Lab Exercise 6: Induction, Transformer, and Motor
Principles
Contents
6-1
6-2
6-3
6-4
P RE - LAB A SSIGNMENT . . . . . . . . . .
I NTRODUCTION . . . . . . . . . . . . . . .
E QUIPMENT . . . . . . . . . . . . . . . . .
E XPERIMENT . . . . . . . . . . . . . . . .
6-4.1 Electromagnetic Induction Principles
6-4.2 Transformer Basics . . . . . . . . . .
6-4.3 Motors . . . . . . . . . . . . . . . .
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Objective
To examine experimentally the basic principles governing electromagnetic induction and its applications such as transformers and motors.
General concepts to be covered:
• Relationship between induced voltage and changes in magnetic
flux.
• Determining the inductance of coils.
• Examine the impact of core shape on the performance of a
transformer.
• Determining the relationships between transformer primary-tosecondary number of windings ratio and the input-to-output
currents (and voltages) ratio.
• Determining the dependance of a-c and d-c motor speeds on the
power-supply’s parameters such as voltage and frequency.
1
2
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
6-1
P RE - LAB ASSIGNMENT
Discussion of the basic principles governing magnetic induction, the operation of motors
and transformers are covered in Chapters 5 and 6 of the textbook “Fundamentals of Applied
Electromagnetics”, by Professor F.T. Ulaby. The student is expected to have read the
material in these Chapters. More details regarding the different types of transformers and
motors can be found in specialized textbooks, located in the Engineering library, and in
articles on the web. The student is encouraged to learn more about practical transformers
and motors. Below is a sample list of resources that can be sought:
Books:
• “Electric Machinery” by A.E. Fitzgerald, Charles Kingsley, Jr., and Stephen D.
Umans.
• “Electric Machinery and Transformers” by Irving Kosow.
• “Electric Machinery Fundamentals” by Stephen J. Chapman
• “Electric Motors and their Controls: An Introduction” by Tak Kenjo
• “Fractional and subfractional horsepower electric motors: available types, basic
operating principles, selection and maintenance” by Cyril G. Veinott and Joseph E.
Martin
Web-sites:
• http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.
html
• http://www.mae.ncsu.edu/courses/mae732/shih/00$_$motors.pdf
6-2
I NTRODUCTION
The first application that comes to mind when magnetic induction is mentioned is power
generation. The discovery of the simple action of moving a conducting wire across a
magnetic field (which results in the generation of current in the wire) has provided us with
the means to reliably and consistently transform mechanical energy to electrical energy.
This in turn has contributed to historic advancements in technology and dramatic changes
in our way of life over the past 100 years or so. Solenoids, power transformers, and
motors are also among the well known applications that are based on the principles of
magnetic induction. Many more, less famous, applications of magnetic induction are in
daily use. Unfortunately, they cannot be covered in this lab. One example is induction
heating, whereby a large conducting coil carrying alternating current is used to heat an
object positioned at the coils center. Another example is magnetic inductive testing of
wire cables, whereby perturbations in the existing magnetic field around the wire, induced
by wire damage (such as wire fracture and corrosion), are detected. Nowadays, sensors
based on measuring the induced current through a conducting sheet that is subjected to a
perpendicular magnetic field, known as Hall-effect sensors, are used for highly accurate
gear-tooth sensing, direction detection, linear sensing, speed sensing, position sensing, and
contact-less switches. Interested students can learn about these applications and others that
are based on magnetic induction principles by searching the internet.
This lab exercise is divided into three parts: Induction, transformers, and motors.
6-3
EQUIPMENT
6-3
3
E QUIPMENT
Item
Multimeter
Digital Function Generator
Digitizing Oscilloscope
DC Power Supply
Motor Assembly
Coil Set (6 coils + 2 iron cores)
Motor Assembly
Permanent Magnetic Rod (2” and 3” long)
Nail
Spring
LCD Tachometer
6-4
E XPERIMENT
6-4.1
Electromagnetic Induction Principles
Part #
HP 973A
PI-9587C
HP 54645A
HP E3620A
SE-8658A
SF-8617
SE-8658A
HANGAR 9
Review of Basic Concepts
To fully understand how induction occurs in coils and be able to explain the different
phenomenas that will be observed in this lab, the following set of basic concepts, that
describe the relationship between the magnetic field and charge, are summarized below:
• A stationary charge does not generate a magnetic field. Only an electric field is
generated. In addition, a magnet has no effect on a stationary charge.
