Module: Robotics - Starsstars.eng.usf.edu › Robotics Module › Lesson 2 Motor Sch… · Web viewThe torque continues until the coil’s poles reach the opposite poles of the

Module: Robotics

Topic Area: Simple Machines

Benchmark/Lesson: SC.B.1.2-1/2; SC.B.1.2-4/5; SC.C.2.2.1– Electric Circuits

SC.B.1.2/1; SC.C.2.2/1; SC.C.2.2/3 – Electric Motor Schematics

SC.B.1.2.5 /SC.C.1.2/1; SC.C.2.2-1/3 – Robotics Schematics

________________________________________________________________________

Lesson 2: Motor SchematicsObjective

The purpose of this lesson is to encourage students to understand the term electric motor

and recognize the difference between an Alternate Current (AC) motors and Direct

Current (DC) motor. The student should be able to identify or recognize electric motor

parts. In addition, the students will be able to work with math concepts of Motor like:

angular velocity, power and torque. This will also give the student the basic knowledge of

electric motor design and components.

Lesson Background]

Electric motor is a machine which converts electric energy into mechanical energy.

It spins many parts of a household, sometimes this rotary motion is obvious, as in a fan or

mixer, but often it’s disguised, as in the agitator of a washing machine or the window

opener of an automobile. Motors come in a variety of shapes and sizes, each appropriate

to its task. A variety of electric motors, provide the power of robots to move material,

parts, tools, or specialized devices with various programmed motions. The word power is

define as the time rate of supplying or absorbing energy. The power associate with

current flow through an element is given by:

p = power in Watts or Joules per seconds (J/sec)

v = voltage in Volts

i = current in Amps

Robotics NSF/USF STARS M3L21

Some motors operate from direct current and can be used with batteries while others

require alternating current. There are even motors that work on either type of current they

may work with gas, or other type of fuel.

In this lesson, we’ll examine the basic mechanisms of electric motors. We’ll look at what

makes a motor turn and how motors differ from one another. Several of the questions that

will try to answer during this background are the following:

How magnetic forces can cause something to spin?

The rotor of an electric motor needs a torque to start it spinning. The rotor is a set of

thin metal plates stacked together, with thin copper wire coiled around each of the three

poles of the armature. This torque is normally produced by magnetic forces, exerted

between magnetic poles on the rotor and those on the motor’s stationary shell. Torque is

a force that produces or tends to produce a rotation or torsion <an electric car motor

delivers torque to the drive shaft>; also: the effectiveness of such a force consists of the

product of the force and the perpendicular distance from the line of action of the force to

the axis of rotation.

The equation for torque for an electric motor is given by:

T = torque of the motor (lbf*inch.) or (N*m)

r = radius of the motor shaft, pulley or whatever is attached to the motor

shaft [inches (inch.) or centimeters (cm)].

F = the force created by the motor (lbf. or N)

The motors supplied in robotics kits are designed for a specific torque and are listed as:

Large motors - 216 in-oz at 56 rpm (a little less than 1 revolution per second)

Small motors – 34 in –oz at 113 rpm (a little less than 2 revolutions per second)

These attractive or repulsive forces pull or push the outer components of a rotor,

producing torques that make the rotor spin faster and faster until friction or the objects

attached to it reduce its net torque to zero. After that point, the rotor turns at a steady

angular velocity.


Both the rotor and the motor’s fixed shell are magnetic. This angular velocity

corresponds to amount of revolutions per second or per minute. A revolution is

progressive motions of a body round an axis.

Why can not a motor be built exclusively from permanent magnets?

But while permanent magnets are often used in electric motors, at least some of a motor’s

magnets must be electromagnets. The electromagnet go his name because is magnetize

by the electrical current from the power source. That is the main reason that motor can

only keep turning if some of its magnetic poles change or move as the rotor spins. In that

way, while the rotor turns to bring opposite magnetic poles as close together as possible,

the poles keep changing or moving so that the rotor finds itself perpetually chasing the

optimum arrangement of poles.

What determines which way a motor rotate?

