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

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 (Z_a) (Figures 125/126).

Primary circuit	U₁ applies
	I₀ flows (idling current)
Secondary circuit	Z_a = ¥
	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 I_Fe

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.

No-load curve

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 U_1n. 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:

U_1n = 220 V; I₀ = 0.5 A; P₀ = 40 W; R₁ = 3.

What percentage of winding losses are contained in idling power?

Solution:

P₀ = P_VFe + P_VW

P_VW = 0.75 W

P_VFe = P₀ - P_VW = 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.