• Charges moving in a specific direction and at constant speed will generate a constant
magnetic field. It will generate also a constant electric field. However, the two fields
are uncoupled. If the stream of charges (or current line) is alternating in direction
and varying in strength over time, then so will be the generated magnetic and electric
fields. However, in this case both fields will be coupled.
• When a magnet is near a charge moving at a constant speed (or near a constant
current line), a force is generated that is perpendicular to both the magnetic field and
the velocity vector of the charge (or the current line). The prescribed system has, in
effect, two magnets that are present near one another and the result is either a pulling
or pushing force between them. It should be noted that the magnetic field generated
by the magnet can change the direction of motion of a charged particle but it cannot
change its speed.
• Magnetic fields can produce an electric current in a closed loop, but only if the
magnetic flux linking the surface area of the loop changes with time. This is the most
important principle governing magnetic induction and is mathematically described
by Fraday’s Law. The key to the induction process is change. The voltage, called
also the electromotive force (emf), across the closed loop can be generated under any
of the following three conditions:
1. A time-varying magnetic field linking a stationary loop; the induced emf is then
called the transformer emf.
4
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Figure 6-1: Setup used to demonstrate motion-induced current. A magnet is suspended by
a spring and inserted halfway inside a coil. (Courtesy of Pasco Scientific)
2. A moving loop with a time-varying area (relative to the normal component of
the magnetic field) in a static magnetic field; the induced emf is then called the
motional emf.
3. A moving loop in a time-varying magnetic field.
• In all cases of magnetic induction, an induced voltage will cause a current to flow
in a closed circuit in such a direction that its magnetic effect will oppose the change
that produced it. This is also known as Lenz’s law.
Motion-Induced Current
Setup
• Connect the 3200-turn coil to Channel 1 of the oscilloscope. Orient the coil such that
its hollow center and the side with the 3200 label are pointed upward.
• Use the Volt/Div knob of Channel 1 to adjust the voltage scale to 500-mV/div and the
Time/Div knob to adjust the time scale 500-msec/div. Trigger the Oscilloscopes data
collection using Channel 1. Set the trigger level to 0-Volt.
• Attach the machine nut to the N-pole of the magnetic rod. Insert one side of the
spring into the nut. The Notched side of the magnetic rod is the N-pole side.
• Lift vertically the free side of the spring. If necessary, adjust the spring, nut, and
magnetic rod, such that the rods axis is vertical.
Procedure
1. Gently insert the S-pole of the magnetic rod (the free side) halfway into the hollow
center of the coil as shown in Fig. 6-1. Record what you observe on the Oscilloscopes
display. Provide an explanation.
2. Gently pull down on the side of the spring near the nut. Adjust the height of
the spring/magnetic rod and repeat the process if necessary to insure that the rod
oscillates vertically in a uniform fashion inside the coil and does not touch the table
underneath the coil. Print a copy of the signal displayed by the Oscilloscope. Adjust
the Time and Amplitude scales if necessary and determine the frequency of the signal.
Is it sinusoidal? Provide an explanation.
Questions
6-4
EXPERIMENT
1. Assuming that you have a repeatable linear actuator that moves the magnet in and
out of the coil at a controllable rate, what can be done to increase the frequency of
the output signal? What changes can be made to the system to increase the output
voltage and current values?
2. What if the coil used in this experiment were connected to a repeatable linear actuator
that moves the coil between the N- and S-poles of a strong, stationary, permanent
magnet. The magnet is composed of two flat plates facing each other with the coil
positioned in between. As the coil is moved forward along its axis, it comes closer
to one pole, and as it is moved back, it comes closer to the opposite pole. Can this
system produce electricity? [Hint: consider the direction of winding with respect to
the magnetic field lines]. If not, then how can it be modified to produce electricity?
