Download 213Lab11-transformer

Phy 213: General Physics III Lab
Instructor: Tony Zable
1
Experiment: Transformers
A transformer is a device that changes the AC voltage and current. It works based on the
electromagnetic induction principles, and consists of 2 coils: the primary coil and the
secondary coil.
Primary coil is connected to an AC power supply. The coil has a ferromagnetic core that
enhances the magnetic field generated by the AC current. Since the AC current
periodically changes its direction, the magnetic field will also change its direction
following a sinusoidal equation. The changing magnetic flux generates induced emfs in
the secondary and the primary coils
Secondary coil is also wound around a ferromagnetic core and it is placed inside the
magnetic field generated by the primary coil (usually on top of the primary, on the same
ferromagnetic coil). The changing magnetic field of the primary coil induces an
electromotive force in the secondary coil, according to Faraday’s Law:
Esec ondary  N sec ondary
 B
t
where Nsecondary is the number of loops in the secondary coil (the changing magnetic field
is produced by the primary coil). The magnetic flux is the same for both coils, so the emf
induced in the primary is
E primary  N primary
 B
t
.
Since the resistance of the coils is almost zero, the equation of the transformer becomes
E primary
Esec ondary

N sec ondary
N primary
If Nsecondary>Nprimary, then Esecondary>Eprimary, this is called a step-up transformer.
If Nsecondary<Nprimary, then Esecondary<Eprimary, this is called a step-down transformer.
For high quality transformers, the resistance is small and the induced emf is
approximately equal to the terminal voltage V (Eprimary ~ Vprimary & Esecondary ~ Vsecondary), or
Vprimary
Vsec ondary

Nsec ondary
N primary
Since the total energy is conserved, the electrical energy in the secondary has to be
equal to the electrical energy in the primary. In the ideal case,
Pprimary=Psecondary
or,
VsecondaryIsecondary=VprimaryIprimary.
Therefore,
Vsecondary/Vprimary=Iprimary/Isecondary.
Note: the ratio of the currents is inverse to the ratio of the voltages.
PCC-Cascade Campus
Department of Medical Technology & Science
Phy 213: General Physics III Lab
Instructor: Tony Zable
2
It also should be noted that some energy is lost as heat in the coils and inside the
ferromagnetic core. The efficiency of the transformer shows how much of the input
energy (primary) is transferred to the secondary coil

Psec ondary
Pprimary

Vsec ondaryI sec ondary
V primaryI primary
OBJECTIVES
 Build a step-down transformer
Measure experimental values for current and voltage in the primary and secondary
coil
 Find the transformer ratio using the slope of the Vsecomdary versus Vprimary graph
Calculate and compare the efficiencies of the transformer with the ferromagnetic
core and without the ferromagnetic core.
MATERIALS




AC adjustable power supply
Digital multimeter
Magnetic wire
switch



Ferromagnetic core
switch
connecting wires
PRELIMINARY QUESTIONS
1. A step-down transformer is labeled “Primary 120V, Secondary 12.6 V”. What is the
“turns ratio” for this transformer?
2. Sketch below the graph Vprimary vs. Vsecondary obtained by applying to the primary an
increasing AC voltage (4,6,8,10,12 V respectively). What is the significance of the
slope of this graph?
PROCEDURE
1. Build the transformer primary coil by winding 150 turns of wire around the
ferromagnetic core; cover the primary coil with tape (insulator), leaving the ends
accessible for connecting to the power supply. On the same core, on top of the
primary coil, build the secondary coil by winding 50 turns of wire.
PCC-Cascade Campus
Department of Medical Technology & Science
Phy 213: General Physics III Lab
Instructor: Tony Zable
3
2. Without plugging in the power supply, connect the primary coil to the AC Power
supply.
3. Prepare the Digital Multimeter: plug the black lead into the COM terminal of
the Multimeter and the red lead into the V terminal. Put the switch in the “AC”
position. Turn the multimeter on by setting it to the 20V scale
4. Have your instructor check your circuit and then turn on the power supply. Adjust
the power supply to 5V and measure the terminal voltages across the primary and
secondary coils. Record your data in the table below.
5. Disconnect the multimeter and turn it off. To measure the currents, the
multimeter has to be connected in series in the primary circuit and then in the
secondary circuit. Move the red lead to the 10A terminal of the multimeter. Switch
to the 10A scale. Connect the multimeter in series in the primary circuit and
measure the current Iprimary. Connect the multimeter to the secondary coil and
measure the current Isecondary. Record the values in the table below.
6. Repeat the voltage measurements (not the current measurements!) for 4 more
values of the primary voltage. Record values in Table 2.
7. Using Graphical Analysis, plot the graph Vsecondary vs. Vprimary. Is the graph linear?
Using the Linear fit feature, find the linear fit equation.
Slope = ________±_______
Y-intercept = ________±_______
8. Cut and paste the graph into Word and print the graph.
9. Remove the ferromagnetic core and repeat step 5 (the measurements of voltage
and current for the primary voltage V=5V). Record your data below in Table 1.
DATA TABLE 1:
(For Vprimary = 5V, record the measurements with and without the ferromagnetic core)
With ferromagnetic core :
Without ferromagnetic core :
Vprimary
Vprimary
5V
5V
Vsecondary
Vsecondary
Iprimary
Iprimary
Isecondary
Isecondary
Pprimary
Pprimary
Psecondary
Psecondary
Efficiency ()
Efficiency ()
PCC-Cascade Campus
Department of Medical Technology & Science
Phy 213: General Physics III Lab
Instructor: Tony Zable
4
DATA TABLE 2:
Trial
Vprimary (V)
Vsecondary (V)
ANALYSIS
1. What is the efficiency of the transformer with and without the ferromagnetic core?
Record your results in Data Table 1 (above).
2. What can you conclude from the efficiency values in (1)?
3. What is the significance of the slope of the Vsecondary vs Vprimary graph? Compare to the
theoretical value Nprimary/Nsecondary.
4. The step-down transformer you constructed lowered the voltage. How does the
secondary current compare to the primary current? Is this consistent with the theory?
Explain.
PCC-Cascade Campus
Department of Medical Technology & Science