Pasco Scientific SE-8657 User manual

©1996 PASCO scientific $7.50
012-06247A
8/96
MOTOR ACCESSORY
Instruction Manual and
Experiment Guide for the
PASCO scientific
Model SE-8657
Includes
Teacher's Notes
and
Typical
Experiment Results

012-06247AMotor Accessory
i
®
Table of Contents
Section ...................................................................................................... Page
Copyright and Warranty, Equipment Return ................................................... ii
Introduction ...................................................................................................... 1
Equipment ........................................................................................................ 1
Table 1. Equipment Options for Experiments 1 - 4 ..........................................2
Operation ......................................................................................................... 3
Assembly—Motor Accessory onto the Variable Gap Magnet ......................... 4
Assembly—Motor Accessory onto the Coils and Cores Set ............................ 5
Suggested Uses ................................................................................................ 6
Experiment 1: Operation of the DC Motor ...................................................... 7
Experiment 2: Operation of AC and DC Generators .................................... 13
Experiment 3: Operation of an AC Synchronous Motor ...............................19
Experiment 4: Operation of the Universal Motor .......................................... 25
Teacher’s Guide ............................................................................................. 29
Technical Support .......................................................................................... 34

Motor Accessory 012-06247A
ii
®
Copyright, Warranty and Equipment Return
Please—Feel free to duplicate this manual
subject to the copyright restrictions below.
Credits
Author: Jim Housley
Editor: Sunny Bishop
Copyright Notice
The PASCO scientific SE-8657 Motor Accessory
manual is copyrighted and all rights reserved. How-
ever, permission is granted to non-profit educational
institutions for reproduction of any part of the manual
providing the reproductions are used only for their
laboratories and are not sold for profit. Reproduction
under any other circumstances, without the written
consent of PASCO scientific, is prohibited.
Limited Warranty
PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of one
year from the date of shipment to the customer. PASCO
will repair or replace, at its option, any part of the product
which is deemed to be defective in material or workman-
ship. The warranty does not cover damage to the
product caused by abuse or improper use. Determi-
nation of whether a product failure is the result of a
manufacturing defect or improper use by the customer
shall be made solely by PASCO scientific. Responsi-
bility for the return of equipment for warranty repair
belongs to the customer. Equipment must be properly
packed to prevent damage and shipped postage or
freight prepaid. (Damage caused by improper pack-
ing of the equipment for return shipment will not be
covered by the warranty.) Shipping costs for return-
ing the equipment, after repair, will be paid by
PASCO scientific.
Equipment Return
Should the product have to be returned to PASCO
scientific for any reason, notify PASCO scientific by
letter, phone, or fax BEFORE returning the product.
Upon notification, the return authorization and
shipping instructions will be promptly issued.
When returning equipment for repair, the units
must be packed properly. Carriers will not accept
responsibility for damage caused by improper
packing. To be certain the unit will not be
damaged in shipment, observe the following rules:
➀The packing carton must be strong enough for the
item shipped.
➁Make certain there are at least two inches of
packing material between any point on the
apparatus and the inside walls of the carton.
➂Make certain that the packing material cannot shift
in the box or become compressed, allowing the
instrument come in contact with the packing
carton.
Address: PASCO scientific
10101 Foothills Blvd.
Roseville, CA 95747-7100
Phone: [phone]
FAX: [phone]
email: [email protected]
web: www.pasco.com
NOTE: NO EQUIPMENT WILL BE
ACCEPTED FOR RETURN WITHOUT AN
AUTHORIZATION FROM PASCO.

1
012-06247A Motor Accessory
®
Equipment
Introduction
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78D$L($C#8#372135$G>2"$2">5$7BB7372E5H
The Motor Accessory includes
- armature with split commutator at one end and a
dual slip-ring commutator at the other
- brush holder
- shaft
- wrench/retaining nut
- maintenance items
- manual
- ceramic magnet
Additional Equipment Required:
- Variable Gap Magnet (EM-8641) or
- Coils and Cores Set (SF-8616)
Safety precautions
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>9$2">5$4E33#82$>5$#N4##D#D$13$>9$B1G#3$>5$7BB@>#D$4182>8E1E5@6F$#5B#4>7@@6$>9$2"#$73:72E3#$>5$812$31272>8CH
!"#$:1213$>5$>82#8D#D$18@6$913$>82#3:>22#82$1B#372>18H
+$L>54188#42$786$B1G#3$51E34#$G"#8#J#3$2"#$:1213$>5$21$?#$@#92$E8722#8D#DH
brush holder
armature
brushes
shaft
split ring
commutator
dual slip-ring
commutator
wrench/
retaining nut

