                            Electrical Machines - Basic Vocational Knowledge (IBE - Deutschland; 144 pages)  Introduction  1. General information about electrical machines  2. Basic principles  3. Execution of rotating electrical machines  4. Synchronous machines  5. Asynchronous motors  6. Direct current machines  7. Single-phase alternating current motors  8. Transformer   8.1. Transformer principle    8.1.1. Operating principle of a transformer    8.1.2. Voltage transformation    8.1.3. Current transformation   8.2. Operational behaviour of a transformer   8.3. Three-phase transformer

#### 8.1.1. Operating principle of a transformer

Transformers are stationary electrical machines which transmit energy from systems with certain current and voltage values into systems with generally different current and voltage values but with identical frequency.

Two separate windings are on the same iron core.

Following connection to alternating voltage U1 there is a standstill current I0. The magnetomotive force Θ = I0• N1 generates a magnetic alternating flow (Φ1) in the iron core.

The input and output winding of an alternating voltage are induced in accordance with the induction law. A self-induction voltage U10 arises in the input winding. It is counter-positioned in accordance with Lenz’s law on applied voltage. During idling operation - because of mutual induction - there arises the output voltage U20 which is simultaneously the terminal voltage U2.

U1~→ I0~→ Θ0~→ Φ1~→ U20~

The value of the induced voltage is derived from the following equation:  max. flow density AFe limb cross-section U0 induction voltage f frequency N number of turns
The induction voltage increases along with the number of turns, the magnetic flow density in the iron core, the iron cross-section and the frequency.

Example:

Which maximum flow density occurs in an iron core of 16 cm2 cross-section when a voltage of 380 V (50 Hz) is applied to the primary coil with 980 turns?

Given: AFe = 16 cm2; N1 = 980; U1 = 380 V; f = 50 Hz

Sought: Solution:     ≈ 1.09 V • s • m-2 ≈ 1.09 T

The iron core evidences a maximum flow density of 1.09 T.   