A transformer is a stationary electrical device that uses the principle of electromagnetic induction to convert alternating current (AC) at one voltage level into AC at another voltage level of the same frequency. In power systems, transformers are critical electrical components that play a vital role in the economical transmission, flexible distribution, and safe use of electrical energy. In addition, transformers are widely used in electrical testing, control systems, and specialized electrical equipment.
Transformers operate based on the principle of electromagnetic induction. Consequently, their structure consists of two (or more) mutually insulated windings wound around a common iron core, interconnected via a magnetic circuit. Thus, like rotating motors, transformers also rely on a magnetic field as a medium. One of the two windings is connected to an AC power source and is called the primary winding, while the other is connected to the load and is called the secondary winding. When the primary winding is connected to an AC power source, an AC current flows through it under the influence of the applied voltage, generating an alternating magnetic flux in the iron core at the same frequency as the applied voltage. This alternating magnetic flux links both the primary and secondary windings; according to the law of electromagnetic induction, an electromotive force is induced in the secondary winding. With this electromotive force, the secondary winding supplies power to the load, thereby transferring energy.
The figure shows a schematic diagram of a single-phase transformer operating under no-load conditions. It consists of two windings with different numbers of turns wound around a closed core. The core is constructed from laminated silicon steel sheets. The winding on the left side of the core is called the primary winding (also known as the first winding or main winding), with N₁ turns. The winding on the other side is called the secondary winding (also known as the secondary winding or auxiliary winding), with N₂ turns.
When the secondary side is open-circuited and the primary side is connected to an AC voltage U₁, a current I flows through the primary winding. This current is commonly referred to as the no-load current. The no-load current generates a no-load magnetic force, resulting in a magnetic flux Φ passing through the core. At this point, induced emf is generated on both the primary and secondary sides.
For the primary side:
E₁ = 4.44fN₁Φm (1-1)
For the secondary side:
E₂ = 4.44fN₂Φm (1-2)
Where E₁ - self-induced emf (V);
E₂ - mutual-induced emf (V);
f - power supply frequency (Hz); N₁ - number of turns in the primary winding;
N₂ - number of turns in the secondary winding;
Φm - maximum value of the alternating main magnetic flux (Wb).
Under no-load conditions, the terminal voltage on the secondary side is U₂ = E₂, and the voltage ratio between the two windings is
where Ku - voltage ratio of the transformer.
When N₁ > N₂, Ku > 1; in this case, U₁ > U₂, meaning the transformer's output (terminal) voltage is lower than the input (feed) voltage. Such a transformer is called a step-down transformer. When N₁ < N₂, Ku < 1; in this case, U₁ < U₂, meaning the transformer's output voltage is higher than the input voltage. Such a transformer is called a step-up transformer.
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