Parts of a transformer
Definition
Transformer
A transformer is a device that uses electromagnetic induction between two coils on a shared iron core to change the size of an alternating potential difference.
- A transformer is built from a primary coil, a secondary coil and a core of soft iron that both coils are wound onto.
- The primary coil is connected to the input supply, and the secondary coil is connected to the output circuit.
- The two coils are insulated from each other and from the core, so there is no complete circuit joining the primary to the secondary and no charge passes between them.
- Soft iron is used for the core because it is easily magnetised and demagnetised, so the magnetic field in the core follows the changing current in the primary almost instantly.
- The core is normally built from thin sheets of iron, called laminations, with insulation between them.
- On a circuit diagram the two coils are drawn on either side of two parallel lines that represent the core, with no wire crossing between them.
How a transformer works
Definition
Electromagnetic induction
Electromagnetic induction is the production of a potential difference across the ends of a conductor when the magnetic field through that conductor changes.
- An alternating current in the primary coil produces a magnetic field in and around that coil.
- Because the current is constantly changing in size and reversing in direction, the magnetic field it produces is also constantly changing in size and reversing in direction.
- The soft iron core carries this changing magnetic field round to the secondary coil, so almost all of the field produced by the primary passes through the secondary.
- The changing magnetic field through the secondary coil induces an alternating potential difference across its ends.
- A current flows in the secondary only when its circuit is complete, so an unloaded transformer still has a potential difference across its secondary.
- The output has the same frequency as the input, because the field in the core completes one full change for every cycle of the primary current.
- Energy is transferred from the primary circuit to the secondary circuit by the changing magnetic field alone, which is why the two circuits can be kept electrically separate.
Why transformers need alternating current
Definition
Alternating current
Alternating current is a current that repeatedly reverses its direction of flow.
- A direct current in the primary coil produces a magnetic field that is steady in both size and direction.
- A steady field through the secondary coil is not changing, so nothing is induced and the output stays at zero for as long as the direct current flows.
- A brief potential difference does appear across the secondary at the instant a direct supply is switched on or off, because the field in the core grows or collapses at that moment.
- A transformer therefore cannot raise or lower the potential difference of a battery, and needs a supply that is changing all the time.
- This is one of the reasons that electricity is generated and transmitted as alternating current rather than direct current.
Energy losses in a real transformer
Definition
Dissipation
Dissipation is the transfer of energy to the thermal store of the surroundings, where it becomes spread out and can no longer be used usefully.
- The coils are made of wire with resistance, so the currents in them heat the wire and energy is dissipated to the surroundings.
- Thick copper wire is used to keep this resistance low, which is why the coils of a large transformer are heavy.
- The changing magnetic field also induces unwanted currents in the iron core itself, and these currents heat the core.
- Building the core from thin insulated laminations rather than one solid block restricts those unwanted currents and reduces the heating.
- Repeatedly magnetising and demagnetising the core transfers energy to it as well, which is another reason soft iron is chosen rather than steel.
- Large transformers are cooled by oil and fins so that the energy dissipated does not raise their temperature to a damaging level.
- A well-built transformer wastes only a small fraction of the power passing through it, which is why transformer questions normally treat one as being 100%100\%100% efficient.
Practical
Demonstrating that a transformer needs alternating current
- Aim: to show that a transformer produces an output only when the current in its primary coil is changing, and to show that the core carries the field between the coils.
- Apparatus: demountable transformer kit with a laminated C-core and yoke, two coils of known turns such as 600600600 turns and 300300300 turns, low-voltage alternating supply, low-voltage direct supply with a tapping key or switch, two multimeters set to measure alternating and direct potential difference, low-voltage lamp with holder, oscilloscope or data logger, and connecting leads.
- Variables: the type of supply connected to the primary coil is the independent variable, the potential difference across the secondary coil is the dependent variable, and the two coils, their turns, the core and the supply setting are controlled.
- Method, setting up:
- Slide the two coils onto the C-core, clamp the yoke firmly across the open ends so the core forms a closed loop, and check that no wire joins the two coils.
- Connect the primary coil to the low-voltage alternating supply set to a low value, and connect the secondary coil to a multimeter.
- Record the number of turns on each coil and the setting of the supply before taking any readings.
- Method, using an alternating supply:
- Switch on the alternating supply and record the potential differences across the primary and the secondary coils.