The coil in motor starts off with its magnetic poles arranged horizontally. Since opposite

poles attract, the coil experiences a torque that acts to spin the coil counterclockwise. The

coil undergoes angular acceleration and begins to rotate counterclockwise. The torque

continues until the coil’s poles reach the opposite poles of the stationary magnets. There

is then no torque on the rotor because the magnetic forces act directly toward or away

from the axis of rotation. The rotor is in a stable equilibrium. But just as the opposite

poles reach one another, the current in the coil reverses directions. Now the coil’s

magnetic poles are close to like poles of the stationary magnets and like poles repel one

another. The rotor is suddenly in the worst possible orientation and must turn upside

down to be optimally oriented again. Because it already has angular momentum, the coil

continues its counterclockwise rotation and once again experiences a counterclockwise

torque from the magnets. This process happens over and over like a cycle until the motor

is shutdown.

What does a motor turn mean?

Turns refers to the number of times wire is wrapped around the motor armature. The

fewer the turns in the motor armature, higher rotational speed can be achieve RPM or

top-end speed. For that reason if you want more speed choose a motor with fewer turns.

Keep in mind, though, that fewer turns requires greater battery draw, and therefore,


reduced battery run time. In addition, for big monster trucks, you'll need motors with a lot

of torque, not just speed. A lot of torque or power is needed to move the huge tires and

many gears of a huge truck. For monster trucks, motors in the 14 to 17-turn range provide

plenty amount of torque and are the best electric motor to use.

What does a motor wind mean?

A wind refers to the number of strands of wire wrapped around the motor armature. The

fewer number of winds the faster the acceleration higher top end speed you will have.

The number of winds, however, only affects motor performance to a very small degree.

For all practical purposes, you can simply ignore the number of winds on a motor and

focus upon the turns. Turns are really the important thing to worry about when you are

buying a motor.

Direct Current (DC) Electric Motors

They just work with batteries or direct current from a house or building plug. In most

DC electric motors, the rotor is an electromagnet that turns within a shell of stationary

permanent magnets. Now going back to the concept of direct current it is defined as the

electric current flow that travels just in only one direction. To make the electromagnet

stronger, the rotor’s coil contains an iron core that is magnetized when current flows

through the coil. The rotor will spin as long as this current reverses each time its

magnetic poles reach the opposite poles of the stationary magnets. The most common

way to produce these reversals is with a commutator. A simpler commutator has two

curved plates that are fixed to the rotor and connected to opposite ends of the wire coil.

The rotor of a DC motor turns at a speed that’s proportional to the voltage drop through

its coils. Those coils have little electric resistance and allow a large current to flow while

the rotor is at rest. But once the rotor is turning, the changing magnetic fields around the

spinning rotor cause it to experience electric fields. These electric fields oppose the flow

of current through the rotor, extracting energy from that current and lowering its voltage.

DC electric motors of this sort are used in a wide variety of battery operated devices,

from toy cars to electric screwdrivers. Unfortunately, their brushes experience

mechanical wear as their rotors turn and eventually wear out and must be replaced. The


brush allows the electric current to flow into the rotor once it touches one of these plates

and leaves the rotor through a second brush that touches the other plate.

Furthermore, the brushes produce mechanical interruptions in the flow of current and

these interruptions often create sparks. Motors with brushes are unsuitable for some

environments because their sparking can ignite flammable gases. Sparking also produces

radio waves so that an automobile’s DC motors often interfere with its radio reception.

Figure 1 Direct Current Motor Components

Induction Alternating Current (AC) Motors

Some alternating current motors have rotors that are neither permanent magnets nor

conventional electromagnets. Alternating current means the electrical current is

alternating directions in a repetitive pattern. AC is created by generators in power plants,

and other sources. When an aluminum rotor is exposed to altering magnetic fields,

created by the electric currents that flow through it is called induction.

AC motors that use induction to create magnetism in their rotors are called induction

motors. They provide lots of torque, start easily, and are inexpensive They are the

simplest type of AC motors and commonly used in everything from household fans,

washing machines, refrigerator compressors, industrial pumps to cable-lift elevators, and

table saws. An induction motor works by moving a magnetic field around the rotor.

