Page 36 - Electric Machinery Fundamentals

12 ELECTRIC MACHINERY FUNDAMENTALS

/ /

rr=~-
i

N

II

FIGURE 1-5
Determining the polarity of a magnetomotive force source in a magnetic circuit.

Like the voltage source in the electric circuit, the magnetomotive force
in the magnetic circuit has a polarity associated with it. The positive end of the rnmf
source is the end from which the flux exits, and the negative end of the mmf source
is the end at which the flux reenters. The polarity of the mmf from a coil of
wire can be determined from a modification of the right-hand rule: If the fingers
of the right hand curl in the direction of the current flow in a coil of wire, then
the thumb will point in the direction of the positive mmf (see Figure 1- 5).
In an electric circuit, the applied voltage causes a current I to flow. Simi-
lar1y, in a magnetic circuit, the applied magnetomotive force causes flux 4> to be
produced. The relationship between voltage and current in an electric circuit is
Ohm's law (V = IR); similarly, the relationship between magnetomotive force and
flux is

(1- 28)

where
g = magnetomotive force of circuit
4> = flux of circuit
CQ = reluctance of circuit

The reluctance of a magnetic circuit is the counterpart of electrical resistance, and
its units are ampere-turns per weber.