Environmental Engineering Reference
In-Depth Information
gate drivers. Furthermore, topology 'C' has low conduction losses and is amenable
to solid-state integration. A disadvantage of 'C' may be the need for switching
snubbers.
5.3
Interior permanent magnet
There has been a great deal of writing on IPM machines during the past three
decades since their inception as an energy conservation improvement of line-start
IMs. During the 1970s, the buried magnet machine was subject to intense research,
leading to it being proposed as an alternative to high efficiency IMs in low power
applications of 2-25 horsepower (hp) [7,8]. Conventional line-start IMs suffer from
continuous
I
2
R
losses in the rotor and the consequent
I
2
R
losses in the stator
necessary to supply the rotor magnetizing currents. This, coupled with the avail-
ability of improved ferrite magnets, and then of NdFeB rare earth permanent
magnets, contributed to the increased interest in a line-start buried magnet machine
for commercial and industrial low power applications. In this early work, attention
was focused on inrush current demagnetizing effects on the buried magnets, par-
ticularly at elevated temperatures. Ferrite magnets were susceptible to demagneti-
zation effects at cold temperatures, and rare earth magnets were susceptible to
demagnetization effects at hot temperatures in a line-start application.
In recent years, attention has shifted to use of the IPM as the machine of choice
for electric traction applications, particularly in the power split hybrid propulsion
architecture. This choice is motivated by the IPM's wide CPSR under field
weakening control and the inherent need for wide CPSR in the power split archi-
tecture [9-20].
The most pervasive application of IPMs has been in white goods applications,
such as refrigerators, washing machines and other household appliances, where the
losses noted above from IMs were counter-productive in an energy conscious
environment and where the durability issues of brush-type universal motors are
questionable. The IPM gives the white goods designer the flexibility to eliminate
rotor losses, reduce stator losses and realize approximately 10% additional torque
from the reluctance component inherent in the IPM. In applications where adjus-
table speed is required, such as the appliances noted as well as in air conditioning
equipment, these benefits are cost-effective.
IPMs today fall into two broad categories depending on the permanent magnet
employed: weak magnet IPMs and strong magnet IPMs. The strong magnet IPM
may be more suitable to line-start applications such as large fans and industrial
equipment for which asynchronous start-up and synchronous running is beneficial.
Since continuous operation is at a synchronous speed, the magnets can be sized to
provide ac synchronous machine performance at near unity power factor at rated
conditions. Traction drives, on the other hand, have gravitated to the weak magnet
IPM. This has been a somewhat surprising trend because a weak magnet IPM is in
reality a VRM or, more precisely, a reluctance machine that happens to have some
magnet content. The reasons for
this are threefold:
(1) hybrid propulsion
