Electrical Machines
Module 1: Transformer Types and
Applications
PREPARED BY
IAT Curriculum Unit
April 2012
© Institute of Applied Technology, 2012
ATE1230: Electrical Machines
Module 1: Transformer Types and
Applications
Module Objectives
Upon successful completion of this module, students should be able to:
1. Describe the function of the transformer and give its application.
2. Describe the operation of transformer and give its schematic
symbol.
3. Describe how to calculate the output voltage of a transformer.
4. Describe how to calculate the efficiency using input and output
powers.
5. Discuss the basic categories of the transformers.
6. Discuss the function of the control transformer.
7. Design a control transformer circuit to provide a given output
voltage.
Module Contents:
Topic
Page No.
1.1
Transformer Construction and Principle of Operation
1.2
Transformer Types
10
1.3
Control Transformers
12
1.4
Lab Activity 1
16
1.5
Lab Activity 2
19
1.6
Lab Activity 3
23
1.7
Lab Activity 4
26
1.8
Review Exercises
32
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1.1
Transformer Construction and Principle of Operation
Definition: A Transformer is an electrical device that converts AC
electricity from one voltage level to another. A typical small size
transformer is shown in Figure 1.1.
Figure 1.1: A typical Transformer
.
Operation of Transformer and its Schematic Symbol
A Transformer consists of three basic components:
Primary Coil- An electrical wire wrapped around the core, as shown in
Figure 1.2.
Secondary Coil- An electrical wire wrapped around the core, as shown in
Figure 1.2.
Core- A ferromagnetic material that is capable of conducting a magnetic
field. Iron is a common material.
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Figure 1.2: Construction of a Typical Transformer
A transformer uses the principle of mutual inductance to create an AC
electrical voltage in the secondary coil from the alternating electrical
current flowing through the primary coil.
The principle of mutual inductance says that when two electrical coils are
placed near to each other, AC electrical current flowing in one coil induces
an AC voltage in the other coil. This occurs because electrical current
creates a magnetic field around the first coil which in turn induces a
voltage in second coil, as shown in Figure 1.3.
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Figure 1.3: Mutual Inductance Concepts
The transformer improves the efficiency of the transfer of energy from
one coil to another by using a core to concentrate the magnetic field. As
shown in Figure 1.4, the primary coil creates a magnetic field that is
concentrated by the core and induces a voltage in the secondary coil. The
voltage induced in the secondary coil can be used to drive a load.
Figure 1.4: Operation of a Transformer
What makes the transformer useful is its ability to make the voltage at
the secondary coil different from the voltage at the primary. This occurs
when there is a difference between the numbers of times each of the coils
is wound around the core.
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Figure 1.5 shows the schematic symbol of a transformer.
Figure 1.5: Schematic Symbol of a Transformer
Conduct Lab Activity 1 on Page Number 16
1.1.1 Output Voltage of a Transformer
To determine the voltage output generated in the secondary coil of a
transformer we only need to know two things: the input voltage and the
turns ratio.
The turn’s ratio is the number of turns (times the wire is looped around
the core) in the primary coil compared to the number of turns in the
secondary coil. This ratio is determined by dividing the number of turns
in the primary by the number of turns in the secondary as follows:
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The voltage output of a transformer is then calculated using the
following formula:
This is an important relationship because it allows us to decide, which
transformer to select for an application and troubleshoot one to decide if
it is good.
Examples :
1. A transformer has a primary voltage of 230v and turns ratio of 5:1.
Calculate the secondary voltage .
VP / VS = TP / TS
230/ VS = 5 /1
VS = 46 V
2. A transformer has 200 turns in the primary, 50 turns in the
secondary, and 120 volts applied to the primary (Vp). What is the
voltage across the secondary (V s)?
VP / VS = TP / TS
120/ VS = 200/50
VS = 120 X 50 / 200 = 30 V
3. There are 400 turns of wire in an iron-core coil. If this coil is to be
used as the primary of a transformer, how many turns must be
wound on the coil to form the secondary winding of the transformer
to have a secondary voltage of one volt if the primary voltage is five
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volts?
VP / VS = TP / TS
5 / 1 = 400 / TS
TS = 1 x 400 /5 = 80 turns
4. A 12 volts transformer has 20 turns in the primary, 5 turns in the
secondary. What is the voltage across the primary side (VP)?