The stationary shell surrounding the rotor contains a sophisticated electromagnet called a

stator. While the stator does not move, the field it produces does. Through a clever use

of various electromagnetic devices, the stator is able to create magnetic poles that move


BRUSHES

in a circle and travel around and around the rotor. Since the pole’s motion is based on

current reversals of the AC power line, the North Pole usually travels once around the

rotor for every full cycle of the current. A South Pole also circles the rotor on the

opposite side. They induce magnetic poles in the rotor and the rotor experiences magnetic

torques. The rotor begins to spin, rotating faster and faster until it’s turning almost as

rapidly as the moving magnetic poles of the stator.

Math Skills

Measurement

Addition and multiplication

Data analysis

Science Skills

Forming a question

Hypothesizing

Communicating

Observing

Recognizing variables

Inferring

Predicting outcome

Interpreting data

Correlating data

Engaging Questions

1. What is an electrical motor?

2. What is purpose of using a brush in a Direct Current Motor?

3. List the basic components necessary to build a Direct Current Motor?

4. What does a stator do in an AC Motor?

5. What is the meaning of torque and how is produces in a motor?

6. What is the meaning of electrical induction?

7. What is the meaning of electromagnets?

Students Pre-Lab Activity (two pre-lab activities are suggested, teacher may choose

one or both)


1. The first activity consists in an Electric Motor Labeling according his use in a

motor. The teacher has to define all the basic components for both types of electric

motors (AC/DC) and each function before the student continue with the labeling activity.

2. The second activity consists of a search of all Electric Motors At Home. The

students should walk through their houses and counts all the motors they find. The

teacher has to explain and offer some examples before the students carry on this activity.

3. The third activity consist of


E

DC Electric Motor

Name: ________________________ Date: ___________________________ Pre Lab Activity #1 Electric Motor Labeling

Instructions: Match the different letters from each electric motor with the bottom table parts.

AC Electric Motor

K

DC Parts AC Parts

____ Commutator ____ Armature or Rotor

____ Cooper Winding ____ Stator

____ Axle ____ Laminate Core

____ Brushes ____Shell

____ Coil ____ Axle

____ Field Magnet ____Copper Winding

Kitchen Bathroom

1. _________________ 1. ____________________

2. _________________ 2. ____________________

3. _________________ 3. ____________________

4. _________________ 4. ____________________

5. _________________ 5. ____________________

6. _________________ 6. ____________________

Living room or Family room Garage

1. _________________ 1. ____________________

2. _________________ 2. ____________________

3. _________________ 3. ____________________

4. _________________ 4. ____________________

5. _________________ 5. ____________________

6. _________________ 6. ____________________

Bedroom Utility room

1. __________________ 1. ____________________

2. __________________ 2. ____________________


Name: ________________________ Date: ___________________________

Pre Lab Activity #2 Electric Motors At Home

Instructions: Walk through your house and make a list of all the things that you think use electric

motors.

3. __________________ 3. ____________________

4. __________________ 4. ____________________

5. __________________ 5. ____________________

6. __________________ 6. ____________________

This motor is based on a wonderful design by Bill Beaty, but this version uses only things

found around a typical house.

List of Materials

1. 2 empty soda cans.

2. 1 Styrofoam cup

3. A simple ball-point pen

4. 2 pieces of aluminum foil

5. 2 paper clips

6. A hot glue gun or regular glue

7. Cellophane tape.

8. 2 piece of wires

9. 2 alligator clips

10. Sharp blade

How to manage the experiment

1. The first step consists of spreading glue on the outside area of the

Styrofoam cup to set a thin layer of aluminum foil. Press the foil

against the cup to eliminate the wrinkles.

2. Then cut a half inch strip out of the foil on both sides, so you

have two patches of foil, one on each side of the cup.

3. The next photo shows the bottom of the pen, cut off with a sharp

knife. The cut end that is facing down has a little dimple that is

perfect to place the tip of the pen.


Name: ________________________ Date: ___________________________

Pre Lab Activity #3 Rotary Electric AC Motor

Instructions: Walk through your house collecting the items of the specified list. These materials are

important to build the rotary motor.