2
Motor Accessory 012-06247A
®
Ta ble 2.xls
Exp erimentO ptions
M otorA c c e ss ory (
S
E-8657)
V ari a ble G a p M a g n e t (EM-8641)
C oilsa n d C ores
S
e t (
S
F-8616)
L
o w V olt a g e A C / D C Po w e r
S
u p ply (
S
F-9584) *
Po w erA m p lifier( C I-6550 A ) ( C I-6552 A)
Digit a lFu n c tio n G e n e ra tor/ A m plifier(PI-9587)
M ultim e t e r(e .g.
S
B-9623) orA m m e t er(
S
F-9569)
G a lv a no m e t er(
S
F-9500) orM ultim e t e r(
S
B-9623)
V olt a g e
S
e nsor( C I-6503)
S
cie n c e W orksh o p 300 or500 Int e rfa c e
S
cie n c e W orksh o p 700 or6500 Int erfa c e
Digit a lPh ot o g a t e Tim e r(
S
F-9215 A)
Digit a l
S
tro b osc o p e (
S
F-9211)
Exp eri m e nt 1: D C M otor
n o c o m p ut erint e rfa c e x x x
n o c o m p ut erint e rfa c e x x x x
c o m p ut erint e rfa c e x x x x x x
c o m p ut erint e rfa c e x x x x x
Exp eri m e nt 2: A C / D C G e n era tor
n o c o m p u t erint e rfa c e x x x
c o m p ut erint e rfa c e x x x x x
Exp eri m e nt 3: S yn c hro n o usA C M o tor
n o c o m p ut erint e rfa c e x x x x x
c o m p ut erint e rfa c e x x x x x x
c o m p ut erint e rfa c e x x x x x x
Exp eri m e nt 4: Univ ersalM o tor
n o c o m p u t erint e rfa c e x x x
c o m p ut erint e rfa c e x x x x x
c o m p ut erint e rfa c e x x x x x
L
o w V olt a g e D C Po w er
S
u p ply (
S
E-9720)
(
S
E-9712) orsimil ar*
EquipmentO ptions
forExp erimental
Setupswith the
PASC O SE-8657
MotorA c c essory
OR
OR
OR
OROR
OR
OR
!"#$%&'(&&)*+,-.%/0&1-0,2/3&425&)6-%5,.%/03&'&7&8
➤NOTE: Although the instructions for experiments in this manual are for mechanical setups
with specific PASCO equipment, the experiments in this manual may be set up in a variety of ways,
depending upon the equipment you have available. They can all be done with or without the
PASCO Science
Workshop computer interface. Table 1 lists the equipment suggested for optional
experimental setups. You may be able to substitute other equipment for the PASCO models listed
in this table.
*If your power supply does not have the capability to quantify output current, you will need
to measure it using an ammeter, or preferably, by calculating it from the voltage drop
across a small value series resistor. (This option avoids the potential for damage to a
sensitive ammeter.) It is important to limit the current to a maximum of 1 A to avoid
damaging the armature.

3
012-06247A Motor Accessory
¨
source. This is impractical at frequencies
much above 30 Hz, and some students may
require assistance even a lower frequencies.
Maintenance and Storage
- A small box is provided for storing the parts of
the motor not installed on either the Variable
Gap Magnet or Coils and Cores Set.
- The commutators and brushes will experience
wear, oxidation, and pitting and will require at-
tention from time to time. Rotate the armature
slowly by hand and monitor current flow or
sense the force developed to determine whether
proper contact is occurring between brushes and
commutator. To restore proper operation, clean
the contacts with emery paper or shift the
brushes somewhat to expose new surfaces.
- Careless installation of the armature onto the
shaft might bend the brushes. You can easily
bend them back into their original shape with
finger pressure.
➤NOTE: If you are using a PASCO CI-
6502A Power Amplifier (for the CI-6500
Interface System), the distorted waveform light
will turn on during operation of the motor, but
no damage is being done to the Power Ampli-
fier; you can ignore the light.
Starting the motor
- The motor is not self-starting. Immediately after
you apply the power, start the motor manually by
grasping the black plastic bushing at the top of
the armature assembly between your thumb and
forefinger and spinning the armature.
- With the Motor Accessory configured as either a
DC or universal motor, almost any attempt you
make at spinning the armature will result in suc-
cessfully starting the motor; only the direction of
the spin is important.
- When configured in an AC synchronous mode,
the motor must be spun at a speed that approxi-
mately matches the frequency of the power
Operation
Options for electrical connections
- Banana-style plugs may be inserted into openings
in the ends of the black plastic brush holder.
- Large alligator clips may be attached to the brass
posts that hold the brushes.
- Small alligator clips may be attached directly to
the ends of the brushes where they protrude from
the slits in the brass posts.
Options for Power Sources
It is important to limit the current of the power source
to 1.0 A to avoid damaging the coils of the armature.
Either choose a power supply that can be set to deliver
a maximum current of 1.0 A, or use your power
source connected in series with a multimeter or
ammeter to monitor the output current. (Alterna-
tively, to avoid possible damage to a sensitive
ammeter, you can measure the voltage drop across
a low-value series resistor, such as a 0.51 ohm, 1
watt resistor, and calculate the output current.) You
will also need to adjust and measure the output
voltage, so if your power supply does not have this
capability, you will need a multimeter or voltmeter.
(See Table 1 for specific suggestions for power
sources.)