VP / VS = TP / TS
VP / 12 = 20/ 5
VP = 12 x 20 / 5 = 48 V
Conduct Lab Activity 2 on Page Number 19
1.1.2 Input and Output Power of a Transformer
Ideally, the output power of a transformer should be the same as
the input power. However, because of the construction of a transformer,
there is usually some amount of power loss between the primary and the
secondary. The “Power In” equals the “Power Out” plus any “Losses”, as
shown in Figure 1.6.
This power lost by a transformer is actually converted into heat, which
must be dissipated by the transformer. Large transformers used by utility
company substations actually have cooling systems that use pumps to
circulate oil around the transformer core to help dissipate this heat.
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Figure 1.6: Power through a Transformer
The heat generated by a transformer can be found by calculating the
efficiency of the transformer. This can be calculated with the following
formula:
Conduct Lab Activity 3 on Page Number 23
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1.2
Transformer Types
Most transformers fall into one of two categories:

Isolation transformers

Autotransformers
Isolation Transformers
An isolation transformer, such as the one shown in Figure 1.7, has
primary and secondary windings that are physically and electrically
isolated. The isolation transformer is magnetically coupled, not electrically
coupled.
Figure 1.7: An Isolation Transformer Schematic
This characteristic is very important. Since there is no electrical
connection between the primary and secondary, the transformer is
basically a filter between the two. Voltage spikes that might occur on the
primary are greatly reduced or eliminated in the secondary. If the primary
is shorted somehow, any load connected to the secondary is not
damaged. Most electrical devices or machines that operate on AC use an
isolation transformer to provide the desired operating voltage, as well as
protection. Some isolation transformers produce an output voltage that is
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equal to the primary voltage. Traditional TV monitors make use of this
type of isolation transformer to protect the picture tube from voltage
spikes on the main power lines.
Autotransformers
An autotransformer as shown in Figure 1.8 uses only one coil for the
primary and secondary. It uses special connections on the coil called taps
to produce the different ratios and voltages.
Autotransformers are used frequently by power companies to provide
voltage regulation to large power lines by providing a small increase or
decrease to the line voltage as required.
Figure 1.8: Single phase tapped Autotransformer Transformer
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1.3
The Control Transformer
A control transformer is a type of transformer that is often used to reduce
voltage from the main power line in the plant to a lower voltage that
operates a machine’s electrical control system. The most common type of
control transformer has two primary coils and one secondary coil, as
shown in Figure 1.9.
Figure 1.9: A Typical Control Transformer Configuration
In most cases, the control transformer is used to reduce the main or line
voltage of 240 VAC or 480 VAC to a control voltage of 120 VAC. Notice
that the primary windings are crossed. This is done so that metal links
can be used to connect the primaries for either 240 or 480 VAC operation,
as shown in Figure 1.10.
To get a control voltage of 120 VAC from a line voltage of 240 VAC, the
primaries are connected differently than if the line voltage is 480 VAC. If
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the supplied line voltage is 240 VAC, the two primaries must be
connected in parallel, as shown in Figure 1.10.
Figure 1.10: Connecting a Control Transformer Primary for 240 VAC
Operation
Figure 1.11 is actually the same as Figure 1.10, except that it has been
redrawn to allow you to more easily see that the primary coils are
connected in parallel.
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Figure 1.11: Primaries Connected in Parallel
If the supplied line voltage is 480 VAC, the primaries are connected in
series, as shown in Figure 1.12.
Figure 1.12: Connecting a Control Transformer Primary for 480 VAC
Operation.
Figure 1.13 is actually the same as Figure 1.12, except that it has been
redrawn to allow you to more easily see that the primary coils are
connected in series.
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Figure 1.13: Primaries Connected in Series
The reason the primaries are connected in either parallel or series is to
create a different turns ratio. To understand this, let’s assume that each
primary has 200 turns and the secondary has 100 turns.
If these coils are connected in parallel, as shown in Figures 1.10 and
1.11, the effective turns of the two primaries is still 200 turns, the same
as if there was only one primary. If the secondary is 100 turns, the turns
ratio is then 2:1. This means an input voltage of 240 VAC will create an
output voltage of 120 VAC.
In contrast, if the primary coils are connected in series, as shown in
Figures 1.12
and 1.13, the effective turns of the two primary coils in
series will be 400, making the turns ratio 4:1. This would cause an input
voltage of 480 VAC to create an output voltage of 120 VAC.
Conduct Lab Activity 4 on Page Number 26
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1.4
Lab Activity 1
Objective: To Connect and Operate a Transformer.
Procedure:
In
this
procedure,
we
will
connect
and
operate
a
transformer. Then we will use a DMM to show that a voltage is induced in
the secondary.
