet assembly move?Did the motor stop?

c. Start and stop the motor several times and notice theaction of the magnetic starter.

2. Connect 2 start-stop stations to the magnetic starter andcontrol the motor from each station.

a. Follow the diagram below to make the connections.Start and stop the motor several times.

b. Does the current to the motor flow through the start-stop stations? If not, what current flow does flowthrough the stations?

c. How does this current flow characteristic simplify thewiring requirements?

Start-Stop PushbuttonStations

Power Supply 120 V.

Heater

TI

,_ _ _ _ _ _ _ _I -1T2

II I

I

I

lIHolding Coil 1

C240 V 1

1

120 V 1

1

I1

1

r JI Load

d: Would the above control mechanism protect the motor ifit became overloaded? How?

31

39

OH I

THE OPERATION OF THE ELECTROMAGNETIC

MOTOR STARTER

PERMANENT-TYPE MAGNET

ELECTROMAGNET

32

40

BIMETAL

STRIP

HEATER

NO CONTACT

BEFORE HEATING

COMMON

NO CONTACT

AFTER HEATING

33

41, . L

OH 2

UNIT: Electrical Sensing Devices

Lesson 5: Using Automatic Sensing Control Devices

WHIM_

When appropriate, temperature, humidity, pressure, motion,light and time can be used to control the operation ofelectrical equipment. The temperature drops so we wouldlike the heater in the shop to come on or we want the fanin the dairy barn to come on automatically when the humidityreaches a certain level. We could even use a timer to turnon the coffee pot. All of this can be done if oneunderstands how to include the sensing devices in thecircuit.

References:

Fundamentals of Electricity for Agriculture, GustafsonSuggestions for Teaching Electricity and Basic Controls Used

in AgricuTEiral Producation

Ob actives:

Given discussion, individual study, teacher demonstrationand student application, the students will be able to:

a. Describe where the various sensing devices can be used.

b. Describe the purpose and components of the varioussensing devices.

c. Wire sensing control devices into control circuits.

Interest Approach:

Show a photocell to the class. Discuss with them how itworks and where it it is used commercially.

Rey Questions, Problems Teaching Techniques andand Concerns Information

1. What are the most commonly a. The sensing device is usuallyused sensing control named after the variable thatdevices? is used to activate the

device.

1. Thermostats: Controlledby air temperature.

2. Humidistats: Controlledby changes in moisture orhumidity.

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2. What are somecharacteristics of somesensing devices?

3. Identify the manner inwhich thermostatsoperate.

4. Photocell: Controlled bylight intensity.

a. The devices are capable ofsensing a change.

1. Each device is sensitive toa specific condition(variable).

2. As the device reacts tochange, it mechLnicallyactuates an electric con-tact or switch.

3. The device is set to reactto a specific conditionwithin a range of sensi-tivity.

4. They may operate on livevoltage or on low voltagecircuits.

5. The devices are rated inboth volts and amperes.

a. Thermostats operate on three(3) basic principles. When athermostat is purchased, it isset to operate at pre-settemperature ranges.

1. Bimetallic thermostat(OH 1).a. Different metals

expand and control atdifferent rates when thetemperature changes.

b. Two dissimilar metalsare bonded together.As the temperaturechanges, the bimetallicbends to open or closethe contact points.

2. Liquid-gas thermostat.a. Consists of a liquid

filled wafer or bellowsmechanically connectedto an electrical contactmechanism.

4. Thermostats areclassified accordingto usaye.

5. Examine a thermostat.

b. A temperative changecauses the gas to expandor contract, thus open-ing or closing thecontact points.

3. Hydraulic thermostat (OH 1)a. Consists of a liquid

filled tube mechanicallyconnected to an electri-cal contact.

b. As the temperaturechanges, the liquidexpands or contractsclosing or opening theswitch points.

a. Heating thermostats.1. Used to control heating

equipment.

2. The contact points close asthe temperature cools.

3. When temperature risesabove a pre-set value, thecontact points automatic-ally open.

b. Cooling thermostats.

1. Used for fans or othercooling equipment.

2. The contact points close asthe temperature raises.

3. When the temperature getsback to the desired level,the points open and deener-gize the system.

a. Provide a thermostat to eachstudent or group of students.Provide them with a copy of

Worksheet 1. Have them :om-plete the worksheet by answer-ing the following questions.

1. What is the differencesbetween a thermometer anda thermostat.

p 6. Examine a humiditycontroller.

7. Examine a humidistat.

MidiagVIW

2. Locate the following partsof the thermostat.a. Nameplate label.b. Switch.c. Temperature scale plate.d. Temperature adjustment

knob.e. Temperature sensing

element.

3. Does the electricityconnect to anything otherthan the switch?

4. What voltage may be appliedto this switch.

5. What is the full loadampere ratings of thisswitch.

6. What is the range of tem-perature adjustments?

7. What is the differentialin degrees F?

a. These devices are commonlycalled humidistats. They mayuse human hair or othermaterial which responds tochanges in humidity. Thesensing element is connectedto the contact points.

a. Provide a humidistat to astudent or group of students.Provide them with a copy ofWorksheet 2. Have themanswer the following ques-tions. (OH 2)

1. What is the humidity rangeof this controller?

2. What is the voltage ratingof the switch?

3. How many full load amperesmay be connected directlythrough the switch.

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8. Examine a timing controldevice.