4
Motor Accessory 012-06247A
¨
Motor Accessory onto the Variable Gap
Magnet
➀Be sure you have the flat iron pole pieces placed on
the two neodymium magnets of the Variable Gap
Magnet. The larger threaded portion of the shaft
screws easily, without tools, into the threaded hole
in the magnet base. Insert the threaded end of the
shaft from above, screwing it in until 1 mm, or
slightly less, of the threaded portion remains above
the upper surface of the base.
➁Turn the magnet over and screw the retaining nut
onto the smaller diameter threaded portion of the
Assembly
shaft that protrudes through the bottom of the mag-
net base. (Note that the retaining nut has a metric
thread, size M6-1.0.) Use firm finger pressure. If
this should prove inadequate, tighten the nut some-
what more with a wrench. If an appropriate wrench
is not at hand, use a heavy metal object to tighten
the nut by tapping the edge of the nut. Do not use
a pole piece of the magnet to tighten the nut be-
cause that might mar the finish of the pole piece.
Do not over tighten.
➂Working from above, press the brush holder onto
the smooth, enlarged portion of the shaft. Apply
increasingly firm pressure equally to each side of
the brush holder while rotating the brush holder
back and forth. If this action loosens the retaining
nut, tighten it more tightly, as described in step 2.
Check to be sure the brush assembly is seated as far
down on the shaft as it will go.
➃Gently lower the armature onto the shaft. To make
a DC motor, the split ring commutator should be
down; for an AC motor, the dual slip-ring commu-
tator should be down. Carefully rotate the arma-
ture back and forth to separate the brushes and al-
low the commutator to slip down between them. If
necessary, insert a pencil or similar object down
between the brushes. Use only the most delicate
force to avoid bending the brushes and necessitat-
ing adjustments or repairs.
➄Adjust the gap of the Variable Gap Magnet so there
is approximately 1 mm of clearance between the
pole pieces and the armature when it is rotated by
hand.
➅Refer to the instructions included in experiments 1-
4 for details of the electrical connections.
92025&:;;%3325<=>"5,"#$%&?"-&9"@/%0&:33%.#$<
armature
wrench/retaining
nut
brushes
brush holder
dual slip-ring commu-
tator
split ring
commutator
(this end down for
AC motor)
(this end down for
DC motor)
leave 1mm
exposed at
installation
shaft
magnet base
flat pole pieces
neody-
mium
magnet

5
012-06247A Motor Accessory
¨
400
400
Motor Accessory onto the Coils and Cores
Set
➀Begin with the U-shaped core, with the coils and
any other parts removed. The smaller threaded
portion of the shaft screws easily into the threaded
hole in the core so the shaft is between the poles of
the core. Use the wrench provided to tighten the
shaft by gripping the flats on the larger threaded
portion. The small wrench limits the torque that
can be applied. If an ordinary wrench is used, be
careful not to over tighten.
Note: Do not discard the small wrench; it
is essential as a retaining nut when the Motor
Accessory is used with the Variable Gap
Magnet.
➁Working from above, press the brush holder
onto the smooth, enlarged portion of the shaft.
Apply increasingly firm pressure equally to each
side of the brush holder while rotating the brush
holder back and forth. If this action loosens the
shaft, tighten it as described in step 1. Check to
be sure the brush assembly is seated as far down
on the shaft as it will go. Orient the brush
holder perpendicular to the base of the Coils and
Cores apparatus.
➂Place the two 400-turn coils from the Coils and
Cores Set onto the poles of the core.
➃Gently lower the armature onto the shaft. The split
ring commutator should be down for use as a uni-
versal motor. Carefully rotate the armature back
and forth to separate the brushes and allow the
commutator to slip down between them. If neces-
sary, insert a pencil or similar object between the
brushes to separate them. Use only the most deli-
cate force to avoid bending the brushes and neces-
sitating adjustment or repairs.
&92025&:;;%3325<=A2,$3&"/B&A25%3&:33%.#$<
U-shaped base
400-turn coil
split ring commutator

