Electrical System
Motoring mode
Electrica1
MechanicaI
Machine
System
Pelec - - -
Generating mode
Pele<:
Pmech
Figure 11-5 Electric machine as an energy converter. There are two basic principies that govern electric machines' operation to convert between electric energy and mechanical work: I) A force is produced on a current-carrying conductor when it is subjected to an externally-established magnetic field. 2) An emf is induced in a conductor moving in a magnetic field. 11-3-1 Electromagnetic Force Consider a conductor of length R in Fig. 11-6a. The conductor is carrying a current i and is subjected to an externally-estab/ished magnetic field of a uniforrn flux-density B perpendicular to the conductor length. A force f.m is exerted on the conductor due to the electromagnetic interaction between the external magnetic field and the conductor current. The magnitude ofthis force
/ ,m
is given as (I 1-9)
f.m=Bi R y .........
~
lN)
ITJ[A}[m)
As shown in Fig. 11-6a, the direction of the force is perpendicular to the directions of both i and B. To obtain the direction of this force, we will superimpose the flux lines produced by the conductor current, which are shown in Fig. 11-6a. The flux lines add up on the right side of the conductor and subtract on the left side, as shown in Fig. I 1-6b. Therefore, the force f.m acts from the higher concentration of jlux /ines
10
the lower
concentralion, that is, from right to left in this case. externa! B field
v f.m
f \
"
f-
"
~) ) r-
f,.
/ resuhant
(b)
Figure 11-6 Electric force on a current-carrying conductor in a magnetic field.
11-5