3. What if the magnet, described in the previous question, is replaced with a permanent
magnet constructed in the form of a cylindrical shell (the shell is split into two half
cylinders: one for N-pole and the other for S-pole) and the coil is moved in and out
along the axis of the cylindrical shell? Can the system produce electricity in that
case? [Hint: consider the direction of winding with respect to the magnetic field
lines]. If not, then how can it be modified to produce electricity?
Solenoid: The DC Case
Procedure
1. Connect the 400-turn coil to the DC power supply. Set the voltage on the power
supply to +1.5 V. Switch off the power of the DC supply momentarily.
2. Insert a nail half way inside the coils hollow center, as shown in Fig. 6-2. Release
the nail. Switch on the DC supply. Document what happens to the nail. Provide an
explanation for the observed response.
3. Insert your finger inside the coils hollow center and push the nail half way out while
the DC supply is still ON. Remove your finger and document what happens to the
nail. Provide an explanation for the observed response.
4. Place the N-pole of the magnetic rod halfway into the coils center. Use the side of
the coil labeled “400”. Release the magnet and document what happens to the rod.
Provide an explanation for the observed response.
5. Remove the magnet and re-insert it following the same procedure outlined in the
previous step, except insert this time the south-pole first. Release the rod and
document what happens to the rod. Provide an explanation for the observed response.
Solenoid: The AC Case
Procedure
6. Connect the 400-turns coil to the low-impedance port of the digital function generator
(DFG). Switch-on the DFG and select the sinusoidal waveform option.
5
6
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Figure 6-2: An Iron nail is partially inserted into a coil which in turn is connected to a DC
power supply (Courtesy of Pasco Scientific)
7. Set the frequency on the DFG to 2-Hz and its amplitude to 5-Volts. Use the
Oscilloscope to validate that the voltage across the coil is 5-Volts peak-to-peak. Keep
the DFG switched-on.
8. Place the magnetic rod inside the coil. Hold the rod gently with your finger tips.
Document what happens to the rod. Provide an explanation for the observed response.
9. Release the rod and document what happens. Provide an explanation for the observed
response.
10. Increase the frequency to 20-Hz. Insert the magnetic rod halfway once more. Hold
gently and observe. Release the rod and document what happens to the rod. Provide
an explanation for the observed responses.
11. Remove the magnetic rod and place it adjacent to the coils side and parallel to its
axis. Document what happens to the rod. Provide an explanation for the observed
response.
12. Insert the straight iron rod (has square cross-section) inside the coil and center it.
Place, as before, the magnetic rod to the side of the coil. Document the rods response.
Provide an explanation for the observed response.
Questions
1. For the AC solenoid case, what is the expected response of the iron nail and the rod
if a triangular waveform was used instead of a sinusoidal one? What if a square
waveform of the same frequency was used?
2. Based on the magnetic induction principles observed above, what setup would you
recommend to use so that an object can be suspended in free space? Will it need DC
or AC supply voltage and why?
Measuring the Inductance of Coils
Consider the circuit in Fig.6-3a. It consists of a primary coil connected to a squarewave voltage supply and a secondary coil connected in parallel to a capacitor of known
capacitance C. The circuit can be used to measure the inductance of an unknown coil (the
coil labeled as secondary coil in the figure) as follows: when a square wave voltage of
relatively low frequency is applied to the primary coil, the magnetic field induced by the
excited coil exhibits sudden changes as the voltage is flipped from positive to negative. As
a result, the induced voltage in the secondary coil, whose inductance is L1 , exhibits a free
damped oscillation in the L1C circuit. The frequency of oscillation, fo , can be measured by
6-4
EXPERIMENT
7
(a)
(b)
Figure 6-3: Experimental setup used to measure the inductance of a coil. The primary coil
is connected to a square-wave power supply while the secondary coil (device under test)
is connected in parallel to a known capacitor. The coil and capacitor form an LC resonant
circuit.
an oscilloscope. It is straight forward to show that fo can be related to L1 and C through the
following simple relation:
1
fo = √
(6.1)
2π L1C
The theoretical self-inductance expression for a solenoid (derived using the magnetic field
at the end points of the solenoid) is:
L1 = µ
N2
A
2l
(6.2)
where N is the number of turns in the coil, l is the length of the coil, and A is the cross
sectional area of the coil.