1. Perform
the
following
sub steps
to
connect
and
supply
power
to the transformer.
A. Connect the transformer to the power supply as shown in Figures
1.14
and
1.15.
This
transformer
has
two
primaries
and
two
secondaries. In this application, we will connect the two primary
windings in parallel.
B. Place the AC/DC selector switch in the AC position.
C. Turn on the T7017 power supply.
Figure 1.14: Operating a Transformer
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Figure 1.15: Schematic of Transformer Connections
2. Set the DMM to measure AC volts and measure
the voltage
across the primary, as shown in Figure 1.16.
Figure 1.16: Measurement of Primary Voltage
Primary voltage = ___________________________ (VAC)
3. Now measure the voltage across one of the secondaries, as shown in
Figure 1.17.
Secondary Voltage = _________________________ (VAC)
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Figure 1.17: Measurement of Secondary Voltage
4. Measure the voltage across the other secondary.
Secondary voltage = _________________________
(VAC).
It should be similar to the value in step 3.
5. Turn off the power supply.
6. Disconnect the circuit.
7. Store all components.
8. Summarise your findings
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1.5
Lab Activity 2
Objective:
To
Calculate
the
Secondary
Coil
Voltage
of
a
Transformer.
Procedure: In this procedure, we will determine the voltage output of
each secondary coil given the number of turns of the primary, the
number of turns of each secondary and the primary coil voltage.
1. Perform the following sub steps to determine the secondary voltage
of the transformer shown in Figure 1.18.
A. First, calculate the turn’s ratio of the transformer.
TR = ________________________________
Figure 1.18: Calculating Secondary Voltage
B. Next, calculate the secondary voltage of the transformer.
The schematic shows that the input voltage is 120 VAC.
VS ________________________________
(VAC)
2. Calculate the turns ratio and the secondary voltage of the transformer
shown in Figure 1.19.
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TR _______________________________
VS ________________________________
(VAC)
Figure 1.19: Transformer Circuit
3. Calculate the turns ratio and the secondary voltage for the circuit
shown in Figure 1.19, if the primary has 400 turns and the
secondary has 100 turns.
TR _______________________________
VS ________________________________
(VAC)
4. Calculate the secondary voltage for the circuit shown in Figure 1.19
and described in step 3 if the input voltage is 480 VAC.
VS ________________________________
(VAC)
A transformer can also have more than one secondary coil, as shown
in Figure 1.20. The amount of voltage induced in each secondary is
calculated using the same formula used for a transformer with a
single secondary.
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Figure 1.20: A Step-Down Transformer
5. Perform the following sub steps to calculate the voltage output of
each secondary in Figure 1.20.
A. Calculate the turns ratio for each secondary.
B. Calculate the voltages for each secondary.
6. Perform the following sub steps to calculate the voltage output of each
secondary in Figure 1.21.
A. Calculate the turns ratio for each of the secondary in Figure 1.21.
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B.
Calculate the voltage of each secondary.
Figure 1.21: Voltage Output of Each Secondary Calculation
7. Summarise your findings and differentiate between a step-down
and a step-up transformer.
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1.6
Lab Activity 3
Objective: To Calculate the Transformer Power Loss.
Procedure: In this procedure, you will load a transformer and take
measurements which will allow you to calculate the power in and out of
the transformer. You will then use these actual power values to
calculate the efficiency of the transformer. This will show that the
transformer does in fact lose power.
1. Connect the circuit shown in Figure 1.22. Set the DMM to
measure AC current (you will want to use the mA input jack).This
circuit uses the resistor module which has two 220 ohm
resistors. These resistors are connected in series to create a load
of 440 ohms.
Figure 1.22: Measuring the Current in the primary circuit
2. Place the AC/DC selector switch on the power supply in the AC
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position.
3. Turn on the power supply and record the reading displayed by
the DMM in the space provided. This is the transformer primary
current.
Transformer primary current is equal to = ______________(mA)
4. Use the analog voltmeter on the T7017 to measure the voltage
across the primary and record this value below.
Transformer primary voltage = __________(VAC)
5. Turn off the power supply.
6. Now move the DMM to measure the current in the secondary, as
shown in Figure 1.23.
Figure 1.23: Measure the Secondary Current
7. Turn on the power supply and record the reading displayed by
the DMM in the space provided. This is the transformer
secondary current.
Transformer secondary current =________(mA)
8. Use the analog voltmeter on the T7017 to measure the voltage
across the secondary and record this value below.
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Transformer secondary voltage _________ (VAC)
9. Turn off the power supply.
10. Use the data you obtained in steps 3 and 4 to calculate the
power being used by the primary.