6
Motor Accessory 012-06247A
¨
Operation as a DC motor
The Motor Accessory can be used with the Variable
Gap Magnet to demonstrate the operation of a DC
motor ( Experiment 1). Students can explore relation-
ships between motor speed and voltage, as well as
between direction of armature rotation and polarity,
developing key concepts including: action of the split
ring commutator, dependence of speed on voltage,
dependence of direction of rotation on polarity, right-
hand rule, and direction of current flow from positive
to negative poles.
Action of AC and DC generators
Spinning the armature by hand while it is connected to
a sensitive DC meter or to the Signal Interface II
shows the action of an AC generator, as well as the
rectifying action of the commutator in a DC generator
(Experiment 2).
Operation of a synchronous AC motor
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07C8#2$755#:?@>#5$G>@@$1B#372#$>8$5684$G>2"$<.$78D$VR
WX$Q78D$192#8$G>D#3$378C#S$5>C87@5$Q*NB#3>:#82$VSH
'2ED#825$478$#NB@13#$2"#$3#@72>185">B$?#2G##8$&(
J1@27C#$78D$:1213$5B##DF$75$G#@@$75$?#2G##8$&($4E33#82
93#YE#846$78D$:1213$5B##DH$$!"#6$478$418DE42$D#27>@#D
#NB@1372>185$19$2"#$B3#4>5>18$19$5684"318>5:$19$&(
4E33#82$78D$:1213$5B##D$G>2"$7$%&'()$'I+UZ<<
L>C>27@$'231?1541B#$13$%&'()$0*+UZ<.&$L>C>27@
%"121C72#$13$?6$1?5#3J>8C$2"#$5231?1541B>4$#99#42$19$78
13D>8736$9@E13#54#82$@7:B$72$5#@#42#D$:1213$5B##D5H$$&5
7$3#5E@2F$2"#6$D#J#@1B$M#6$4184#B25F$>84@ED>8C$2"#
>8D#B#8D#84#$19$&($:1213$5B##D$78D$J1@27C#F$D#B#8+
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742>18$19$7$DE7@$5@>B+3>8C$41::E27213H
Suggested Uses
Operation as a universal motor
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9E842>185$75$7$E8>J#357@$:1213F$1B#372>8C$18$?12"$&(
78D$L($B1G#3$5EBB@>#5$Q*NB#3>:#82$;SH$'2ED#825$$478
#NB@13#$2"#$3#@72>185">B5$19$4E33#82$D>3#42>18$78D$D>3#4+
2>18$19$2"#$:7C8#2>4$9>#@DF$2"#$#99#42$19$4"78C#5$>8
J1@27C#$78D$&($4E33#82$93#YE#846$18$:1213$5B##DF$78D
2"#$#99#42$19$4"78C#5$>8$L($J1@27C#$18$:1213$5B##DH
Additional possibilities
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478$?#$E5#D$21$D#2#3:>8#$2"#$5B##D5$19$:7N>:E:$B1G#3
78D$:7N>:E:$#99>4>#846$19$7$L($:1213$?6$J736>8C$2"#
@17D$G">@#$5>:E@278#1E5@6$:#75E3>8C$2"#$5B##DF$213YE#F
78D$73:72E3#$4E33#82H$$T8$2">5$#NB#3>:#82F$61E$478
:#75E3#$2"#$:1213\5$5B##D$G>2"$7$B"121C72#$13$5231?1+
541B#H
!"#$01213$&44#55136[(1>@5$78D$(13#5$755#:?@6$7@51
478$?#$E5#D$21$D#:1852372#$5#3>#5+G1E8DF$5"E82+G1E8DF
78D$"652#3#5>5+5684"3181E5$:1213$5#2EB5H