Setup
• Connect the 3200-turn coil to the low impedance port of the digital function generator
(DFG).
• Connect Channel 1 of the Oscilloscope to the DFG. Use 24-inch long BNC cable and
the BNC-to-plug-in adapter.
• Switch-on the DFG. Adjust the frequency to 500 Hz and select the square waveform
for output.
• View the waveform on the Oscilloscope. Set the Time/Div to 200-microseconds per
division. Set Trigger source to be Channel 1 and set the trigger level to 1 Volt. Press
8
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
the Voltage button on the Oscilloscopes front panel and use the soft keys below the
screen to select 1 for source and Vp-p for Voltage measurements. Use the verticalposition knob of Channel 1 to position the signal at 0-Volt level.
• Adjust the amplitude of the DFG to 10 Volts peak-to-peak. Use Channel 1 on the
Oscilloscope to measure voltage. Adjust the Volt/Div knob, as necessary, to display
the square waveform properly.
Procedure
1. Position the 1600-turn coil next to the 3200-turn coil as shown in Fig. 6-3a. Connect
the 10-nF capacitor to the 1600-turn coil. Connect Channel 2 of the Oscilloscope in
parallel with the capacitors terminals.
2. Use the Volt/Div and position knobs of Channel 2 to display the signal properly. The
signal on Channel 2 should exhibit a damped oscillatory behavior.
3. Use the Volt/Div (of Channel 2), Time/Div, and Horizontal Delay knobs as needed in
order to measure the period of the oscillatory signal. Record the period in seconds.
4. Repeat the above three steps by successively replacing the 1600-turn coil with the
800, 400, and 200-turn coils, while keeping the 10-nF capacitor connected as before
in all cases. Measure the period for each coil.
5. Repeat Steps 1 through 3 using the 800-turn coil (keep the 10-nF capacitor connected
as before). Print a hardcopy of the displayed data (input square wave signal and the
LC response). Measure the period of the oscillatory signal. It should be consistent
with the result observed in Step 4.
6. Insert the straight iron core (it has a square cross-section) through both coils and
center it. Use the Volt/Div and position knobs of Channel 2 to display the signal
properly. Print a hardcopy of the displayed data. Measure the period of the oscillatory
signal.
7. Use a ruler and measure the physical dimensions of one of the coils.
Analysis
1. In Step 4 above, compute the resonance frequency and from it compute the inductance
for each of the coils.
2. Compute the theoretical inductance values for all coils under test. Use the physically
measured dimensions in your calculation. Compare with the experimentally
measured values.
3. For steps 5 and 6 above, compute the resonance frequency and use it to compute the
inductance of the 800-turn coil in each case. Compare and explain the results.
Questions
1. What would be the resonance frequency if the setup in Step 6 was slightly modified
such that the straight iron core was inserted only into the 3200-turn coil?
6-4
EXPERIMENT
9
Figure 6-4: In a transformer, the directions of I1 and I2 are such that the flux Φ generated
by one of them is opposite that generated by the other.
2. If, instead of the DFG, you had a different square waveform source with minimum
frequency value of 5-kHz, then what changes you need to make in order for the
experimental setup to operate as before? Explain.
3. Optional:What uncertainties does this measurement procedure have? And what do
you recommend for improvements?
6-4.2 Transformer Basics
The purpose of this part is to familiarize the student with the basics of transformers, such
as the impact on the output voltage and current by the core shape, source frequency, coil
structure, and the ratio of number of turns of the secondary coil to number of turns of the
primary coil on the output voltage and current.