Primary Power ________________________(Watts)
11. Use the data you obtained in steps 7 and 8 to calculate the
power being used by the load on the secondary.
Secondary Power__________ ______________(Watts)
12. Use the actual power values that you calculated in steps 10 and
11 to calculate the efficiency of the transformer. Record your
answer in the space provided.
Transformer Efficiency =________________________ %
13. Calculate the input-output voltage ratio:……………………………….
14. Calculate the input-output current ratio:……………………………….
15. Compare the two ratios obtained in steps 13 and 14. Explain the
relationship between current, voltage and truns ratios?
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1.7
Lab Activity 4
Objective: To design a control transformer circuit to provide a
given output voltage.
Procedure Overview
In this procedure, you will determine how the primaries of a control
transformer should be connected to produce a desired output given the
line voltage and number of turns of each coil. You will then connect a
transformer to deliver their desired output.
1. Redraw the control transformer schematic shown in Figure 1.24 in
the space provided below. Make the connections so that the output
voltage is 120 VAC.
Figure 1.24: Control Transformer Schematic
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2. Redraw the schematic shown in Figure 1.25 with the transformer
connected to produce a turns ratio of 1:1. Use both primary and
secondary coils.
Figure 1.25: Transformer on the Transformer Module
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Draw your circuit below.
Figure 1.25 above shows the transformer on the T7017 transformer
module. Notice that the transformer has two primaries and two
secondaries. Also, the primary coils are not crossed.
3. Connect the circuit you drew in step 2 on the T7017. Use the 24 VAC
jacks to supply power to the transformer.
4. Turn on the power supply and use the DMM to measure the input and
output voltages. Record these values below:
Input Voltage ____________________________________ (VAC)
Output Voltage ___________________________________ (VAC)
You should find an input voltage of approximately 26.5 VAC and an output
voltage of approximately 35 VAC. The output voltage is actually greater
than the input voltage. This is because transformers of the size used on
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the T7017 are rated for the output voltage at a specific output current.
In the next four steps, you will add a load to the secondary of the
transformer.
This
load
is
sized
to
draw
rated
current
from
the
transformer. You will then re-check the input and output voltages.
5. Turn off the power supply.
6. Add a 440 ohm load to the secondary of the transformer, as shown in
Figure 1.26.
Figure 1.26: Load for Transformer Secondary
7. Turn the power supply back on.
8. Use the DMM to measure the input and output voltages. Record these
values below:
Input Voltage ________________________________________ (VAC)
Output Voltage _______________________________________ (VAC)
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Calculate the input-output voltage ratio:.........................................
9. Turn off the power supply.
10. Solve the following design problem.
With the primary connected in parallel, determine how the transformer
secondary should be connected to produce a turns ratio of 2:1.
Draw the circuit below.
11. Connect your circuit on the T7017.
12. Add a 110 ohm load to the secondary of the transformer, as shown in
Figure1.27. This will properly load the transformer for this ratio.
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Figure 1.27. Load for Transformer Secondary
13.Turn the power supply back on.
14. Use the DMM to measure the input and output voltages, record these
values below:
Input Voltage ________________________________________ (VAC)
Output Voltage _______________________________________ (VAC)
Calculate the input-output voltage ratio:.........................................
15. Turn off the power supply.
16. Disconnect and store all components.
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1.8 Review Exercises
1. ___________________is the ability of one coil to induce voltage in
another coil.
2. A(n) ___________________ is an
electrical
device
that
converts AC electricity from one voltage level to another.
3. The
_______________coil
of
the
transformer
creates
a
magnetic field that is concentrated by the core.
4. The voltage induced in the _________________ coil can be used
to drive load.
5. The
_______________
of
a
transformer
determines
the
relationship between the primary coil and the secondary coil.
6. Many machines require a(n) _________________ to step down the
line voltage to the machine’s operating voltage.
7. Because of the construction of a transformer, there is usually a
small amount of ____________ between the primary power and
the secondary power.
8. If a transformer is operated at something other than rated load and
voltage, the _____________ will be lower.
9. A(n) ______________ transformer is commonly used to reduce the
line voltage from 480 VAC or 240 VAC to 120 VAC.
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10. Determine the voltage across the load resistor.
11.
Determine the current in the secondary.
12.
A 230 V to 12 V bell transformer is constructed with 800 turns on
the primary winding. Calculate the number of secondary turns and
the primary and secondary currents when the transformer supplies a
12 V 12 W alarm bell.
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