Transformers
PES 216 Advanced Physics Lab II
Purpose of the experiment
•
What is a transformer and what practical use does it have?
•
Discover the basic principles of a transformer.
•
FYI
FYI
The number of possible ways of playing the first four moves per side in a game of chess is
318,979,564,000.
Transformers - 1
Table of Contents
Background
3
Lab Procedure
5
Equipment List
Banana cables
Pasco Basic Coil Set
Variable AC Power Supply
Digital Multi-meter
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Background
Transformer
A transformer is a device that transfers energy
from one electrical circuit to another by
magnetic coupling. A transformer comprises of
at least two coupled windings, and usually a
magnetic core to concentrate magnetic flux. A
voltage applied to one winding creates a timevarying magnetic flux in the core, which
induces a voltage in the other windings. Varying
Three-phase pole-mounted stepdown transformer.
the relative number of turns in the windings determines the ratio of their voltages, thus
transforming the voltage from one circuit to another.
The transformer principle was demonstrated in 1831 by Faraday, though practical designs
did not appear until the 1880s. Within less than a decade, the transformer was
instrumental during the "War of Currents" in seeing alternating current systems triumph
over their direct current counterparts, a position that has remained dominant. The
transformer permits the economic transmission of power over long distances. Amongst
the simplest of electrical machines, the transformer is also one of the most efficient, with
large units attaining effiencies of 99.75%. Transformers come in a range of sizes from a
thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawattrated units used to interconnect portions of national power grids. All operate with the
same basic principles.
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Basic principles:
Coupling by mutual induction
To help understand the basic
principles of the transformer
let’s look at the ideal case.
The hypothetical ideal
transformer consists of two
windings of zero resistance
around a core of negligible
reluctance (no loss of
magnetic energy). A voltage
applied to the primary
An ideal step-down transformer showing magnetic
flux in the core
winding causes a current,
which develops a
magnetomotive force (MMF) in the core. The current required to create the MMF is
termed the magnetising current; in the ideal transformer it is considered to be negligible.
The MMF drives flux around the the core. An electromotive force (EMF) is induced
across each winding, an effect known as mutual inductance. The windings in the ideal
transformer have no resistance and so the EMFs are equal in magnitude to the measured
terminal voltages. In accordance with Faraday's law of induction, they are proportional to
the rate of change of flux:
and
where:
and
and
and
are the induced EMFs across primary and secondary windings,
are the numbers of turns in the primary and secondary windings,
are the time derivatives of the flux linking the primary and secondary
windings.
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In the ideal transformer, all flux produced by the primary winding also links the
secondary, and so
, from which the well-known transformer equation
follows:
The ratio of primary to secondary voltage is therefore the same as the ratio of the number
of turns.
Procedure
The following is a picture of the experimental setup:
AC volt
meter
Iron Core
Primary
Coil
Secondary
Coil
AC power supply
Figure 1: Experimental Setup
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Part A: Transformer Basics
1. Set up the coils and core as shown in Figure 1. Typically the coil to which the input
voltage is applied is referred to as the Primary Coil and the coil which supplies the
output voltage is the Secondary Coil. Use a 400-turn coil as the primary and the other
400-turn coil as the secondary. To attach coils within the iron core loosen the bolt on
the core until the iron cross bar can be removed.
2. Connect the AC power supply to the primary and adjust the input voltage to 6 volts
ac, using the meter to monitor the voltage. Note: make sure the meter is setup to read
AC volts. The frequency is not critical 100 Hz (the default value displayed on the AC
supply when first turned on) will be fine.
3. Now that the input voltage has been set move the AC volt meter to the secondary coil.
Measure the ac output voltage and record your results in Table 1.
4. Turn off the ac power supply and replace the 400-turn secondary with the 200-turn
coil. Measure the ac output voltage and record your results in Table 1.
5. Repeat step (3) with the 800-turn coil as the secondary.
6. Calculate the Turns Ratio and ac voltage ratio in Table 1.
Primary
Turns, N
p
Secondary
Turns, N
s
Turns
Ratio
N /N
s
p
Primary
ac
Voltage,
V
p
400
400
400
•
Secondary ac
Voltage,
V
s
ac
Voltage
Ratio,
V /V
s
p
200
400
800
What does Table 1 indicate about the relationship between the ac Voltage Ratio and
the Turns Ratio?
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•
Using Graphical Analysis, make a graph of the V /V (on y-axis) vs N / N (on xs
p
s
p
axis).
•
Generate a best-fit straight line which fits your data. The slope of this graph for an
“Ideal Transformer” should be 1. How close to ideal did you come?
•
Calculate the percent difference between your transformer and the ideal case. What
could have caused the discrepancy?
Part B: Effect of Transformer Cores
Now that we have seen how a transform works let’s now examine how the induction
between the coils is depended on the material and shape of the core.
1. Using air as the medium between the two coils, place two 400-turn coil face-to-face
as shown in the diagram below. Apply the same 6 VAC to the primary coil and
measure the voltage output on the secondary coil.
2. To improve the mutual induction, an iron core can be introduced to focus the
magnetic field into the secondary coil. Using the cross piece from the U-shaped core,
the induced voltage is increased. Measure this increase using the same setup as
above.
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3. Numerous modifications of the cores can be investigated. In each case record the
ratio of secondary voltage to primary voltage.
4. In the above experiments the input voltage and the number of coil turns where held
constant and the configuration of the core was changed. Explain the outcome of each
case, include the ratio of secondary voltage to primary voltage as evidence of your
conclusions.
Transformers - 8