Review of Transformer Basics
The transformer shown in Fig. 6-4 consists of two coils wound around a common magnetic
core. The coil of the primary circuit has N1 turns and that of the secondary circuit has N2
turns. The primary coil is connected to an a-c voltage source V1 (t) and the secondary coil
is connected to a load resistor RL . In an ideal transformer the core has infinite permeability
(µ = ∞), and the magnetic flux is confined within the core. The directions of the currents
flowing in the two coils, I1 and I2 , are defined such that, when I1 and I2 are both positive,
the flux generated by I2 is opposite that generated by I1 .
On the primary side of the transformer, the voltage source V1 generates a current I1
in the primary coil, which establishes a flux Φ in the magnetic core. The flux Φ and the
voltage V1 are related by Faraday’s law:
V1 = −N1
dΦ
,
dt
(6.3)
V2 = −N2
dΦ
.
dt
(6.4)
and, similarly, on the secondary side,
The combination of Eqs. (6.3) and (6.4) gives
V1 N1
=
.
V2 N2
(6.5)
In an ideal lossless transformer, all the instantaneous power supplied by the source
connected to the primary coil is delivered to the load on the secondary side. Thus, no
power is lost in the core, and
P1 = P2 .
(6.6)
10
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Since P1 = I1V1 and P2 = I2V2 , and in view of Eq. (6.5), we have
I1 N2
=
.
I2 N1
(6.7)
Thus, whereas the ratio of the voltages given by Eq. (6.5) is proportional to the
corresponding turns ratio, the ratio of the currents is equal to the inverse of the turns ratio.
The theoretical expressions that define the relationship between the voltage, current, and
power of the primary winding and the voltage, current, and power of the secondary winding
of a transformer are based, in part, on the following important assumptions: (1) the strength
of the magnetic flux density generated by the primary winding is equal to the strength of the
flux density affecting the secondary winding, (2) the wires used in constructing the windings
do not contribute to power loss or energy storage, and (3) the iron core, common to both the
primary and secondary windings, is lossless. In practice, these assumptions are sometimes
violated and a more complicated model for the transformer is used. Experiments that can
be used to characterize an actual, low-performance transformer, is beyond the scope of this
lab. However, it will be shown in this lab that despite the imperfections in the transformers
used, certain important relationships are still maintained.
Effect of Coil Ratio on Voltage and Current
Procedure
1. place the 400-turn coil on the middle leg of the W-shaped core and use it as the
primary coil. Connect the coil’s terminals, in series, to the DFG and an Ammeter. Set
the ammeter to measure AC current and its scale to milli-Ampere. Connect Channel 1
of the Oscilloscope in parallel to the source’s terminals to measure the input voltage,
V1 . Set the DFG to sinusoidal waveform output at 4.6-Vrms and 60-Hz.
2. Place the 200-turn coil on the right leg of the W-shaped core base. Place the upper
W-shaped core back on top of the base. Connect a second Ammeter in series with the
secondary coil and set it to measure AC current and set its scale to milli-Ampere. In
effect, this ammeter is measuring the short circuit current through the secondary coil.
Measure the current through the primary coil, I1 , and through the secondary coil, I2 .
3. Disconnect the ammeter attached to the secondary coil. Connect the coil’s terminals
to Channel 2 of the Oscilloscope. In effect, Channel 2 of the Oscilloscope is
measuring the open-circuit voltage across the secondary coil, V2 .
4. Repeat steps 2 and 3 using the 400, 800, and 1600-turn coils as secondary coils in
place of the 200-turn coil. Measure I1 and I2 using the short circuit configuration of
Step 2 above and V2 using the the open-circuit configuration of Step 3.
5. Cascade on the same core leg different combinations of coils, connected in series, as
shown in Fig. 6-5, in order to create secondary coils with total number of turns equal
to 600 (200 + 400), 1000 (800 + 200), and 1200 (800 + 400). Repeat Steps 2 and 3
above and measure I1 , I2 , and V2 . Make sure that when pairing the coils in series they
are placed on the core leg with their labels facing upward.
Analysis
6-4
EXPERIMENT
Figure 6-5: Setup used to measure the current through secondary coil. Two coils are used
in series to generate the desired number of windings in the secondary coil side (Courtesy of
Pasco Scientific).
