12.3 ROTATING MAGNETIC FIELDS 525
Phase a
Phase a
Phase b
Phase b
Phase c
Phase c
Total
(a)
(b)
Total
θ
− 90 ° − 30 ° 0 ° 30 ° 90 ° 150 ° 210 ° 270 °
θ
150 ° 210 ° 270 °
120 °
30 ° 90 °
− 30 °
0 °
− 90 °
F
Instant in time
ia
ib
ic
Instant in time
ic
ia
ib
Figure 12.3.2Generation of a rotating mmf.(a)Spatial mmf distribution at the instant in time when the
a-phase current is a maximum.(b)Spatial mmf distribution at the instant in time when theb-phase current
is a maximum.
rotating in opposite directions at the same angular velocity given bydθ/dt=ω. The vertical
components of the two rotating vectors in Figure 12.3.3(b) always cancel, and the horizontal
components always yield a sum equal toBmcosωt, the instantaneous value of the pulsating
vector.
This principle is often used in the analysis of single-phase machines. The two rotating fluxes
are considered separately, as if each represented the rotating flux of a polyphase machine, and
the effects are then superimposed. If the system is linear, the principle of superposition holds
and yields correct results. In a nonlinear system with saturation, however, one must be careful in
reaching conclusions since the results are not as obvious as in a linear system.