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

#### 8.1.2. Voltage transformation

A few field lines already close before reaching the output coil (Figure 125) so that flow Φ1 can be divided into a maximum flow ΦK which saturates both coils and a leakage flow ΦS.

The leakage flow may be ignored in regard to the unloaded transformer (idling). Therefore the following applies:

According to the transformer equation

and

.

If we relate both equation then

Shortening gives us

During idling no current flows into the output winding, thus there is no voltage decrease. Consequently the induced voltage U20 equal to the terminal voltage U2 (Cp Figure 125):

Figure 125 - Transformer principle

1 Input winding/upper voltage winding/primary winding, 2 Output winding/under voltage winding/secondary winding

U20 = U2

In the event of minimal idling current I voltage decrease in the input winding is negligibly minimal. We therefore have

U10 = U1

which results in

The voltages behave like the numbers of turns.

The interrelationship of the numbers of turns is known as the transformation ratio Ü. We have:

The rated voltages U1n and U2n are indicated on the rating plate of the transformer.

Example:

What secondary terminal voltage arises in a transformer where 380 V is applied to the primary winding of 980 turns and the secondary winding has 594 turns?

Given: U1 = 380 V; N1 = 980; N2 = 594

Sought: U2

Solution:

U2≈ 230 V