4. Then it is necessary to glue the cut end of the ball paint pen to the center of the

bottom of the cup as shown on the next picture. Because the cup is going to be

spinning on the tip of a pen.

5. The construction of the motor start with a paper plate, and

glue the bottom of a ball point pen to the center of the plate, maintaining the pen

straight up.

6. The next step consists of gluing two soda cans upside-down onto the plate, leaving

enough room between them. In that way the Styrofoam cup can rotate easily without

touching either can. ** Note there should be about half inch gap between the cup and

each can.

7. Straighten two of the bends paper clip into an S shape

and tape them to the cans as shown. Bend the wires,

leaving enough room to place the cup on top of the pen.

8. Now place the cup upside-down onto the pen tip. Make

sure the dimple fits onto the pen tip. The wires should

be about a half inch away from the cup. Nothing should

be touching the cup except the point of the pen.

9. Once everything is you can connect a wire from the can on the right to a large sheet

of aluminum foil pressed against the screen of a TV (or a computer with a CRT

screen).

10. The second end of the can left can be connected to a good ground connection, such as

a metal frame or you can just hold to this end, since your body is a good enough

ground for this little motor.

11. When you turn on the television, the foil will pick up a high voltage, and the little

motor will start spinning. As it slows down, turn the television off, and the motor will


get another kick, and spin faster. To keep the motor spinning it is necessary to turn off

and on the TV for a while.

How this motor spinning occurs?

First of all the can on the right hand side is charge with high voltage from the TV screen.

During this process a flux or push of electrons from the TV screen to the aluminum foil

occurs. This electrical “pressure” moves away from the wire, and hit the aluminum foil,

where the electrons leave the air molecules and collect on the foil. The electrons in the

can on the left are repelled by the electrons in the cup foil. With the right can repelling,

and the left can attracting, the foil is pulled to the left, and the cup rotates. This represents

the basic physical concept of an Alternate Current Induction Motor.

Teacher’s Procedure

1. The day prior to starting the activity has the students do the pre-lab engaging

questions and one of the pre-lab activities. {Consider: (1) Allowing the students take it

home for homework and include a section for parental involvement.

2. Checks to make sure students have done the pre-lab activity, allow students to keep

the pre-lab until they have completed the experiment and drawn conclusions.

3. Take 10-15 minutes to review each lecture on “Direct Current Electric Motor” and

“Alternate Current Electric Motor”.

4. Explain in detail the activity. As you explain the activity point out each piece of

material.

How to manage the experiment

1. Place the students in group of four or five.

2. Have all materials per table and have one person from each group gather the needed

material for his/her group.

3. Provide the data sheet attached to the experiment.

4. Check to make sure that all the materials include in the Direct Current Motor Kit are

available. Write on the board all the steps necessary to assemble the motor and

explain certain important details to avoid problems.

5. Observe that each group is following the written or the instructions manual. At the

same time walk around the room and provide assistance when it is needed with


connections and diagrams. Provide plenty time for the students to set up their model

(approximately 35 minutes).

6. Notify to the student when it is time to pick up and have each group return their

materials in an organized fashion to the back of the room.

7. Some children should need help to actually hook the wires to the battery, switch, and

other motor parts. Have your resource instructor help you but give them advance

warning that you will need their assistance this day.

Materials

1. Direct Current Electric Motor kit it includes:

2 coils of copper wire,

Base

Field pole or magnets,

Armature or core

Brushes

Fasteners, screws, pin, clips,

etc…

Instruction booklet with theory

and experiments included

3 to 6 v batteries

Multi-meter for voltage and

current

Student Procedure

Students group will begin the experiment verifying first if they have all the materials

including the instruction booklet to assemble the motor. They will follow the teacher

instruction all the time. Follow these steps when conducting the experiment:

1. Place students in groups of 4 or 5.

2. Set-up stations for each group with all necessary materials.

3. Layout all the materials in a counter and set them in order according to the

instruction booklet. The students should check the booklet and classify all the

materials require per step. In this way they will have the opportunity to get

familiar with the Direct Current Motor.