1. Compute the ratios I1 /I2 , V2 /V1 , and N2 /N1 using the measured data. Plot I1 /I2 as
a function of N2 /N1 . Also, plot V2 /V1 as a function of N2 /N1 .[Hint: You should
expect a close to linear relationship between both the voltage and current ratios, and
the coils-turn ratios. However, the slopes of the resulting lines might not be equal to
unity.] Derive linear equations for the resulting data. Comment on your results.
2. Use the current and voltage ratios at each coils-turn ratio to compute the following
ratio: (I1 /I2 ) / (V2 /V1 ) and plot this new ratio as a function of N2 /N1 . Comment on
the resulting plot.
Questions
1. Suggest a way to improve the transformer structure used in this experiment so that its
performance approaches the ideal (theoretical) performance.
6-4.3 Motors
The electromagnetic generator is the converse of the electromagnetic motor. The principles
of operation of both instruments may be explained with the help of Fig. 6-6. A permanent
magnet is used to produce a static magnetic field B in the slot between the two poles of the
magnet. When a current is passed through the conducting loop, as depicted in Fig. 6-6(a),
the current flows in opposite directions in segments 1–2 and 3–4 of the loop. The induced
magnetic forces on the two segments are also opposite, resulting in a torque that causes the
loop to rotate about its axis. Thus, in a motor, electrical energy supplied by a voltage source
is converted into mechanical energy in the form of a rotating loop, which can be coupled to
pulleys, gears, or other movable objects.
If, instead of passing a current through the loop to make it turn, the loop is made to
rotate by an external force, the movement of the loop in the magnetic field will produce a
m , as shown in Fig. 6-6(b). Hence, the motor has become a generator, and
motional emf, Vemf
mechanical energy is being converted into electrical energy.
In practice, the windings of an AC generator (or motor) are connected to two slip-rings
located on the generator’s (or motor’s) shaft, as shown in Fig. 6-7. Thin metallic sheets,
called brushes, touch the slip-rings and connect the generator (or motor) to the outside
circuitry. If DC mode of operation is desired, then a different commutator system is used.
In this system, the two slip-rings are replaced by a split-ring. One side of the winding on
11
12
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Figure 6-6: Principles of the a-c motor and the a-c generator. In (a) the magnetic torque on
the wires causes the loop to rotate, and in (b) the rotating loop generates an emf.
the shaft is connected to one of the split-ring halves while the other side of the winding
is connected to the second half of the split-ring. Two brushes are also used, but they are
positioned at opposite locations around the shaft. This way, the two brushes can never touch
simultaneously the same half of the split-ring. The two brushes will alternate in touching
the two halves as the windings rotate; hence creating a rectification effect on an otherwise
AC-generated current. This commutator system works also for DC-motors.
Over the years, there have many been variations to the simple generator-motor structure
described above resulting in a variety of design parameters, models, etc. Variations to the
simple structure include, among other things, the shape and number of permanent magnets
used, the number of windings on the armature, and in the case of DC mode the number of
sections in the split-ring used. Examination of the different designs is beyond the scope
of this experiment. However, the general formulas that describe the relationship between
speed, electromotive force, number of windings, etc. are given as follows:
For the DC case, the induced emf between the brushes can be expressed as:
Vem f =
2 φ P ω N
2 π a
(6.8)
where Vem f is the induced emf, φ is the flux per pole in webers, P is the number of poles, N
is the total number of turns on the armature, a is the number of parallel armature paths, and
ω is the angular velocity in radians/second.
6-4
EXPERIMENT
13
Figure 6-7: A loop rotating in a magnetic field induces an emf. AC current is coupled to
outside circuitry through a system of slip-rings and brushes.
For the AC case, the induced emf between the brushes can be expressed as:
Vem f = A ω N B0 sin(ω t)
(6.9)
where Vem f is the induced emf, A is the surface area of the windings, B0 is the magnetic
field due to the magnet, N is the total number of turns on the armature, and ω is the angular
velocity in radians/second.