9. Study a timer'verycarefully.

4. Locate the sensing element.What is it made of? DON'TTOUCH THE SENSING ELEMENT!Explain how humidityaffects the sensingelement.

5. Notice the two (2) wiresconnected to the switch.Is one neutral? Is theswitch installed in thecurrent-carrying (hot)conductor or the neutral?

a. These devices are commonlycalled time clocks or timeswitches. They usually con-sist of an electric clockmechanically connected to aset of points. The pointsautomatically open andclose at definite time inter-vals.

a. Provide a 24-hour timer toeach student or one for agroup of students. Have thestudents answer the followingquestions. Provide each stu-dent with a copy of WorksheetNo. 3.

1. What is the "rating" ofthis time clock?

2. How many amperes may beconnected to the load cir-cuit of the switch?

3. How do you set the dial forthe correct time of day?How many control operationscan be performed in a 24-hour period?

4. Locate the terminal strip.Refer to the wiring diagraminside the front cover ofthe case and then identifythe following.

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10. What are thecharacteristics of thephoto electric cell?

11. Examine a photocell.

a. The numbered terminalwhere the ungrounded(hot) line and loadwires would beconnected.

b. The numbered terminalswhere the grounded(neutral) llne and loadwires would beconnected.

a. The photocell is generallyused for lighting applica-tions.

1. Street lights can be auto-matically turned on andoff.

2. A farmstead light could beactivated by a photocell.

3. Electric operated garagedoors use a photocell.

a. Provide a photo cell for eachstudent or groups of students.Provide each student a copyof Worksheet No. 4. Have themanswer the following ques-tions.

1. What is the voltage ratingof the unit?

2. How many watts can be con-nected directly to theunit.

3. Locate the glass lenceopening. What do yousuppose is the purpose ofthe louver inside the lens?

4. Is the red colored wireused for connecting line-inwire or for the load wire?

Application and Followup:

A series of problems have been taken from the texts thatwill allow the students to utilize the knowledge gainedthroughout the lessons.

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Worksheet 1

THERMOSTAT WORKSHEET

Study and examine the thermostat very carefully and then answerthe following questions. After you have answered the questions,your instructor will discuss the answers with you.

1. What is the difference between a thermometer and athermostat?

2. Locate the following parts of the thermostat?

a. Nameplate label.b. Switch.c. Temperature scale plate.d. Temperature adjustment knob.e. Temperature sensing element.

3. Does the electricity connect to anything other than theswitch?

4. What voltage may be applied to this switch?

5. What is the full load ampere rating of this switch?

6. What is the range of temperature adjustments?

7. What is the differential in degrees F?

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Worksheet 2

WORKSHEET ON THE HUMIDISTAT

Study and examine the humidistat very carefully and then answerthe following questions. After you have answered the questions,your instructor will discuss the answers with you.

1. What is the humidity range of this controller?

2. What is the voltage rating of the switch?

3. How many full load amperes may be connected directly throughthe switch?

4. Locate the sensing element. What is it made of? DON'TTOUCH THE SENSING ELEMENT. Explain how humidity affects thesensing element.

5. Notice the two wires connected to the switch. Is oneneutral? Is the switch installed in the current-carrying(hot) conductor or the neutral?

Worksheet 3

WORKSHEET ON THE TIMER CONTROL

Study and examine the timer device very carefully and then answerthe following questions. After you haVe answered the questions,your instructor will discuss the answers with you.

1. What is the "rating" of this time clock?

2. How many amperes may be connected to the load circuit of theswitch?

3. How do you set the dial for the correct time of day? Howmany control operations can be performed in a 24-hourperiod?

4. Locate the terminal strip. Refer to the wiring diagraminside the front cover of the case and then identify thefollowing:

a. The numbered terminal where the ungrounded (hot) lineand load wires would be connected.

b. The numbered terminals where the grounded (neutral)line and load wires would be connected.

Worksheet 4

WORKSHEET ON THE PHOTOCELL

Study and examine the photocell very carefully and then answerthe following questions. After you have answered the questions,your instructor will discuss the answers with you.

1. What is the voltage rating of the unit?

2. How many watts can be connected directly to the unit?

3. Locate the glass lence opening. What do you suppose is thepurpose of the louver inside the lens?

4. Is the road colored wire used for connecting line-in wire orfor the load wire?

4351

TO CONTROLCIRCUIT

TYPES OF THERMOSTATS

BIMETALLIC STRIP

SPRING

CONTACTS

ADJUSTMENTSCREW

BIMETALLIC THERMOSTAT

PIVOT

TO ------777-71CONTROLCIRCUIT

CONTACTS*

ADJUSTMENT_,,jSCREW

%OW/m.o. aim

OH 1

EXPANSIONELEMENT

CAPILLARY TUBE

EXPANDING FLUID BULB

HYDRAULIC THERMOSTAT

5244

TOCONTROLCIRCUIT

A HUM ID I STAT

ADJUSTMENT

MATERIAL CHANGINGLENGTH WITH CHANGEIN HUMIDITY

5 3

45

OH 2