- Replace the multimeter with the lamp and observe that the lamp lights even though no wire connects it to the supply.
- Connect the oscilloscope across the secondary and sketch the trace, noting that the output alternates at the same frequency as the input.
- Method, using a direct supply:
- Switch off, replace the alternating supply with the direct supply at the same setting, and connect the multimeter set to measure direct potential difference across the secondary.
- Close the tapping key and hold it closed, then record the steady reading on the secondary, which stays at zero.
- Watch the meter or the oscilloscope closely while the key is closed and again while it is opened, and record the brief kick of the reading at each switching moment.
- Tap the key repeatedly and note that a reading appears only at the instants of switching, not while the current flows steadily.
- Method, testing the role of the core:
- Return to the alternating supply, then loosen and remove the yoke so that the core is no longer a closed loop, and record the new secondary reading.
- Lift one coil off the core entirely and hold it near the other, then record how small the reading becomes.
- Measurements and processing: tabulate the supply type, the primary potential difference and the secondary potential difference for each arrangement, and note whether the output is steady, alternating or momentary.
- Expected pattern: the alternating supply gives a steady alternating output, the direct supply gives no output except at the moments of switching, removing the yoke reduces the output sharply, and separating the coils reduces it almost to nothing.
- Sources of uncertainty: a loose yoke leaves an air gap that varies between runs, the coils may not be pushed fully onto the core, the meter may be too slow to show the switching kick, and the coils warm up and change resistance during a long run.
- Improvements: clamp the yoke to the same tightness each time, use an oscilloscope or data logger rather than a meter so the switching kick is visible, and keep each run short so the coils stay cool.
- Safety: use only low-voltage supplies, switch off before rearranging the core or the coils, do not touch the coils after a long run because they become hot, and keep the tapping key closed only briefly so the primary is not overheated.
Describing the transformer chain
- State the input: an alternating potential difference is applied across the primary coil, driving an alternating current in it.
- State the field: the alternating current produces a magnetic field that is constantly changing in size and direction.
- State the link: the soft iron core carries the changing magnetic field to the secondary coil.
- State the output: the changing field through the secondary coil induces an alternating potential difference across it.
- Every link in that chain carries a separate mark, so leaving out the core or the word changing costs marks even when the rest of the answer is right.
Example
Why a phone charger hums
- A charger plugged into a 230 V230\ \text{V}230 V mains socket contains a transformer whose primary coil carries an alternating current at 50 Hz50\ \text{Hz}50 Hz.
- The magnetic field in the core therefore builds up and collapses 100100100 times each second, twice in every cycle of the current.
- Each build-up and collapse pulls the laminations of the core very slightly together and lets them spring apart again.
- The laminations vibrate at 100 Hz100\ \text{Hz}100 Hz and push on the surrounding air, which is heard as a low hum.
- The charger also becomes warm because the coils have resistance and unwanted currents are induced in the core.
- A charger left plugged in with nothing connected still hums, because the primary current and the changing field are present even when the secondary circuit is open.
Exam technique
Writing a transformer explanation
- Use the word changing every time you mention the magnetic field, because a field that is merely present induces nothing.
- Name the soft iron core as the part that carries the field from one coil to the other, rather than saying the field simply reaches the secondary.
- State that the coils are not electrically connected when a question asks how energy crosses the transformer.
- For a question about a direct supply, say that the field is steady, so nothing is induced, and add that a brief output appears at switch-on and switch-off.
- When asked why a transformer becomes warm, name a specific loss such as resistance heating in the coils rather than writing about energy being wasted.
Common Mistake
- Do not write that current passes from the primary coil to the secondary coil, because the two circuits are separate and only the magnetic field links them.
- Do not say that a transformer works on a battery supply, since a steady current produces a steady field and induces nothing.
- Do not describe the core as a conductor of electricity between the coils, because its job is to carry the magnetic field.
- Do not claim that a transformer changes the frequency of the supply, since the output frequency always equals the input frequency.
- Do not use hard steel or a plastic core in a description, because soft iron is needed so the field can change as fast as the current does.
Self review
- Name the three main parts of a transformer and state what each one does.
- Describe the chain of events from an alternating current in the primary coil to an induced potential difference across the secondary coil.
- Explain why a transformer connected to a battery gives no steady output.
- Explain why the core of a transformer is made of soft iron rather than steel.
- Give two reasons why a real transformer becomes warm in use.
