Page 108 - Electric Machinery Fundamentals

84 ELECTRIC MACHINERY FUNDAMENTALS

FIGURE 2-13
The total excitation current in a transformer.

• •
+ +

FIGURE 2-14
A real transfonner with a load connected to its secondary.

while a current flowing into the undotted end of a winding produces a negative
magnetomotive force. Therefore, two currents flowing into the dotted ends of
their respective windings produce magnetomotive forces that add. If one CUlTent
flows into a dotted end of a winding and one flows out of a dotted end, then the
magnetomotive forces wil1 subtract from each other.
In the situation shown in Figure 2-14, the primary current produces a posi-
tive magnetomotive force gjip = Np ip• and the secondary current produces a neg-
ative magnetomotive force CJf s = - Nsi . Therefore, the net magnetomotive force
s
on the core must be
(2- 31)