7
012-06247A Motor Accessory
¨
EQUIPMENT NEEDED:
•Motor Accessory •multimeter
•Variable Gap Magnet •patch cords
•low voltage DC power supply, limited to 1 A •small piece of masking tape
Purpose
The purpose of this experiment is to demonstrate the operation of the DC motor in terms of
basic concepts of electromagnetism.
Theory
The Variable Gap Magnet is a permanent magnet possessing a north pole and a south pole
that interact with the north and south poles of the armature (an electromagnet when con-
nected to an electric current). Like poles repel, while unlike poles attract. The armature
rotates until its north pole is as close as possible to the south pole of the permanent magnet
(and also as far as possible from the north pole). Instead, if the rotational speed of the
armature matches the frequency of the alternating current, the direction of current in the
armature will reverse at that instant, so that the torque continues to act in the original direc-
tion.
A better explanation involves an understanding of fields. The variable gap magnet pro-
duces a magnetic field that passes through the gap between the pole pieces. When current
passes through the turns of the armature in the presence of the field, forces act to cause a
torque that rotates the armature. Inertia carries the armature past the position of no torque
to the point where the torque would force the armature back in the other direction. How-
ever, at that point the commutator reverses the direction of current in the armature so the
torque continues to act in the original direction.
Setup
➀Be sure you have the flat iron pole pieces placed on the two neodymium magnets of the
Variable Gap Magnet. (The iron pole pieces spread the magnetic field over a wider
area.) Screw the larger threaded portion of the shaft into the threaded hole in the magnet
base. Insert the threaded end of the shaft from above, screwing it in until 1 mm, or
slightly less, of the threaded portion remains above the upper surface of the base.
➁Turn the magnet over and screw the retaining nut onto the smaller diameter threaded
portion of the shaft that protrudes through the bottom of the magnet base. Use firm
finger pressure. Do not over tighten.
Experiment 1: Operation of the DC Motor

8
Motor Accessory 012-06247A
¨
➂Working from above, press the brush
holder onto the smooth, enlarged portion
of the shaft. Apply increasingly firm
pressure equally to each side of the brush
holder while rotating the brush holder
back and forth. If this action loosens the
retaining nut, tighten it more tightly, as
described in step 2.
Check to be sure the brush assembly
is seated as far down on the shaft as it
will go.
➃Gently lower the armature onto the shaft
with the split ring commutator down.
Carefully rotate the armature back and
forth to separate the brushes and allow
the commutator to slip down between
them. If necessary, insert a pencil or
similar object down between the brushes.
Use only the most delicate force to avoid
bending the brushes and necessitating
adjustments or repairs.
➄Adjust the gap of the Variable Gap
Magnet so there is approximately 1 mm
of clearance between the flat pole pieces
and the armature when it is rotated by
hand.
➅Connect the positive terminal of the DC
power supply to one end of the brush
holder with a red patch cord by plugging
the banana terminals into each.
➆Connect the negative terminal of the DC
power supply to the other end of the
brush holder with a black patch cord.
Do not turn the power on.
Figure 1. Installation of the Motor Accessory
onto the Variable Gap Magnet
armature
wrench/retaining
nut
brushes
brush holder
dual slip-ring commu-
tator
split ring
commutator
leave 1mm
exposed at
installation
shaft
magnet
base
Figure 2. Experimental Setup
flat pole pieces
6 AMP MAX
RESET
ON
OFF
PAS CO sc ie nt ifi c
METER
PUSH FOR
CURRENT
DC CURENT
ADJUST
DC VOLTAGE
ADJUST 2
4
6
8
12 14 16
18
20
22
24
10
AC VOLTAGEADJUST
2 - 24 VOLTS AC OUTPUT
0 - 24 VOLTS DC OUTPUT
8 AMP MAX
MODEL SF-9584 LOW VOLTAGE AC/DC POWER SUPPLY
wire connected to
the + terminal of the
power supply

