Transformers: Coils in disguise

1. Transformers: Coils in disguise Learning Objectives: What is a Transformer? How is a current induced in the secondary coil? Is there a relationship between the number of coils and the output voltage?

2. I wonder how the charger charges the toothbrush up! They are both made of plastic so the charger should not be able to transfer any electric charges to the toothbrush, right? I think it might work with electromagnetic induction, but I cannot imagine how. One would need to open one up and see! How can we induce a current in a circuit?

3. Wires, coils and solenoids

4. Transformers Quiz What name can we give to a transformer that has more windings in the primary coil than in the secondary coil? Well done! A Step Down transformer has less windings in the secondary coil than in the primary coil. Step Up Transformer Step Down Transformer Megatron Optimus Prime

5. Transformers Quiz Why does a transformer only work if an A.C. Voltage is input across the Primary Coil? Well done! An A.C. current will generate a changing magnetic field in the Primary Coil and this will induce a changing (alternating) current in the secondary coil because of Lenz’s Law. D.C. Current is not high enough to induce a magnetic field in the Primary Coil D.C. currents are too dangerous for the user A changing magnetic field is needed to induce a current in the Secondary Coil A magnetic field cutting through the secondary coil is needed to induce a current

6. Transformers Quiz Why is a laminated core useful in a transformer? Well done! A laminated core is not essential, but it highly improves its performance, because it channels all the magnetic field lines generated around the Primary Coil to the Secondary Coil. It’s there to hold the primary and secondary coils together It carries the current from the primary to the secondary coil It channels the magnetic field lines to the Secondary Coil It ensures no magnetic field lines reach the secondary coil

7. Transformers Quiz What is the relationship linking V1, N1, V2 and N2? Well done! The Primary Voltage divided by the Secondary Voltage equals the no of Primary Windings divided by the no of Secondary Windings.

8. Transformers Quiz When is the relationship from the previous question false? Well done! If the transformer is not 100% efficient the power output in the Secondary Coil would not be the same as the input power in the Primary Coil and the equation would not work. When the transformer is not 100% efficient When the Secondary Voltage is too high When there are too many windings in the Primary Coil When the transformer is 100% efficient

9. Transformers Quiz What is the Secondary Voltage in a transformer when V1 = 20 V, N1 = 100 and N2 = 2000? Well done! V1 / N1 x N2 = 20/100 x 2000 = 400 V V2 = 400 V V2 = 200 V V2 = 20 V V2 = 4000 V

10. Transformers Quiz What is the Secondary Voltage in a transformer whose input power P = 1500 W and Secondary Current = 30 A? Well done! The output power = input power and P = VI, so: V = P/I = 1500/30 = 50 V V2 = 50 V V2 = 45 V V2 = 10 V V2 = 45000 V

11. Card Games How do transformers work? Blue TeamRed Team A current flows in the primary coil An AC current flows in the primary coil Most field lines are lost but some reach the secondary coil A fixed magnetic field runs through the iron core The field lines cut through the secondary coil & a Vs is induced The iron core stays magnetised, so a Vs is induced The iron core channels the field lines to the secondary coil A changing magnetic field is induced in the primary coil The motor effect causes the secondary coil to move A current flows in the primary coil A current flows in the primary coil Most field lines are lost but some reach the secondary coil A changing magnetic field is induced in the primary coil A fixed magnetic field runs through the iron core An AC current flows in the primary coil The motor effect causes the secondary coil to move The iron core stays magnetised, so a Vs is induced The field lines cut through the secondary coil & a Vs is induced The iron core channels the field lines to the secondary coil The iron core channels the field lines to the secondary coil The iron core stays magnetised, so a Vs is induced The motor effect causes the secondary coil to move An AC current flows in the primary coil A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil A changing magnetic field is induced in the primary coil The field lines cut through the secondary coil & a Vs is induced A current flows in the primary coil A changing magnetic field is induced in the primary coil An AC current flows in the primary coil Most field lines are lost but some reach the secondary coil A fixed magnetic field runs through the iron core The field lines cut through the secondary coil & a Vs is induced The motor effect causes the secondary coil to move The iron core stays magnetised, so a Vs is induced The iron core channels the field lines to the secondary coil 1 2 1 2 A current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil An AC current flows in the primary coil The field lines cut through the secondary coil & a Vs is induced A fixed magnetic field runs through the iron core Most field lines are lost but some reach the secondary coil The motor effect causes the secondary coil to move The iron core stays magnetised, so a Vs is induced A current flows in the primary coil A current flows in the primary coil An AC current flows in the primary coil Most field lines are lost but some reach the secondary coil A fixed magnetic field runs through the iron core The field lines cut through the secondary coil & a Vs is induced The motor effect causes the secondary coil to move The iron core stays magnetised, so a Vs is induced The iron core channels the field lines to the secondary coil A changing magnetic field is induced in the primary coil The iron core stays magnetised, so a Vs is induced A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil The motor effect causes the secondary coil to move The field lines cut through the secondary coil & a Vs is induced Most field lines are lost but some reach the secondary coil An AC current flows in the primary coil A fixed magnetic field runs through the iron core A current flows in the primary coil A fixed magnetic field runs through the iron core The motor effect causes the secondary coil to move Most field lines are lost but some reach the secondary coil The field lines cut through the secondary coil & a Vs is induced The iron core stays magnetised, so a Vs is induced An AC current flows in the primary coil A changing magnetic field is induced in the primary coil The iron core channels the field lines to the secondary coil 4 3 4 3

12. What have we learnt? • What is a transformer? A Transformer is made of two coils and a laminated soft iron core. • How is a current induced in the secondary coil? The primary coil carries an A.C. current which induces a changing magnetic field cutting through the secondary coil, where an A.C. current is induced. • Is there a relationship between the number of coils and the output voltage? Learning outcomes

13. Homework Answer the following questions. • How many windings would a laptop transformer have in the primary coil, if V2 = 15 V and N2 = 250? (Hint: What is the mains voltage?) • The power output on the transformer box is rated at 90 W. What is the working current of the laptop? • What is the total resistance of the laptop? Homework