176 



ALTERNATING CURRENTS 



producing the flux <. (See Par. 80.) This quadrature current is 

 called the exciting or magnetizing current of the transformer. As 

 the energy current which is in phase with the back emf . is small, the 

 quadrature current is very nearly equal, numerically, to the total 

 no-load current. Therefore, the no-load current is often called 

 the exciting current of the transformer. The back emf. is nearly 

 constant for all loads, as it differs from the terminal voltage only 

 by the primary impedance drop, which is small. Therefore the 

 flux and hence the exciting current are practically independent 

 of the load. 



This exciting current produces a flux < in the core, the direction 

 of the flux being as shown (corkscrew rule). The value of this 

 flux must be such as to make the induced primary emf. practically 



FIG. 172. Simple transformer, load applied to secondary. 



equal to the primary line voltage. This primary induced emf. 

 is a back emf. and is therefore in opposition to the primary im- 

 pressed voltage. 



Now apply a load to the secondary. As a result a current 1% 

 flows in the secondary. The direction of this current must be 

 such as to oppose the flux <. This is in accordance with Lenz's 

 law that an induced current always has such a direction as to 

 oppose the cause which produces it. If the secondary current 1 2 

 were producing the flux <, then by the corkscrew rule the current 

 would flow in at the upper terminal, Fig. 172. Since 7 2 opposes 

 the flux </>, it must actually flow out at the upper terminal. The 

 secondary current 7 2 then tends to reduce the value of the flux 

 in the transformer core. If the flux is reduced, the back electro- 

 motive force of the primary is also reduced, and hence more 

 current will flow in the primary to supply the increase in power 

 due to the load on the secondary. This is the sequence of reac- 



