Civil Engineering Reference
In-Depth Information
FIGURE 6-6
Equivalent electrical circuit of induction machine for performance calculations.
Under the steady state operation at slip “s”, the induction generator has
the following operating speeds:
•
stator flux wave speed
N
s
•
rotor mechanical speed
N
= (1 - s) · N
r
s
•
stator flux speed with respect to rotor
s · N
s
•
rotor flux speed with respect to stator
N
+ s · N
= N
(6-5)
r
s
s
6.2.4
Equivalent Circuit for Performance Calculations
The theory of operation of the induction machine is represented by the
equivalent circuit shown in
Figure 6-6
.
It is similar to that of the transformer.
The left-hand side of the circuit represents the stator and the right hand side,
the rotor. The stator and the rotor currents are represented by I
,
respectively. The vertical circuit branch at the junction carries the magnetiz-
ing (or excitation) current I
and I
1
2
, which sets the magnetic flux required for the
electromagnetic operation of the machine. The total stator current is then the
sum of the rotor current and the excitation current. The air-gap separation
is not shown, nor is the difference in the number of turns in the stator and
rotor windings. This essentially means that the rotor is assumed to have the
same number of turns as the stator and has an ideal 100 percent magnetic
coupling. We calculate the performance parameters taking the stator winding
as the reference. The actual rotor voltage and current would be related with
the calculated values through the turn ratio between the two windings. Thus,
the calculations are customarily performed in terms of the stator, as we shall
do in this chapter. This matches the practice, as the performance measure-
ments are always done on the stator side. The rotor is inaccessible for any
routine measurements.
o