Description of Motor Used in the Experiment
The AC/DC motor assembly to be used in this experiment is shown in Fig. 6-8. It is often
called the universal motor due to its capacity to operate as an AC and as a DC motor. The
main components comprising the experimental motor system are: the shaft, the armature,
the permanent magnet, and brushes. The shaft is used to hold the armature in place while it
rotates. The armature consists in this system of two coils wound around an iron rod, a split
ring commutator on one side (used for DC motor operation) and a dual slip-ring commutator
on the other side (used for AC motor operation). The N- and S-poles of the permanent
magnet are held in place around the armature using the U-shaped wooden structure, as
shown in Fig. 6-8. The two brushes connect the motor armature, via the commutator, to the
power source, whether it is DC or AC.
Handling and Initial Assembly of the Motor:
The motor assembly should be for the most part assembled and ready for use. An important
task that the student will have to do is to set the motor to operate in either DC or AC
mode. When operating in DC mode, the split ring commutator should be down, and when
operating in AC mode, the dual slip-ring commutator should be down. If the armature is
already in place over the shaft but the wrong type commutator is touching the brushes, then
the student should remove the motor and place the correct commutator type down. To do
so: first unscrew the retaining nut and remove it off of the shaft. Then, carefully hold the
upper portion of the armature (above the coils section) with your finger tips. Gently rotate
the armature back and forth and be careful not to bend the brushes. Once the armature is
free from the brushes, pull it out off of the shaft, flip it to the desired position and carefully
14
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Figure 6-8: Various components comprising the experimental, permanent magnet AC/DC
motor assembly (Courtesy of Pasco Scientific)
place it on the shaft. Notice that the weight of the armature and the pulling forces of the
permanent magnet might result in the armature slipping away and plunging down on the
brushes. Hold tightly and gently lower the armature. Rotate the armature back and forth
to separate the brushes and allow the commutator to slip down between them. If need be,
insert a pencil between the brushes to momentarily separate them and allow the armature
through. Screw back the retaining nut over the shaft.
Starting the Motor:
• The motor is not self starting. Once you prepare it for DC or AC operation and apply
power to it, then you need to start the motor manually by grasping the black plastic
bushing at the top of the armature (above the coils) using your fingertips and spinning
the armature.
• In case of DC operation, only the direction of spin is important. Pay close attention
to the direction of winding of the coils on the armature, which brush is connected
to the positive terminal of the power supply, and the direction of the magnetic field
generated by the permanent magnet in order to figure out in which direction to spin
the armature. Use the small magnet (small cylindrical magnet with its N-pole painted
in white) to determine which side of the permanent magnet in the motor assembly is
N-pole.
• When the AC synchronous mode is used, the motor must be spun at a speed that
approximately matches the frequency of the power source. Experience has shown
that a source frequency 10 to 15-Hz is practical for starting the motor manually.
Once the motor is in motion, the source frequency can be changed and the motor will
continue to spin.
Safety Precautions:
6-4
EXPERIMENT
• Keep fingers and loose cloth items away from the spinning armature.
• Limit current to no more than 1-A. The motor may overheat if this current is exceeded
or if power is applied continuously, especially if the armature is not rotating.
• Disconnect the power source whenever the motor is to be left unattended or is not in
active use.
DC Motors
The purpose of this experiment is to demonstrate the operation of the DC motor and explore
the relationship between motor speed and voltage, and between the direction of rotation and
polarity. A key feature of the DC motor is its split-ring commutator.
Setup
• Make sure that the split ring commutator is down. Follow the procedure outlined at
the beginning of this section on how to remove and install the armature.
Procedure
1. Use the small magnet to determine which side of the permanent magnet in the motor
setup is its N-pole. Note that the white-painted side of the small magnet is the Npole. Connect the DC supply to the motor terminals according to the layout shown
in Fig. 6-9. Set the voltage on the DC supply to 2.50-V. Record the displayed current
value. Document which brush was connected to the positive terminal.
2. With your finger tips holding the upper section of the armatures shaft (above the
windings) spin the armature manually. Record the direction of rotation of the
armature after it spins continuously on its own. Record the displayed current and
voltage values.
