Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.): 8. Transformer: 8.2. Operational behaviour of a transformer: 8.2.3. Loaded voltage behaviour

Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)

8. Transformer

8.2. Operational behaviour of a transformer

		8.2.1. Idling behaviour Idling features
		8.2.2. Short-circuit behaviour
		8.2.3. Loaded voltage behaviour
		8.2.4. Efficiency

8.2.3. Loaded voltage behaviour

In contrast to operational idling, during loading the secondary circuit is closed through an external resistance Z_a (Figure 126). Secondary current I₂ flows. According to the energy conservation law the transformer must also take up commensurate primary power, thus a primary current I₁ also flows.

Primary circuit	U₁ is applied
	I₁ > I₀
Secondary circuit	Z_a < ¥
	I₂ > 0
	U₂ ¹ U₂₀

Voltage curve U₂ = f (I₂)

As the curve in Figure 133 shows, terminal voltage U₂ decreases during loading.

Figure 133 - Voltage behaviour during loaded operation U₂ = f (I)₂

1 U_K small, 2 U_K big

Figure 134 depicts the duplicate circuit diagram for the loaded transformer.

Figure 134 - Duplicate circuit for the loaded transformer with a transformation ratio = 1:1

The duplicate circuit diagram corresponds to a transformer with a transformation ratio

As rated current flows the short-circuit voltage U_K decreases at the internal transformer resistance Z_i as a result of which the terminal voltage U₂ declines by the power decrease of the short-circuit voltage U_K.

Transformers with considerable short-circuit voltage U_K have powerful internal resistors, that is to say pronounced voltage changes as load alters.

U_K = 2...10%	minimal voltage losses
	voltage-rigid behaviour
U_K = 20...50%	considerable voltage losses
	voltage-flexible behaviour

Example:

A 220/42 V transformer has a short-circuit voltage of 10%.

How great is the voltage change between idling and rated current loading?

Solution:

Output voltages at differing loads

Given differing loads with ohmic, inductive or capacitive external resistance gives rise to the dependence of output voltage on load current as shown in Figure 135.

Figure 135 - Secondary terminal voltage depending on the degree and nature of loading

1 Idling, 2 Rated load

Given capacitive load, the output voltage may even be greater than no-load voltage.

The output voltage of a transformer depends on the

- degree of load current I₂
- the magnitude of relative short-circuit voltage
- the nature of the load (ohmic, inductive or capacitive).