Page 47 - Electric Machinery Fundamentals

INTRODucnON TO MACHINER.Y PRINCIPLES 23

7000

6000
v ~
/
5000
'Vi II '\
~
0
.~ 4000 "-
~ '\
:§.
:t "-
3000

2000
"" i'-
( "
1000

a
10 20 30 40 50 100 200 300 500 1000
Magnetizing inlensity H (A • turns/m)
(d)

FIGURE 1-10
(conlinued)

between flux and magnetomotive force. The slope of the curve of flux density ver-
sus magnetizing intensity at any value of H in Figure 1- 1 Db is by definition the per-
meability of the core at that magnetizing intensity. The curve shows that the penne-
ability is large and relatively constant in the unsaturated region and then gradually
drops to a velY low value as the core becomes heavily saturated.
Figure I- IOc is a magnetization curve for a typical piece of steel shown in
more detail and with the magnetizing intensity on a logarithmic scale. Only with
the magnetizing intensity shown logarithmically can the huge saturation region of
the curve fit onto the graph.
The advantage of using a ferromagnetic material for cores in electric ma-
chines and transformers is that one gets many times more flux for a given magne-
tamotive force with iron than with air. However, if the resulting flux has to be pro-
portional, or nearly so, to the applied magnetamotive force, then the core must be
operated in the unsaturated region of the magnetization curve.
I Since real generators and motors depend on magnetic flux to produce volt-
age and torque, they are designed to produce as much flux as possible. As a result,
most real machines operate near the knee of the magnetization curve, and the flux
in their cores is not linearly related to the magnetomotive force producing it. This