The Operation Of A Transformer
Brook Edgar
Teacher
Contents
Explainer Video
Transformers & The National Grid
The National Grid is a network of high-voltage cables, transformers and pylons that connect power stations to consumers over long distances. Transformers are used to change the potential difference, enabling efficient transmission over long distances.
A step-up transformer is used to increase the potential difference, thereby decreasing the current, as high currents would cause heating in the wires, resulting in energy losses to the surroundings.
Wires/cables have resistance and thus power is lost in them due to heating, calculated by the equation, . Low currents result in less power loss in the wires due to heating. The power transmitted from the power station is calculated using the equation, . Therefore, the higher the potential difference, the lower the current transmitted in the cables.
A step-down transformer is used to decrease the potential difference, as high voltages are dangerous to consumers.
Remember: In the UK, standard mains electricity operates at with an AC frequency of .
Transformers only work with alternating current as they rely on electromagnetic induction. A transformer consists of a primary coil and a secondary coil wound around the same iron core. The primary coil is connected to a source of alternating current/alternating potential difference, which creates an alternating magnetic field in the primary coil which is carried by the iron core (iron is an induced magnet). The alternating magnetic field cuts the secondary coil, inducing an emf in the secondary coil due to Faraday’s law.
From GCSE, we know that a step-up transformer has more coils of wire on its secondary arm, and we can calculate the voltage induced across the secondary arm by calculating the ratio of the number of turns of coil on each arm.
Formula:
Worked Example
Complete the gaps to describe how a transformer works:
______________________ in the primary coil, creates a changing magnetic field. This is guided by the iron core to the secondary coil. The alternating magnetic field in the secondary coil ____________________________ proportional to the rate of change of flux. If the circuit is complete, alternating current is generated.
Answer:
The ideal transformer
A transformer with perfect efficiency has no energy loss.
Unfortunately, transformers are not efficient due to eddy currents.
An eddy current is a current set up in a conductor in response to a changing magnetic field. They flow in closed loops in a plane perpendicular to the magnetic field. If the changes in the magnetic field are rapid, very large eddy currents and significant heat is generated.
To increase the efficiency of the transformer by reducing eddy currents, the iron core is laminated, whereby layers of iron are separated with layers of insulator (eg plastic). The core is made of soft Iron as it is easily magnetised and demagnetised, further reducing wasted power.
The efficiency of a transformer can be determined by calculating the ratio of the power output on the secondary side to the power input on the primary side.
Formula:
Worked Example
Complete the gaps to describe how a transformer are made more efficient:
The core is _______________________________to reduce _____________ currents on the surface of the iron core and hence increase efficiency. A ______ iron core must be used so that the direction of magnetisation changes without delay.
Answer:
Worked Example
A transformer with an efficiency of has turns on its primary coil and turns on its secondary coil. When the primary coil of the transformer is connected to a AC supply, the secondary current is .
Calculate the primary current and secondary voltage.
Answer:
Root Mean Square
As transformers operate using alternating current, it is helpful to know what direct current would give the same power output. This is known as the Root mean square current.
Formula:
Root mean square voltage
Root mean square voltage of an alternating voltage is the direct voltage which has the same power as the alternating voltage.
Formula:
Worked Example
The oscilloscope trace shows an alternating voltage. On the y-axis the scale is .
Calculate the following:
Time period
Frequency
Peak voltage
Root mean square value
Answer:
Practice Question
Explain how a transformer works.
-> Check out Brook's video explanation for more help.
Answer:
Alternating current in the primary coil creates a changing magnetic field.
The soft iron core carries the alternating magnetic field to the secondary coil.
The changing magnetic flux induces an emf in the secondary coil.
To increase efficiency, the core is laminated to prevent eddy currents, reducing loss of energy due to heat.