9
012-06247A Motor Accessory
¨
Procedure—Part A
➀Rotate the armature and observe how the segments of the split ring commutator contact the
brushes as the armature turns.
➁Remove the armature from the shaft by grasping it between your thumb and forefinger and
rotating it back and forth while lifting gently. If necessary, insert a pencil between the
brushes to gently separate them to remove the armature.
➂Examine the armature closely and imagine current entering one of the split rings from a
brush. Trace the path of the current through the wire to the coil, through the coil, through
the wire to the coil on the opposite side of the armature, through that coil, and through the
wire to the other split ring and into the second brush. By carefully examining the part of
the coils where the wire emerges from the coil, you can determine the direction in which
the wire is wound on the coil (see Figure 3).
➃Holding the armature in one hand, imagine that the brush from the + lead is touching one
of the split rings of the commutator. Follow the wire from the split ring to the right coil of
the armature and note the direction the wire is wound in the coil. Note where the wire
enters the coil and where it exits.
➄Use the right-hand rule to determine the direction the magnetic field will flow when you
turn on the power: Grasp the coil with your fingers wrapped around the coil in the direc-
tion of the current (Figure 4). (Current direction is described by convention as being from
the positive to the negative lead. Note that this is opposite of the direction of electron
movement—see note on page 10) . Your thumb will point in the direction of the field —
that is, toward the north pole of the coil). Put a small piece of tape on the end of the
armature that will be its north pole when you turn on the power.
➅Follow the wire over to the left coil. Use the right hand rule to find the direction of the
north pole.
Record your observations on Figure 4.
a) In this situation, is the direction of the north pole the same for the right and left coils?
Figure 3. Direction of the Wire Winding on the Coil

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wire connected to the +
terminal of the power
supply
Figure 4. Determining the Direction of the Magnetic Field of the Coil Using the
Right-Hand Rule
When you wrap
your fingers in
the direction of
the flow of the
electric current,
your thumb
points towards
the north pole of
the magnetic
field.
+
+
Draw arrows indicating the direction
of current flow.
Indicate which pole: north (N) or south
(S).
N
➤Note: Here’s why the direction of conventional current is opposite to that of the
direction of electron flow: In the mid-eighteenth century, Benjamin Franklin suggested the
terms positive and negative, and conjectured that electrical current was the movement of
positive “fluid” from positive to negative regions. Although he understood that it was
equally possible that a negative fluid moves from negative to positive, for more than a
century there was no way to resolve the issue. By convention, scientists agreed to describe
the direction of current as being from positive to negative. Not until 1879 did Edwin H.
Hall show that in metals the current was a negative “fluid”; it remained for J. J. Thompson,
R. A. Millikan, and others to demonstrate the existence of electrons, which are the charge
carriers of this “fluid”. This might seem an argument for changing the convention. But
current doesn’t always travel in metals. In gasses, current consists of electrons traveling in
one direction while positive ions move simultaneously in the opposite direction. In
solutions, current consists of oppositely charged ions traveling in opposing directions.
And in certain semiconductors, it is most useful to think of positive “holes” as being the
charge carriers.
Considering this complexity, scientists have found it most useful to continue the conven-
tion begun by Franklin: the “direction” of current is from positive to negative.

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b) Both coils surround a single iron core on the armature, and each coil is capable of
temporarily magnetizing the core when electric current is running through it. Do the
actions of the two coils add to create a greater effect or cancel to create a reduced
effect? (Consider your answer to 6a above.)
➆Turn the armature over 180° and imagine that the brush attached to the + lead is contacting
the other split ring of the commutator. Note the path of the wire from where it is attached
to the split ring to where it enters and exits from the coil on the right side of the armature.
Imagine a current running through the wire and use the right-hand rule to determine the
direction the magnetic field would flow. Is the north pole on the same end of the coil as it
was in step 5?
➇Follow the wire over to the left coil. Use the right hand rule to find the direction of the
north pole.
a) In this situation, is the direction of the north pole the same for the right and left coils?
b) True or False? When the electric current is on, the two coils become electromagnets
with magnetic fields oriented in the same direction, which turns the armature into a single
electromagnet with its force oriented towards that same direction.
c) True or False: In the DC motor, you cannot determine the direction of the magnetic
field of the armature by determining the direction of the north pole of either of the two
coils.
d) What happens to the location of the armature’s north pole as the brush attached to the
+ lead touches the different sides of the split ring commutator?
e) Can you explain why the current in the armature is alternating, despite the fact that the
motor is supplied with direct current. (Hint: think about your answer to 8d.)
➈Gently replace the armature onto the shaft with the split ring commutator down. Carefully
rotate the armature back and forth to separate the brushes and allow the commutator to
slip down between them. If necessary, insert a pencil or similar object between the brushes
to separate them. Use only the most delicate force to avoid bending the brushes and
necessitating adjustment or repairs.
Procedure—Part B
➀Turn on the power. Adjust the output voltage to 6 volts.
➁Use the small cylindrical ceramic magnet to check your predictions from steps 5 and 6

