i8o 



TRANSFORMERS. 



[Exp. 



4. Transformer on Open Circuit. When a transformer is on 

 open circuit, the secondary winding has no current flowing in 

 it and it accordingly has no magnetizing effect on the core. 

 A small current flows in the primary which magnetizes the 

 core. Let us see what determines the magnitude and phase of 

 this open-circuit primary current. 



5. Assuming No Core Loss. The open-circuit diagram for 

 a perfect transformer, in which there are no losses, is shown 

 in Fig. i. The primary electromotive force EP causes a current 

 7 to flow and this current sets up a flux <. This flux, being 

 alternating, causes a counter-electromotive force opposed to the 

 primary impressed electromotive force. When the primary cir- 

 cuit is closed, the current / , and the flux </> which it sets up, 

 assume such values that the counter-electromotive force is just 

 equal* to the impressed electromotive force. 



This primary counter-electro- 

 motive force has, at any instant, 

 the value e' = S^(d^ -=- dt), 

 the equal and opposite im- 

 pressed electromotive force be- 

 ing ep = S 1 (d<f>-r- dt). It will 

 be seen that the electromotive 

 force is zero when the flux 

 is a maximum and that the flux 

 < lags 90 behind the impressed 

 electromotive force Ep, as in 

 Fig. i. 



6. In the absence of core 

 loss, the current 7 is in phase 

 with the flux <, which it produces. When permeability is constant, 

 magnetizing force H is proportional to / and is in phase with 



* The primary resistance on open circuit is very small and can be 

 neglected. 



_ Phase of 

 ~J? and H 



FIG. i. Open-circuit diagram for a 

 transformer with no core loss. 



