With a hollow coil the lines form complete rings. If there is an iron core in the coil it becomes magnetised, and seems to make the field become much stronger while the current is on. The iron core of a transformer is normally a complete ring with two coils wound on it. One is connected to a source of electrical power and is called the primary coil ; the other supplies the power to a load and is called the secondary coil.
The magnetisation due to the current in the primary coil runs all the way round the ring. The primary and secondary coils can be wound anywhere on the ring, because the iron carries the changes in magnetisation from one coil to the other. There is no electrical connection between the two coils. However they are connected by the magnetic field in the iron core.
When there is a steady current in the primary there is no effect in the secondary, but there is an effect in the secondary if the current in the primary is changing.
A changing current in the primary induces an e. If the secondary is connected to a circuit then there is a current flow.
A step-down transformer of 1, turns on the primary coil connected to V a. A step-up transformer with 1, turns on the primary fed by V a. The iron core is itself a crude secondary like a coil of one turn and changes of primary current induce little circular voltages in the core.
Iron is a conductor and if the iron core were solid, the induced voltages would drive wasteful secondary currents in it called eddy currents. So the core is made of very thin sheets clamped together, with the face of each sheet coated to make it a poor conductor.
The edges of the sheets can be seen by looking at the edges of a transformer core. For 9 Resources. The following guidance notes cover these practical collections: Permanent magnets In certain spectacular demonstrations, very large voltages are used to produce long arcs, but they are relatively safe because the transformer output does not supply a large current. If there is no change in primary voltage, there is no voltage induced in the secondary.
One possibility is to connect DC to the primary coil through a switch. As the switch is opened and closed, the secondary produces a voltage like that in Figure 4. This is not really a practical alternative, and AC is in common use wherever it is necessary to increase or decrease voltages. Figure 4. Transformers do not work for pure DC voltage input, but if it is switched on and off as on the top graph, the output will look something like that on the bottom graph.
This is not the sinusoidal AC most AC appliances need. A battery charger meant for a series connection of ten nickel-cadmium batteries total emf of It uses a step-down transformer with a loop primary and a V input.
You would expect the secondary to have a small number of loops. The number of loops in the secondary is small, as expected for a step-down transformer. We also see that a small input current produces a larger output current in a step-down transformer. When transformers are used to operate large magnets, they sometimes have a small number of very heavy loops in the secondary.
This allows the secondary to have low internal resistance and produce large currents. In this case the primary and secondary power is W. Verify this for yourself as a consistency check. So the AC output of the secondary coil needs to be converted into DC. This is done using something called a rectifier, which uses devices called diodes that allow only a one-way flow of current. Transformers have many applications in electrical safety systems, which are discussed in Electrical Safety: Systems and Devices.
A step-up transformer increases voltage and decreases current, whereas a step-down transformer decreases voltage and increases current.
Conceptual Questions 1. Explain what causes physical vibrations in transformers at twice the frequency of the AC power involved. A plug-in transformer, like that in Figure 4, supplies 9.
A cassette recorder uses a plug-in transformer to convert V to What is the maximum input current if the input voltage is V? A multipurpose transformer has a secondary coil with several points at which a voltage can be extracted, giving outputs of 5. What are the numbers of turns in the parts of the secondary used to produce the output voltages?
A large power plant generates electricity at Its old transformer once converted the voltage to kV. The secondary of this transformer is being replaced so that its output can be kV for more efficient cross-country transmission on upgraded transmission lines. If the power output in the previous problem is MW and line resistance is 2. Unreasonable Results The kV AC electricity from a power transmission line is fed into the primary coil of a transformer.
Construct Your Own Problem Consider a double transformer to be used to create very large voltages. The device consists of two stages. The first is a transformer that produces a much larger output voltage than its input. The output of the first transformer is used as input to a second transformer that further increases the voltage. Construct a problem in which you calculate the output voltage of the final stage based on the input voltage of the first stage and the number of turns or loops in both parts of both transformers four coils in all.
Also calculate the maximum output current of the final stage based on the input current. Discuss the possibility of power losses in the devices and the effect on the output current and power. Skip to main content. Search for:. Transformers Learning Objectives By the end of this section, you will be able to: Explain how a transformer works. Example 1. Calculating Characteristics of a Step-Up Transformer A portable x-ray unit has a step-up transformer, the V input of which is transformed to the kV output needed by the x-ray tube.
Discussion for a A large number of loops in the secondary compared with the primary is required to produce such a large voltage.
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