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above. The painted face of the magnet is its North Pole (north-seeking pole). [You can
verify this by hanging the magnet from a thread and observing that the painted face points
toward the North (toward the earth’s north magnetic pole, located in northern Canada).]
With the armature and power supply leads oriented as in Figure 2 and the power turned on,
hold the ceramic magnet near the ends of the armature. If both poles of the ceramic magnet
attract the armature, the pole with the stronger attraction will be the opposite pole.
a) Does the result of this test agree with your predictions in steps 5 and 6?
b) Label each end of the armature in Figure 2 according to whether it is the north or south
pole of the electromagnet.
c) Determine the polarity of the Variable Gap Magnet in the same way. Label its poles “N”
and “S” in Figure 2.
➂Predict the direction the armature will rotate when you release it from the position of Figure
2.
Will the motor rotate clockwise or counter-clockwise?
If the motor does not start up immediately, try turning it by hand in the predicted direction.
If that fails, try turning it in the opposite direction.
➤If the motor does not start in either direction, turn off the power and ask your
teacher for help.
➃Turn off the power and reverse the positive and negative leads to the motor. Before turning
the power on, predict the direction of rotation.
a) Will the motor rotate clockwise or counter-clockwise?
Turn the power back on and immediately try spinning the motor to start it. If it doesn’t start,
try spinning it in the other direction.
b) Explain why the armature turns when you turn on the power.
➄While the motor is running, raise the voltage to approximately 8 volts.
a) What happens to the motor’s rotational speed when you raise the voltage?
b) Is the relationship of the motor’s rotational speed and voltage of the DC current depen-
dent or independent?

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EQUIPMENT NEEDED OPTIONAL EQUIPMENT
• Motor Accessory •Voltage Sensor
•Variable Gap Magnet •computer interface
•multimeter or galvanometer
•patch cords
•small strips of masking tape
Purpose
The purpose of this experiment is to detail the operation of an AC generator and a DC
generator in terms of basic concepts of electromagnetism.
Theory
Motors and generators may be regarded as devices that convert energy from one form to
another. A motor converts electrical energy into mechanical energy. Many designs of
motors work as generators as well: when mechanical energy is input by spinning the shaft,
electrical energy is produced. More than one line of reasoning may be used to predict the
magnitude and direction of the electrical current that is produced. At the most fundamental
level, electrical charges moving across a magnetic field experience a force that is at right
angles to both the direction of motion and the direction of the magnetic field, according to
the vector equation:
F=qV x B
Conductors, of course, contain charges, and moving a conductor sideways across a
magnetic field causes a force on the charges that may make the charges flow the length of
the conductor if it is part of a circuit. The force on the charges can be seen from the
equation to be proportional to both the speed and the strength of the magnetic field.
From this reasoning you can derive Faraday’s law of electromagnetic induction, which
states that a change in the magnetic flux linking a closed circuit will result in an electromo-
tive force (or electric current) in the circuit that is instantaneously proportional to the time
rate of change of the linking flux; however, it is easier to understand Faraday’s law by
observing the action of a generator. In a generator, an electromotive force (emf) that is
proportional to the rate of change is induced in a loop of wire that is in a field of changing
magnetic flux. (The coils of the armature may thought of as many loops connected in
series.)
Experiment 2: Operation of AC and DC Generators

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Surprisingly, the direction of induced current can be determined from the law of conserva-
tion of energy. Due simply to friction, work must be done to rotate a generator. If the
generator is connected to a load and producing electric current, the law of conservation of
energy dictates that additional work must be done to turn the shaft. This is an example of the
reasoning that led to Lenz’s law: the induced current is in such a direction as to produce a
magnetic field that opposes the original magnetic field.
You can demonstrate Lenz’s law to yourself by determining the direction of the magnetic
field of the Variable Gap Magnet and by detecting the direction of the induced electric
current with a galvanometer (or multimeter) as you move the armature through the magnetic
field.
Setup
➀Be sure you have the flat iron pole pieces placed on the two neodymium magnets of the
Variable Gap Magnet. (The iron pole pieces spread the magnetic field over a wider area.)
Screw the larger threaded portion of the shaft into the threaded hole in the magnet base.
Insert the threaded end of the shaft from above, screwing it in until 1 mm, or slightly less, of
the threaded portion remains above the upper surface of the base.
➁Turn the magnet over and screw the retaining nut onto the smaller diameter threaded portion
of the shaft that protrudes through the bottom of the magnet base. Use firm finger pressure.
Do not over tighten.
Figure 1. Experimental Setup
voltmeter or galvanometer

