Transformer external characteristic

The dependence of the secondary voltage U2 on the current I2 at a fixed voltage U1, with cosφ = const



The higher the load current I2, the lower the voltage U2, but within changes, that is, below by a few%.

Load characteristics



P2 - active power given to the consumer,
P1 - active power received from the network source
low-power transformers
electric furnace transformers have



from the experience with the load




- electrical losses
- magnetic losses depend on the load factor

The external characteristic of the transformer is the dependence of the voltage of the secondary winding U ₂ on the current I ₂ . If we ignore the no-load current, then the equivalent circuit of a transformer in nominal mode takes the form shown in Figure 1.

Figure 1 The equivalent circuit of a transformer for the construction of its external characteristics

Considering that Z _kz = R _kz + jX _kz , the voltage on the secondary winding of the transformer can be expressed as follows:

(one)

Construct a vector diagram relative to the current vector as shown in Figure 2.

Figure 2 Vector diagram of currents and voltages of the transformer secondary winding

If the load resistance active then the voltage coincides in phase with the current . Amount . The magnitude of the applied voltage U ₁ = const , it is obvious that when the current changes , the voltage drop Δ U on the internal resistance of the transformer will also change. That is, the external characteristic has a falling character, as shown in Fig. 2.25.