Environmental Engineering Reference
In-Depth Information
A
B
C
a
b
c
Frequency converter
to
A
to
B
to
C
Figure 5.40 Schematic of Hunt winding, self-cascade induction machine
Subsequent applications of the Hunt winding have led to numerous develop-
ments of a class of doubly fed induction and most recently, doubly excited reluc-
tance machines [28-30]. The interested reader is referred to those references.
5.4.3.2 Electronic pole change
The concept of electronic pole changing has been described by various authors
since the early 1970s after the availability of bipolar electronic switches. The fact
that many industrial applications require operation over vastly different speed
regimes had been the early motivation for such research. More recently, and
especially after the early years of research and development on battery electric
vehicles, it became important to extend the limited CPSR of IMs to enable high
torque for vehicle launch and grades, yet maintain sustainable power at relatively
high speeds. The work by Osama and Lipo in the mid-1990s was one such example
of electronic pole changing in which contactorless changeover was realized by
purely electronic control of machine currents [31].
In their work, Osama and Lipo described a contactorless pole changing tech-
nique that was capable of extending the field weakening range of a 4-pole IM. The
machine itself was wound with six coil groups (e.g. phases) with two sets of three
phases each connected to their respective power electronics inverters as shown
schematically in Figure 5.41.
The machine described in Figure 5.41 must be designed to sustain the stator
and rotor flux of its lowest pole number operating mode. For example, if the motor
is a conventional 4-pole IM and 2-pole operation is required, the injected currents
must conform with the values given in Table 5.8. In this table, a minus sign sig-
nifies polarity reversal at the inverter group leg associated with that coil group. If
a single dc supply is used as is shown in the figure, then the neutrals of the two
3-phase groups must remain isolated. That means that neutrals in group (1, 3, 5)
must be isolated from the group (2, 4, 6). Furthermore, the rotor of the IM used for
