Physics
Home > Physics > Core > Motors and generators > Motors and generators: 4. Transformers
9.3 Motors and generators: 4. Transformers
Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 18 Sep 03]
Prior learning: Preliminary module 8.3.
perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced
- When performing investigations, it is
important that you identify and use safe work practices.
Use only low voltage AC, as available from an ordinary
laboratory power supply. This activity must not be
attempted using 240 V AC mains supply. Switch the
current on for the shortest time that allows you to make
observations and record measurements. If the wiring becomes
hot, place a suitable resistive load in series with each
coil to limit the current and avoid the risk of burning
yourself and damaging the equipment. Your teacher may
provide you with an investigation plan, or you could follow
one of the procedures below.
- This investigation involves the
modelling of a transformer. You will need
to construct a circuit that displays the main features of a
transformer, including the physical arrangement of coils
and core, the inductive coupling between the coils and the
electrical properties of both a step-up and a step-down
transformer. You may need to construct several models with
different ratios of turns between the primary and secondary
coils and with different physical arrangements of the
coils.
- When demonstrating how the secondary voltage is produced, you will need to show the conditions under which a secondary voltage is best produced and the relationships between the secondary voltage and the primary voltage.
Sample investigation
Look at a dismantled transformer that has been removed from the circuit of a household appliance to familiarise yourself with the essential parts of a transformer. A transformer usually consists of two coils of wire wound on the same iron core. Identify the primary coil, the secondary coil and the laminated iron core that inductively couples the two coils. The primary coil is the input coil connected to the electricity supply. The secondary coil is the output coil connected to the device using the electricity at a voltage other than the supply. It is often possible to identify the parts without destroying the transformer.
Safety Note!
Under no circumstances should a transformer be connected to the power when it is being removed, dismantled or examined, nor should a transformer be reassembled and used again after examination. If you open it up to take a look at it, throw it away.
------------
Make your own simple model transformer by winding two separate coils of insulated wire onto an iron ring and connecting them into separate complete circuits. Have a significantly different number of turns in each coil.
Connect one coil to the power supply as the primary coil and input a low AC voltage into it. Check with a multimeter whether a voltage is induced in the second coil with the power supply switched on and with it switched off. Compare the output voltage from the secondary coil with the input voltage to the primary coil for several settings of the power supply voltage. Calculate the ratio of the output voltage to the input voltage and compare this with the ratio of the number of turns in each coil.
Reverse the connections to the two coils, so that the secondary coil becomes the primary coil, and repeat the above investigation. Determine which arrangement models a step-up transformer, with secondary voltage higher than primary voltage, and which a step-down transformer.
If you have access to a multi-channel datalogger or a dual-trace oscilloscope, you may be able to simultaneously record or observe the voltage and frequency of both the primary and secondary currents, and also any phase difference between them. Be careful to set the datalogger to a sampling frequency much higher than the input frequency, say 1000 Hz, or the CRO to a much shorter time base, say 1 ms, to avoid the problem of frequency aliasing which can give false readings of frequency.
Use your observations to determine what must happen for a voltage to be detectable in the secondary coil of a transformer. Determine also the relationship between the secondary voltage and the primary voltage in terms of the number of turns in each coil. Comment also on the relationship between the frequency and phase of the primary and secondary voltages.
An alternative method of performing this investigation involves making your model transformer by using a dissectible transformer kit available in many school science laboratories. Such a kit allows you to assemble various configurations of laminated iron core with a limited variety of ready-made coils that fit over the iron core. The number of turns is printed on the outside of each coil, and external terminals allow the coils to be connected into a circuit with a power supply, ammeter, voltmeter, etc. Try various combinations of coils with different core arrangements.
describe the purpose and principles of transformers in electrical circuits
- The domestic supply voltage in Australia is 240 V
single-phase AC. Industrial and commercial supply is
usually 415 V three-phase AC. Many appliances, such as
motors and lights, are designed to operate directly on
these voltages.
- However, many domestic and industrial appliances
contain components that require voltages well below the
supply voltage, such as display panels, printed circuit
boards or semi-conductor devices, which typically require
between 3 V and 24 V. In addition, some imported appliances
run on voltages common in the country of manufacture, such
as 110 V in North America. Transformers are placed in the
circuit between the AC supply and the component to reduce
the supply voltage to that required for the component. Such
a transformer is known as a step-down transformer.
- Some components, such as television picture tubes,
require voltages well above the supply, around 1500 V. A
step-up transformer is placed in the circuit between the AC
supply and the component to raise the voltage to that
required for the component.