3. Switch-off the supply. Swap the wires at the supplys terminal. Switch-on the DC
supply. Make sure that the voltage is 2.5-V. Record the current while the motor is
stationary. Then manually spin the motor. Record its direction of rotation, current
drawn, and voltage.
4. Switch-on the tachometer and select 2 as the number of blades. Position the
tachometer about 1/2 to 1 inch away from the rotating coils. Make sure that the
tachometer is kept horizontally and parallel to the desk plane. Record the displayed
value by the tachometer. To convert the value to revolutions per minute (RPM),
multiply the number by 10. To convert the speed displayed by the tachometer to
revolutions per second (RPS), divide the displayed number by 6.
5. Connect the oscilloscope to motor power terminals. Adjust the Volt/Div and
Time/Div so you can see the waveform of voltage while the motor is spinning. Use the
Time-measuring knob and cursors to determine its frequency of every two periods.
6. Adjust the DC supply voltage to 2.2-V while the motor is spinning. Then increase
the voltage value at increments of 0.2-V until you reach 3.0-V. At each voltage value,
record the drawn current and RPS by tachometer and frequncy from oscilloscope.
Analysis
15
16
LAB EXERCISE 6: INDUCTION, TRANSFORMER, AND MOTOR PRINCIPLES
Figure 6-9: Connecting the motor assembly to the power supply (Courtesy of Pasco
Scientific).
1. Compare between the current drawn in steps 2 and 3 above.
2. Using the principles of electromagnetic induction, justify qualitatively, the directions
of rotation of the motor observed in Steps 2 and 3 above. Look closely at the motor
windings.
3. Plot both the current and the RPS, measured in step 6 above, as a function of DC
voltage. Analyze the resulting graphs and determine the relationships between speed
and voltage and between current and voltage.
Questions
1. The motor must be spun manually for it to start rotating on its own. This is not how
normally DC motors operate. Discuss the reasons for needing a jumpstart and what
can be done to make the motor start on its own.
2. If you were to load the armature, by touching the upper section of the armature with
your fingertips or having it connected to a gear assembly, then what would happen to
voltage, speed, and current.
AC Motor
The purpose of this experiment is to demonstrate the operation of the AC synchronous
motor and explore the relationship between motor-speed, voltage, and current frequency.
I. Observations:
Setup
• Make sure that the dual slip-ring commutator is down. Follow the procedure outlined
at the beginning of this section on how to remove and install the armature.
Procedure
1. Connect in series the motor, the Ammeter, and the DFG. Use Channel 1 of the
Oscilloscope to measure the rms voltage across the DFG terminals. Switch-on the
Ammeter and set it to measure AC current. Set its scale to 10-A.
2. Switch-on the DFG, select the sinusoidal waveform, set its frequency to 30-Hz and
its amplitude to 6-Vrms. Make sure that the drawn current does not exceed 1.0-A.
6-4
EXPERIMENT
3. With your finger tips, carefully attempt to rotate slowly the armature to 45o in
azimuth. Describe your observations.
4. Reduce the DFG amplitude to 3-Vrms while maintaining its frequency at 30-Hz.
Redo Step 3.
5. Change the frequency to 10-Hz and the amplitude to 6-Vrms. Redo Step 3.
II. Parameters Governing AC-Motor Speed:
Procedure
6. Set the frequency to 10 Hz and the amplitude to 6-Vrms. With your finger tips holding
the armatures shaft (above the windings), spin the armature in clockwise direction.
The motor should begin spinning. If not, try few more times before asking the lab
instructor for help. Record the RPS.
7. Slowly lower the amplitude to 3-Vrms. If the motor stops, then jumpstart it manually
once again. Record the RPS.
8. Set amplitude to 6-Vrms. Then increase the frequency from 10-Hz to 22-Hz in 2-Hz
increments and record RPS.
Analysis
1. Discuss your observations in steps 3, 4, and 5.
2. Compare between the RPS measured in Step 6 and Step 7. Is RPS sensitive to the
amplitude of the input voltage?
3. Plot the RPS in step 8 as a function of signal frequency. Discuss the dependence of
RPS on signal frequency.
17