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

8.2.1. Idling behaviour Idling features

A transformer idles where mains voltage U1 remains applied to the primary side whilst no consumer is connected to the secondary side (Za) (Figures 125/126).

 Primary circuit U1 applies I0 flows (idling current) Secondary circuit Za = ¥ I2 = 0 U2 = U20

Idling current

The applied voltage U drives the idling current I0. This is needed to establish the magnetic field Iµ. This lags behind the voltage U1.

Figure 127 - Indicator image for idling operation

1 Iron loss current IFe

The phase position of the idling current I0 to voltage U1 can be determined according to the circuitry of Figure 128.

Figure 128 - Circuitry to determine idling losses

1 Rated voltage

The value of idling current I0 is between 2 and 5 per cent of idling current in big transformers and up to 15 per cent in smaller transformers.

The idling curve I = f (U1) in Figure 129 indicates that no-load current I0 increases proportionally to the input voltage U1. No-load current increases markedly over and beyond the input rated speed U1n. It can, moreover, even attain values greater than the rated current.

Figure 129 - Idling curve of a transformer I0 = f (U1)

Transformers shall not be driven by voltages greater than the rated voltage.

Idling losses (iron losses)

The active power derived from the circuit in Figure 128 can only be transformed into heat in the input winding and iron core as no current flows into the secondary winding during idling. The active power P0, which is converted into heat in the iron core, is made up of eddy current and hyteresis loss.

The following example shows that the iron losses almost always arise during idling.

Example:

The following idling values were measured in a transformer:

U1n = 220 V; I0 = 0.5 A; P0 = 40 W; R1 = 3.

What percentage of winding losses are contained in idling power?

Solution:

P0 = PVFe + PVW

PVW = 0.75 W

PVFe = P0 - PVW = 40 W - 0.75 W = 39.25 W

Thus, the power loss determined during idling is an iron loss.

Iron losses are determined during no-load operation and are independent of load.