Experiment 6d
Class:
Name:
(
) Date:
6d Measuring the voltage ratio of a
transformer
Objective
To measure the voltage ratio of a transformer and compare it with
the turns ratio using a CRO and voltage sensors.
Background information
1
When an a.c. flows through the primary coil of a transformer, the
magnetic field lines going round the core change continuously. This
induces an alternating voltage in the secondary coil which has the
same frequency as the a.c. in the primary coil.
2
The length of the vertical trace on a cathode-ray oscilloscope (CRO)
screen is directly proportional to the voltage it measures.
Apparatus
❏ 1 pair of double C-cores with
a clip
❏ 1 power pack (0–12 V a.c./d.c.)
❏ 2 lamps (2.5 V)
❏ 1 coil of 10 turns
❏ 1 cathode-ray oscilloscope (CRO)
❏ 1 coil of 25 turns
❏ several connecting leads
Procedure
126
1
Set up the apparatus as shown in Figure 6d-1 on p.127:
(a) Construct a transformer with a primary coil of 10 turns and a
secondary coil of 25 turns around two C-cores clipped together.
(b) Connect a 2.5 V lamp each to the primary and secondary coils.
(c) Apply an a.c. voltage of 1 V from an a.c. power supply across
the primary coil. Compare the brightness of the two lamps.
New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
) Date:
Experiment 6d
power pack
2.5 V lamp
2.5 V lamp
secondary coil
(25 turns)
primary coil
(10 turns)
cathode-ray oscilloscope
(CRO) with time base switched off
Fig 6d-1
✎
brighter
The lamp in the secondary coil is __________________
(brighter/dimmer)
than the lamp in the primary coil. This shows that the voltage in the
higher
(higher/lower) than the voltage in
secondary coil is __________________
the primary coil.
Note
If a dual-trace CRO is
used, switch on the time
base. Connect channel 1
across the primary coil
and channel 2 across
the secondary coil to
display the primary and
secondary waveforms at
the same time.
✐ The specimen results
were obtained with
Y-gain setting = 1 V/cm.
2
(a) Connect a CRO across the primary coil. Switch off the time base
of the CRO.
(b) Measure the length of the vertical trace on the CRO screen.
3
Connect the CRO across the secondary coil and measure the length
of the vertical trace on the CRO screen.
4
Record the results below and calculate the voltage ratio of the
transformer.
✎
Results:
Turns ratio of transformer
Length of vertical trace (primary coil)
3.0
lp = _____________
cm
Length of vertical trace (secondary coil)
7.0
ls = _____________
cm
Voltage ratio of transformer
New Physics at Work (Second Edition)
Np
0.4
= _____________
Ns
Vp
l
0.43
= p = _____________
Vs
ls
© Oxford University Press 2007
127
Experiment 6d
Class:
Name:
(
) Date:
Alternative set-up using voltage sensors
Apparatus
❏ 1 pair of double C-cores with
❏ 2 voltage sensors
a clip
❏ 1 data-logger interface
❏ 1 coil of 10 turns
❏ 1 computer with data-logging
❏ 1 coil of 25 turns
program installed
❏ 1 power pack (0–12 V a.c./d.c.)
❏ several connecting leads
❏ 2 lamps (2.5 V)
Procedure
1
Set up the apparatus as shown in Figure 6d-2:
(a) Connect two voltage sensors to a data-logger interface and then
connect the data-logger interface to a computer.
(b) Construct a transformer with a primary coil of 10 turns and a
secondary coil of 25 turns around two C-cores clipped together.
(c) Connect a 2.5 V lamp each to the primary and secondary coils.
(d) Connect one voltage sensor each to the primary and secondary
coils.
(e) Apply an a.c. voltage of 1 V from an a.c. power supply across
the primary coil. Compare the brightness of the two lamps.
monitor
scope
power pack
scope
data-logger
interface
A
1
2
3
B
C
4
voltage
sensor
voltage
sensor
2.5 V lamp
2.5 V lamp
secondary coil
(25 turns)
primary coil
(10 turns)
Fig 6d-2
128
New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
) Date:
Experiment 6d
brighter
The lamp in the secondary coil is _________________
(brighter/dimmer)
than the lamp in the primary coil. This shows that the voltage in the
✎
higher
(higher/lower) than the voltage in
secondary coil is __________________
the primary coil.
2
(a) Run the data-logging program on the computer and display the
voltage recorded in real time by the two voltage sensors.
(b) Measure the peak value of the voltages in the primary and
secondary coils.
3
Record the results and calculate the voltage ratio of the transformer.
✎
Results:
Turns ratio of transformer
Voltage in primary coil
Voltage in secondary coil
Voltage ratio of transformer
Np
0.4
= ______________
Ns
1.0
Vp = ______________
V
2.0
Vs = ______________
V
Vp
0.5
= ______________
Vs
Discussion
Compare the voltage ratio with the turns ratio of the transformer.
Account for any difference between the two ratios.
✎
The voltage ratio is slightly larger than the turns ratio. This is because not all magnetic
field lines pass through the secondary coil.
What will happen to the two lamps if this experiment is repeated with
a d.c. voltage of 1 V across the primary coil? Explain your answer.
✎
Only the lamp connected to the primary coil is on. This is because no current can be
induced in the secondary coil if the current in the primary coil is a steady d.c.
ratio of the voltages
1 The ___________________________________
in the primary and
secondary coils of a transformer depend on the number of
turns in the coils.
equal to
2 The voltage ratio of a transformer is ________________
its turns
ratio, i.e.
New Physics at Work (Second Edition)
primary voltage =
secondary voltage
number of turns in primary coil
number of turns in secondary coil
© Oxford University Press 2007
129