Page 676 - Mechatronics with Experiments

662 MECHATRONICS

+X –X

(a) Magnetic field set-up (b) Magnetic field set-up
when phase -1 is in positive when phase -1 is in negative
cycle cycle
+Y

–Y
(c) Magnetic field set-up (d) Magnetic field set-up
when phase -2 is in positive when phase-2 is in negative
cycle cycle

FIGURE 8.38: AC induction motor operating principle: a two-phase motor example.

phase is a sinusoidal function of time. The induced magnetic field changes magnitude
and direction as a function of current in the phase. It is basically a pulsating magnetic
field in the X direction (Figure 8.38a,b). Next, let us consider the same for phase 2 which
is spatially displaced by 90 degrees from the first phase. The same event occurs except
the direction of the magnetic field is in the Y direction (Figure 8.38c,d). Finally, if we
consider the case where both phases are energized but by 90 electrical degrees apart and
same frequency, the magnetic field would be the vector addition of two fields and it rotates
in space at the same frequency as the excitation frequency (Figure 8.39). Therefore, we
can think of the magnetic field as having a certain shape (that is distribution in space as a
function of rotor angle) as a result of the winding distribution and current, and it rotates in
space as a result of the alternating current in time. In other words, the flux distribution is a
rotating wave.
In a three-phase motor, the windings would be displaced by ± 120 degrees spatially
and electrically excited with the same frequency source except ± 120 electrical degrees
apart. This rotating magnetic field, generated by the stator winding voltage, induces voltage
in the rotor conductors. As a result of Faraday’s induction law, the induced voltage is
proportional to the time rate of change in the magnetic flux lines that cut the rotor. In other
words, if the rotor was rotating mechanically at the same speed as the electrical rotating
speed of the stator field, there would not be any torque generated.