Chapter 5A.C. Machines
Pages 36

Figure 5.1 shows one complete cycle of the fl ux waveform in the core of a transformer. From this diagram the following statements can be made:

Total change of fl ux m m m( ) weber2

time taken for this change T 2


therefore average rate of change of fl ux 2 2

4 m m T T/


in general, e N d f /d t volt, so for either winding:

average emf induced 4 1N T

T fm volt, and since / , then

average emf induced 4m Nf volt and since we are dealing with sinewaves, having a form factor of 1.11, the r.m.s. emf induced is given by:

E Nfm 4 44. volt (5.1)


1 The above equation applies to both primary and secondary induced emfs. For the primary, read Ep and Np , and for the secondary, Es and Ns .