Civil Engineering Reference
In-Depth Information
FIGURE 6-3
Squirrel cage rotor of the induction machine under rotating magnetic field.
the induction machine. The angular speed of the rotating magnetic field is
called the synchronous speed. It is denoted by N
and is given by the following:
s
f
p
(
)
N
=⋅
60
revolutions per minute rpm
(6-1)
S
where f = frequency of the stator excitation
p = number of magnetic pole pairs.
The stator coils are embedded in slots of high-permeability magnetic core
to produce the required magnetic field intensity with low exciting current.
The rotor, however, has a completely different structure. It is made of solid
conducting bars embedded in the slots of a magnetic core. The bars are
connected together at both ends by the conducting end rings (
Figure 6-3
).
Because of its resemblance, the rotor is called the squirrel cage rotor, or the
cage rotor in short.
6.2.2
Working Principle
The stator magnetic field is rotating at the synchronous speed determined
by Equation 6-1. This field is conceptually represented by the rotating mag-
nets in
Figure 6-3
.
The relative speed between the rotating field and the rotor
induces the voltage in each rotor turn linking the stator flux
φ
. The magnitude
