A transformer is an electrical device that changes an alternating input voltage to an output voltage of a different value. Operating on the principle of electromagnetic induction, transformers are essential in powering domestic appliances with appropriate voltages and minimising electrical energy loss during long-distance transmission from power stations to homes and industries.
See how alternating flux induces voltage across coils and how four design modifications minimise transformer energy losses.
Use thicker copper wire to lower the electrical resistance of the coils.
Use a laminated soft iron core made of thin iron sheets insulated from one another.
Use a soft iron core, which requires a smaller amount of energy to magnetise and demagnetise.
Wind the secondary coil directly on top of the primary coil.
A transformer is an electrical device that changes an alternating input voltage to an output voltage of a different value. It operates on the principle of electromagnetic induction.
A direct current produces a steady, non-changing magnetic field. Since there is no change in magnetic flux linking the secondary coil, no voltage is induced across the secondary coil.
Increases voltage: \(V_s > V_p\) and \(N_s > N_p\).
Decreases voltage: \(V_s < V_p\) and \(N_s < N_p\).
Transformer Equation
\(V_p\) = voltage across primary coil (input voltage), \(V_s\) = voltage across secondary coil (output voltage), \(N_p\) = number of turns of the primary coil, \(N_s\) = number of turns of the secondary coil.
Efficiency of a Transformer
An ideal transformer experiences zero energy loss (\(\eta = 100\%\)), so output power equals input power: \(V_p I_p = V_s I_s\).
In an induction cooker, a high-frequency alternating current in a coil beneath a ceramic plate creates a rapidly changing magnetic field. This induces large eddy currents at the metal base of a steel pot, heating the pot directly to cook food efficiently.
Step-down transformers: mobile phone chargers, notebook computer chargers, photocopy machines, welding machines.
Step-up transformers: microwave ovens, defibrillators, X-ray machines.
Electricity generated at the power station (e.g., \(20\text{ kV}\)) passes through a step-up transformer to be stepped up to high voltages (e.g., \(275\text{ kV}\)) for transmission. Stepping up the voltage reduces the transmission current (\(I\)). Since power loss in cables is given by \(P_{\text{loss}} = I^2 R\), a smaller current drastically reduces electrical energy lost as heat in the transmission cables.
During distribution, step-down transformers reduce voltage in stages: main receiving substation stepped down to \(132\text{ kV}\), main distribution substation to \(33\text{ kV}\) (heavy industry), further substations to \(11\text{ kV}\) (light industry), and finally to \(230\text{ V}\) for residential homes.
Answer in your mind, then press “Check Answer”.
Why is an alternating current power supply used instead of a direct current power supply in a transformer?
An ideal transformer connected to a \(240\text{ V}\) supply has \(800\) turns in its primary coil and \(60\) turns in its secondary coil. If it supplies \(27\text{ W}\) of power to an appliance, what is the output voltage and secondary current?
How does a laminated iron core help improve the efficiency of a transformer?
A transformer creates extra energy when it steps up voltage.
Transformers obey the principle of conservation of energy. For an ideal transformer, input power equals output power (\(V_p I_p = V_s I_s\)). Stepping up voltage results in a proportional reduction in current, so power output is never greater than power input.
Eddy currents are always harmful and have no useful applications.
While eddy currents cause unwanted heat loss in transformers, they are deliberately generated in devices such as induction cookers to heat cooking pots directly and efficiently.
Objective: Test your understanding of transformers, energy loss and transmission.
1 Why are step-up transformers used at power stations before transmitting electricity through long-distance cables?
2 Which modification specifically reduces energy loss due to hysteresis in a transformer?
3 For an ideal transformer, the output power is equal to the input power, which can be expressed by the formula \(V_p I_p =\) .
4 In a step-down transformer, the number of turns in the secondary coil is than the number of turns in the primary coil.
Match each cause of energy loss in a transformer to its corresponding reduction method. If using a phone, tap the answer first, then tap the matching box.
There is something wrong with this question.