- It is common for the step-up or step-down transformer to be built into the appliance as part of its power supply. Many appliances contain both a step-up and a step-down transformer to supply voltages required by different components. Many step-down transformers are multi-tapped transformers capable of supplying a range of different secondary voltages from the same primary input voltage.
compare step-up and step-down transformers
- The following table compares step-up and step-down transformers.
| Step-up transformer | Step-down transformer |
|---|---|
| Consists of two inductively coupled coils wound on a laminated iron core | Consists of two inductively coupled coils wound on a laminated iron core |
| More turns in the secondary coil than the primary coil | Fewer turns in the secondary coil than the primary coil |
| Higher output voltage than input voltage | Lower output voltage than input voltage |
| Lower output current than input current | Higher output current than input current |
| Used at power stations to increase voltage and reduce current for long-distance transmission | Used at substations and in towns to reduce transmission line voltage for domestic and industrial use |
| Used in television sets to increase voltage to operate the picture tube | Used in computers, radios, and CD players to reduce household electricity to very low voltages for electronic components |
identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage
- The ratio of primary to secondary voltage in a
transformer is the same as the ratio of the number of turns
in the primary and secondary coils. In a step-up
transformer there are more turns in the secondary coil than
in the primary coil and the secondary voltage is higher
than the primary voltage by the same factor. The reverse is
true for a step-down transformer.
- This relationship is expressed mathematically as:
where Vp is the
voltage in the primary coil, Vs is the
voltage in the secondary coil,
np is the number of turns in the
primary coil and ns is the number of
turns in the secondary coil.
explain why voltage transformations are related to conservation of energy
- The Law of conservation of energy states that energy
cannot be created nor destroyed, but can be transformed
from one form into another. All physical systems obey this
law. The amount of electrical energy entering a transformer
in a certain time must equal the total amount of energy in
all forms leaving the transformer in the same period of
time. That is, power in equals power out.
- In an “ideal” transformer, all of the
magnetic flux produced in the primary coil threads the
secondary coil. Thus the rate of change of magnetic flux
induced by the primary voltage is equal to the rate of
change of magnetic flux inducing an emf in the secondary
coil. The relationship between voltage, number of turns and
rate of change of flux in the primary coil is given by
Faraday’s Law
- The same equation links the number of turns, the rate
of change of flux and the induced voltage in the secondary
coil. Combining equations for both coils, and allowing for
equal rates of change of flux, gives us the so-called
“transformer equation”
- Thus the voltage transformation that occurs in a
transformer is a consequence of the Law of conservation of
energy.
Another consequence of the Law of conservation of energy in an “ideal” transformer is that the power in the secondary coil Ps is equal to the power in the primary coil Pp.
Now P = V I, so that: Pp = Vp Ip = Vs Is = Ps, or
- This means that the ratio of secondary current to
primary current is the inverse of the ratio of secondary
voltage to primary voltage. The secondary current is less
than the primary current in a step-up transformer, and
greater in a step-down transformer.
- Real transformers produce heat because of the resistance of the iron core to induced eddy currents. This represents an energy loss to the system as heat is a form of energy. The power output of a transformer cannot exceed the power input, and the useful electrical power output is less than the input by the amount of the power loss through heating within the transformer.
solve
problems and analyse
information about transformers using:
- In solving problems about transformers
using this equation, you may need to select different
strategies, depending on the amount of given data. If only
one variable is unknown, you may choose to rearrange the
equation as necessary and substitute for known variables.
However, solutions may not be unique if two variables are
unknown, because this equation deals with ratios rather
than actual numbers of turns or voltages. For example, in a
problem to design a transformer for a specific task, you
may know the input voltage and the required output voltage
but you may not know the number of turns in either coil.
Other design features may need to be considered, and other
strategies used to solve the design problem.
- You can analyse information about transformers by using this equation as a mathematical model to make predictions, for instance, about the voltage characteristics of a transformer if you know the number of turns in each coil. You can also analyse the effect on the output voltage of changing the number of turns in the secondary coil while keeping the primary coil the same.
Sample problem
A transformer has 528 turns in the primary coil and 242 turns in the secondary.
- Explain whether this is a step-up or step-down transformer.
- If the input voltage is 240 V AC, describe the output voltage.
Solution:
- The secondary coil has fewer turns than the primary coil so, by the transformer equation, the output voltage is lower than the input voltage. Therefore this is a step-down transformer.
- Rearrange the transformer equation to find
Vs:
Vs = Vp x ns / Np = 240 x 242 / 528 = 110 V AC
Sample analysis
A multi-tapped transformer has a number of different connections to the secondary coil, so that different numbers of turns of the secondary coil can be connected into the external secondary circuit. Turns not connected are left open and do not contribute to the transformer output. If the voltage and power input to the transformer remain fixed, explain how the output voltage and current vary with the different secondary connections or “tappings”.