Fig.1 Shows all the parts of the Direct Current Motor Kit

4. Each group has to provide a list of materials at the end of the experiment.

5. After the assembling process it is necessary to set the batteries and prove if the

motor is working. This could be done by the teacher or someone with enough

experience with this type of equipment.

6. Once each group gets familiar with the motor proceed to answer the experimental

data sheet.

7. The experiment is going to be carrying for two different voltage sources and two

different numbers of turns so the student would have the opportunity to observe

the motor behavior.

8. A fifth test can be carry so the students will use the bigger battery or voltage

source and the maximum number of turns.

9. Record your observations and discuss your findings and answer the entire

experimental data sheet questionnaire.


Scientist Work Sheet

Name: ________________________ Date: _________________________

I. Experimental Data Sheet

DC Motor

Voltage Source(Volts)

Number of turns

Motor Current (Amp)

Motor Power(Watts)

Observations or Remarks

1st test

2nd test

3rd test

4th test

5th test

II. Questions

1. Show all the mathematical steps to determine the electric motor power?


Power FormulaP = V*I 1.

2. 3.

4. 5.

2. When the voltage source is change what happened with the speed of the motor axle? Explain your answer.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. What really happened when the voltage of the source remains the same but the number of turns is decrease? Explain your answer.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 4. What is the physical meaning of power and how is produces in a motor? Explain your answer.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. What happened with the current when the number of turn is change? Explain your answer ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

6. What happened with the current (I) when the voltage (V) of the source is change? Explain your answer ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


7. If the speed of the Direct Current Motor increases, what happened with the torque of the motor it will increase or decrease? Explain your answer ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Student Post Lab Activity

Drawing Conclusion/Discussion Questions

1. Check to make sure the students have completed the “Experimental Worksheet”.

2. Review and discuss what the groups discovered on their own about the motor

behavior after all the different changes. Re-examine any areas of confusion and

make sure that the students understand clearly the concepts.

3. Other suggestion for a post-lab activity includes a writing report describing the Direct Current Motor Experiment.

4. In order to assess students' understanding on an individual basis, have students

work independently on a homework assignment or in class. Doing the following

questions that are posed in the teacher package:

1. How difficult was to assemble and make the motor work? Explain your answer.

2. How this type of motors can help us in future circumstances?

3. What did you learn from this activity?

4. Why are both types of motors useful in our lives?

5. Describe the function of a circuit breaker in any type of motor.

6. Enumerate different part of both type of motors (AC/DC) according the pre-lab activities.

7.

Interdisciplinary Activities (activity involving these subjects)


1. Language Arts

The students will be requires to develop a report explaining the different steps they took

all over the experiment.

2. Math

a. Multiplication skills

b. Data analysis

c. Current and Volt Measurement

using a multi-meter.

3. Critical Thinking, Reading, etc.

a. Answer of multiple question base on observation, deduction and understanding.

b. Writing a report about the previous experimental lesson is suggested. Students

could consult the some books at the library and use the internet as a source.

References/Suggested Sources/Websites

1. http://www.electronics123.com/amazon/catalogue/c4-2.htm

2. Harcourt Science Unit E “Physical Science” 1st edition, Harcourt School Publishers,

Orlando, Boston, Chicago, San Diego

3. http://www.howstuffworks.com/motor.htm

4. http://my.execpc.com/~rhoadley/magdcmot.htm

5. http://www.simplemotor.com

6. http://www.deltasee.org/CTC/Activity%2011%20Build%20It%20Electric%Motor.pdf

7. http://www.engin.umich.edu/labs/csdl/ME350/motors/ac/induction/

8. http://www.orchardproject.org/electric_motor.htm

9. http://www.mnstate.edu/lindaas/educators/MoveMeBabyET.pdf

10. http://www.webping.net/electroDynamic/electrostatic_motor.html


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Module: Robotics - Starsstars.eng.usf.edu › Robotics Module › Lesson 2 Motor Sch… · Web viewThe torque continues until the coil’s poles reach the opposite poles of the