Transformer and Shielding

Transformer

The device which transfers AC electrical power from one circuit to another at a constant frequency with varying voltage is called a Transformer. The voltage can be increased or decreased based on the necessity. The principle on which the transformer works is electromagnetic induction.

Working of a transformer

A transformer has a primary coil and a secondary coil wound on the same laminated soft iron core. The voltage to be varied is applied across the primary coil and the output voltage is got across the secondary coil. The voltage across the primary coil is Vp and the voltage across the secondary coil is Vs. n_p and n_s are the numbers of turns in the primary and secondary coils respectively. Then we get the equation

= =T, here T is called the turns constant

If the value of T＞1, then n_s＞n_p and Vs＞Vp

The output voltage is greater than the input voltage. This type of transformer is called a step-up transformer.

If the value of Tく1, then n_sくn_p and VsくVp

The output voltage is greater than the input voltage. This type of transformer is called a step-down transformer.

For an ideal transformer, the power input must be equal to the power output.  The power input is given by VpIp and the power output is given by VsIs. Here Ip and Is are the currents in the primary coil and secondary coil respectively.

Reasons for power loss in a transformer

• Power loss due to heating: The primary and secondary coils have a certain amount of resistance which produces heat in the circuit. This can be reduced by using wires of suitable thickness.
• Flux leakage: The magnetic flux passing through every turn of the primary coil may not pass through the secondary coil. This can be reduced by winding the coils in a specific manner.
• Eddy current loss: The change in magnetic flux in the core gives rise to induced current on the surface of the core. These currents are eddy currents and these currents heat the core. Eddy currents can be reduced by laminating the core and using an insulating material to separate them.
• Hysteresis loss: As the core undergoes magnetization and demagnetisation many times they get heated up. Materials with narrow hysteresis loop are used to reduce power loss due to hysteresis

Transformer Shielding

A grounded metallic barrier between the primary coil and the secondary coil of the transformer is called shielding. The metallic shield reduces the capacitive coupling and improves the transformers ability to reduce high-frequency noise. Unwanted noise produced in the transformer is diverting to the ground using a shield. Any magnetically permeable material usually copper can be used as the shielding material. The shield is a single turn of conductive foil. The enclosing shield provides a very effective noise reduction by not reradiating the noise signal. The other advantage of using a shield in a transformer is to reduce the lightning spike. If a bolt of lightning strikes the primary winding it creates a spike of 1000 volt. The shield takes the majority of the spikes to the ground and allows only 10 volts to pass through.