Analysis:
As more and more turns of the secondary coil are connected into the circuit, the output voltage increases, since the ratio of secondary to primary turns increases. If the power input is fixed, so is the power output. Thus, as the number of connected secondary turns increases, and the voltage increases, the output current must decrease, since power is the product of voltage and current.
gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome
- Gather information from a range of
sources including text books, popular scientific journals
and the Internet. Refine your searching technique by
looking for articles that specifically mention heating and
cooling in transformers.
- Analyse the information by looking for
cause-effect relationships, such as the cause of the
heating and the undesirable effects of the heating. Look
also for information on mechanisms used for cooling and how
these work to overcome the difficulties of heating.
- Use available evidence to organise your information into a logical progression of ideas. Begin your discussion by identifying the source of the heating and the difficulties this causes. Identify at least two mechanisms by which either the cause of the heating can be reduced or the effects of the heating can be dissipated, and describe how each of these helps to overcome the difficulties of heating.
Sample discussion
A transformer has an iron core to concentrate the magnetic field to achieve the maximum possible inductive coupling between the primary and secondary coils. As the changing flux intersects the core, eddy currents are induced in the iron. Heating occurs because of the rather high resistance of the iron to the eddy currents. This heat represents a power loss to the electrical system and excessive heating can damage or destroy the transformer.
One of the best ways to overcome difficulties of heating in transformers is to reduce the size of the eddy currents. Transformer cores are made of laminated iron, that is, many thin sheets of iron pressed together but separated by thin insulating layers. This limits the circulation of any eddy currents to the thickness of one lamina, rather than the whole core, thus reducing the overall heating effect.
Once the transformer does get hot it must be cooled to prevent overheating. Several strategies have been developed to keep transformers cool:
- Heat-sink fins are added to the metal transformer case so that heat dissipation to the environment can occur more quickly over a larger surface area.
- The transformer case may be made of a black material so that the heat produced internally is efficiently radiated to the environment. Most small transformer rectifier units found around the home are coloured black.
- Pad-mounted transformers at ground level have ventilated cases to allow air to remove heat by convection. They may also have an internal fan to assist air circulation to remove excess heat faster.
- The transformer case may be filled with a non-conducting oil that transports the heat produced in the core to the outside where the heat can be dissipated to the environment. The oil may circulate from hotter to cooler regions by convection alone, or circulation may be assisted by a pump. The case may have design features such as cooling tubes and radiator slats to increase the rate of heat dissipation.
- Large transformers such as at substations are always located in the open or in well-ventilated areas to maximise airflow around them for cooling. These are fitted with a combination of cooling mechanisms including pumps to circulate cooling oil through large radiators, and fans to increase the airflow over the radiators. The fans are often thermostatically controlled and cut in at a specified temperature, usually around 50°C.
gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use
- Gather secondary information about why
transformers are necessary from a range of sources. Your
local power supply authority may have free information
pamphlets to read or an information officer you could
interview. Most power supply authorities also have
informative Internet sites that you could explore. Print
and CD-based encyclopaedias are another possible source of
useful information. Try to find out also what electricity
transmission was like before AC transformers were invented.
- Analyse the information to identify
the connectedness of concepts such as power, voltage,
current and transmission line losses.
- In discussing the need for transformers, draw evidence from your analysis to identify the transmission issues that make transformers necessary and describe the ways in which transformers solve these problems.
Sample discussion
Electricity is typically consumed in homes and industry at 240 V or 415 V. If there were no transformers, electricity would have to be generated and distributed at these same voltages. To supply the power demands of even a small town, the current at these voltages would be very large, leading to large and costly transmission losses and possible overheating of conductors. If the power demand were to increase, the number of conductors would need to multiply, to keep the current per conductor within reasonable limits.
For a large city there would need to be many power stations spaced every few kilometres. If different voltages were needed, these would require separate power stations and separate distribution systems, adding to the network of cabling required. The result would be an expensive, unsightly, unreliable web of cables serving consumers only within a limited distance from each power station.
The use of transformers with AC electricity overcomes many of these problems.
It is more efficient to generate electricity at high voltages, such as 23 kV, than at low voltages. Power stations can run efficiently at their design voltage and different transformers can be used to simply step the voltage up for transmission or down for local use as required.
It is much more efficient to use very high voltages, up to 500 kV, for transmission lines, because at these voltages the currents are relatively small and transmission line losses are less of a problem. The higher the voltage, the smaller the line losses, and the greater the distance of transmission, the more important this saving is. Because the current is smaller at high voltages, fewer, smaller conductors are necessary for any particular power load than at lower voltages. High voltages are easily achieved for economical transmission by the use of step-up transformers.
