Environmental Engineering Reference
In-Depth Information
Interior permanent magnet, or buried magnet design
. The original IPM, known
variously as buried magnet, or interior PM machine, consisted of a single
magnet layer, tangentially oriented along chord lines in the rotor, when it was
first conceived
1
about 30 years ago. The buried magnets provide magnetization
of the stator and minimize the reactive power needed. This design has been used
for line-start appliance adjustable speed applications because of its high power
factor and good torque performance.
●
Asynchronous or induction machine design - the industrial workhorse electric
.
The induction design dates to 1888 when Nikola Tesla first conceived of what
he termed his 'current lag' motor [3]. Trapped in his fourth floor room at the
Gerlach Hotel in New York City during the blizzard of 1888 - the location of
his newly formed Tesla Electric Company funded by venture capitalists when
he resigned from the Edison Electric Company just 3 days earlier - he sketched
out the design of the world's first asynchronous electric machine. His design
was that of an alternating current, 3-phase asynchronous machine - the IM.
●
Interior permanent magnet - flux squeeze design with radial PMs.
Rather than
burying the rotor permanents beneath a soft iron pole shoe, the magnets can
either be inset into the surface with the interpolar gap filled with soft iron, or they
can be sandwiched between larger soft iron wedges as shown in Figure 5.16(d).
This design is particularly attractive for ceramic magnet machines because of
the flux squeezing effect at the airgap of flux collected from the larger surface of
the magnets and effectively focused into the rotor to stator air gap.
●
Synchronous reluctance (SyncRel design).
By simply swapping out the cage or
wound rotor of an IM and inserting a rotor of laminated saliencies, one obtains
the synchronous reluctance design. These 'rain-gutter' style laminations have
low reluctance to stator flux in the direct axis, but high reluctance to flux in its
quadrature axis. The SyncRel machine has received renewed interest in recent
years for use in machine tools and factory automation because of its inert rotor
and synchronous torque-speed control.
●
Variable reluctance machine
. When both stator and rotor laminations are sali-
ent, a class of doubly salient machines is realized. The reluctance machine is
renowned for its completely inert rotor and easy to install bobbin wound stator
coils. The VRM has the power density of IMs but continues to have audible
noise problems unless the stator back iron is reinforced, for example, by being
excessively thick, or includes bosses or other structural enhancements.
●
Electric machines with smooth stators, that is, slotted designs with distributed
windings, can be either 'copper dominated' or 'iron dominated'. When the iron
fraction, or ratio of stator tooth width to stator tooth pitch,
b ¼
W
t
/
t
s
>
0.55, the
machine can be called iron dominated. For the most part, hybrid propulsion
machines are iron dominated in order to operate at magnetic loading values
>
0.7 T.
This trend results from the fact that hybrid propulsion M/Gs have high peak to
1
US Patent 4,139,790 by C.R. Steen, 'Direct Axis Aiding Permanet Magnets for a Laminated Syn-
chronous Motor Rotor', Issued, February 13, 1979.