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➂Working from above, press the brush holder onto the smooth, enlarged portion of the
shaft. Apply increasingly firm pressure equally to each side of the brush holder while
rotating the brush holder back and forth. If this action loosens the retaining nut, tighten it
more tightly, as described in step 2.
➤Check to be sure the brush assembly is seated as far down on the shaft as it will
go.
➃Gently lower the armature onto the shaft with the dual slip-ring commutator down. Care-
fully rotate the armature back and forth to separate the brushes and allow the commutator
to slip down between them. If necessary, insert a pencil or similar object down between
the brushes. Use only the most delicate force to avoid bending the brushes and necessitat-
ing adjustments or repairs.
➄Adjust the gap of the Variable Gap Magnet so there is approximately 1 mm of clearance
between the flat pole pieces and the armature when it is rotated by hand.
Procedure
Part A: AC Generator
➀During the first part of this experiment, the dual slip-ring commutator should be down,
between the brushes. If it is not, remove the armature from the shaft by grasping it between
the thumb and forefinger and rotating it back and forth while lifting gently. Sometimes it
may be necessary to insert a pencil between the brushes to gently separate them so that
they don’t prevent removal of the armature.
➁The cylindrical ceramic magnet may be used to determine the polarity of other magnets.
The painted face of the magnet is its North Pole (north-seeking pole). [You can verify this
by hanging the magnet from a thread and observing that the painted face points toward the
North (toward the earth’s north magnetic pole, located in northern Canada).] Determine the
polarity of the variable gap magnet by holding the ceramic magnet near its rectangular pole
pieces. In the event that both poles of the ceramic magnet attract a pole piece, the stronger
attraction occurs when opposite poles are together. Label the pole pieces N and S using
small strips of tape.
➂Examine the armature closely, and imagine current entering one of the two slip rings from
a brush. Trace the path of the current through the wire to the coil, through the coil, through
the wire to the coil on the opposite side of the armature, through that coil, and through the
wire to the other split ring and into the second brush. By carefully examining the part of
the coils where the leads emerge from the coil, it should be possible to determine the
direction in which the wire is wound on the coil. Can you verify that the current maintains
its same direction of rotation as it leaves one coil and enters the other? This means that the
two coils of the armature act as a single coil. Ask for help if you cannot.
➃Label the end of the armature that connects to the upper slip ring with a small piece of tape.
➄Gently replace the armature onto the shaft. The dual slip-ring commutator should be
down. Carefully rotating the armature back and forth will often separate the brushes and
allow the commutator to slip down between them. Otherwise, inserting a pencil or similar
object between the brushes to separate them may be necessary. Only the most delicate
force should be used to avoid bending the brushes and necessitating adjustments or repairs.

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➅Adjust the gap of the Variable Gap Magnet so that there is approximately 1 mm of clearance
between the pole pieces and the armature when it is rotated by hand.
➆Position the armature so that it is at right angles to the N-S orientation of the Variable Gap
Magnet. Then rotate it by hand 90 degrees until the end of the armature marked with tape is
near the north pole of the magnet. The magnetic field of the magnet may be envisioned as
arrows passing out of the north pole piece and into the south pole piece.
(a) What happens to the amount of this magnetic field that passes through the loops of the
coils during your 90-degree rotation above? If the amount changed, did it increase or
decrease?
(b) What does Faraday’s induction law say about this situation?
➇Continue rotating the armature another 90 degrees.
(a) What happens to the amount of this magnetic field that passes through the loops of the coils
during your 90-degree rotation above? If the amount changed, did it increase or decrease?
(b) What does Faraday’s induction law say about this situation?
(c) How would the induced emf be different during the rotation of step 7, compared to step 8?
➈The forces due to Lenz’s’ law in this equipment are much less than other effects and are not
readily noticeable. Nonetheless, the reasoning involving Lenz’s law allows you to predict the
direction of current. Consider the 180 degree rotation you performed above:
(a) To oppose the motion during the first 90 degrees of rotation, what pole (N or S) would the
taped end of the armature need to be?
(b) To oppose the motion during the second 90 degrees of rotation, what pole (N or S) would the
taped end of the armature need to be?
➉In order to cause the armature to act as you stated in step 9 above, what direction would the
induced current need to move?
➤To answer this, you will need the “right hand rule”, which can be used to predict the
direction of the magnetic field of a coil. Grasp the coil with the fingers wrapped around
the coil in the direction of the current. The thumb will point in the direction of the field.
(i.e., toward the north pole of the coil.) Current direction here is described as being from
the positive to the negative (conventional current). Note that this is opposite of the
direction of electron movement.
(a) Must conventional current enter the coil, or leave the coil, from the upper brush, in order
to make the armature act as you described in 9 (a) above?