Electrical energy is usually consumed at low voltages, but at widely scattered locations. Transformers are used to progressively step the voltage down from the transmission lines to the consumer. Major transmission lines in the national grid typically carry 330 kV. At regional sub-stations, step-down transformers reduce this to 110 kV for regional distribution. Local sub-station transformers step this down further to 33 kV or 11 kV for distribution along suburban streets. Pole-mounted transformers step this down again for supply to houses and factories at 415/240 V. The stepped-down voltage used at each stage of distribution is chosen to balance the power, and hence the current, requirements, and therefore also the transmission losses, against the area over which distribution is required.
explain the role of transformers in electricity sub-stations
- Electricity from power stations is transmitted through
the national grid at very high voltages (up to 500 kV in
Australia). The high voltages are necessary to minimise
energy loss due to resistance in the conducting
transmission wires as the energy is carried over great
distances.
- Transmission lines operate at voltages very much higher
than those required to operate most industrial and domestic
equipment and appliances. These operate at low voltages,
typically 240 V single phase or 415 V over three phases, so
that design is simpler, the cost of insulation is
affordable and operation is safer.
- The role of transformers in electricity sub-stations is to progressively reduce the voltage as it comes closer to the consumer. At each stage, the output voltage is chosen to match the power demand and the distances over which supply is needed.
Example
From the power station to the consumer electricity might be transformed in the following ways: generated in the power station at 23000 V; stepped up at the adjacent sub-station to 330 000 V; transmitted to a Transgrid® substation where it is transformed to 132 000 V; distributed to regional electricity supplier substations and transformed to 33 000 V; further transmitted to a city substation where it is transformed again to 11 000 V; transmitted to local pad or pole mounted transformers and stepped down to 415 V for distribution to consumers.
A typical Transgrid® substation transformer might be rated 330-132 kV and 30 MVA, meaning the primary or input voltage is 330 kV, the secondary or output voltage is 132 kV and the transformer has a power rating of 30 MVA or a capacity to transform 30 MW of electrical power.
The following Internet site offers a downloadable audio
file of an interview about the role of transformers in
substations: 9.3 Motors and
generators Learning Materials Production Centre:
OTEN-DE. [Requires browser plug-in capable of playing .ram
files.]
discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer
- Electricity supplied to homes is typically 240 V AC.
Many domestic appliances are designed to run most
efficiently at this voltage. Such appliances are connected
directly to the mains supply without the need for a
transformer.
- Some appliances contain components that require a
transformer because they operate best at lower voltages
than the mains supply. In a microwave oven, for example,
large, energy consuming parts such as the turntable motor
and the microwave transducer may be connected directly to
the mains, while the control and display panel is supplied
with low voltages from a step-down transformer in a
built-in power supply unit.
- Many small portable appliances, such as personal CD
players and mobile telephones, have been designed to run on
batteries. These require low DC voltages, either as an
alternative to batteries or to recharge the batteries. When
the whole appliance is designed to run at the same low
voltage, a step-down transformer-rectifier may be built
into the plug of the power supply lead that connects to the
mains supply. Alternatively, a normal power lead connects
the mains to a built-in power supply unit that contains a
step-down transformer and a rectifier.
- Appliances such as television receivers and computer monitors contain cathode ray tubes that require voltages well above the mains supply, up to around 25 kV, to accelerate electrons toward the screen. These use a built-in step-up transformer to provide the necessary voltage. The power supply unit may contain both a step-up and a step-down transformer.
discuss the impact of the development of transformers on society
- The development of transformers made it possible to
transmit electrical energy efficiently over great
distances. This has had a range of impacts on society.
- Even very remote communities now have access to
grid-supplied high-voltage electricity which is stepped
down locally by transformers. This has raised living
standards in rural communities through provision of, for
instance, electric lighting, refrigeration and air
conditioning, and increased the scope of rural industries.
- Large cities have been allowed to spread, because
electricity is readily available as an energy source,
thanks to transformers. This has led to social dislocation
in urban “deserts”, as people have moved
further from family and friends and workplaces.
- Industry is no longer clustered around power stations
or other sources of energy. Power stations can be in remote
locations and high-voltage electrical energy can be
distributed almost anywhere, to be stepped down near the
point of use. This has allowed industries to be
decentralised and has facilitated the development of
industrial areas away from residential areas. This has
relocated pollution away from homes, but it means that many
people now spend significant time travelling between home
and work.
- With the development of the transformer, people have changed the way they live, as electricity to every home has become an affordable necessity rather than a luxury.