Environmental Engineering Reference
In-Depth Information
fields of the order of 2.8 MA/m (35 kOe, i.e. cgs units are more common in the
magnet industry). The second reason is that NdFeB magnets have relatively high
bulk conductivity so that the fast magnetizing transient induces high levels of eddy
currents into the magnet slab, thus inhibiting penetration of the magnetizing flux.
Magnetizers for NdFeBmagnets tend to require higher pulse durations (higher stored
energy) to ensure sufficient levels of magnetization. Regardless of the magnet
material, if the magnetizer does not have sufficient magnetizing intensity to push the
magnet well into first quadrant saturation (in its induction,
B
, versus magnetizing
force,
H
, plane), the value of remanence induction will be low and/or there will be
too much variation part to part from the process. Second, if the magnetizing pulse has
insufficient dwell, the magnet may not be uniformly magnetized.
The highest power density electric machines are the brushless dc type. This is
because for a given value of flux in the machine, the flat top of the trapezoid results in
much higher rms value than a sinusoidal flux for the same iron saturation limited
peak value. The same applies for the current - block mode conduction with flat top
waveform has a higher rms value than its corresponding sinusoidal cousin for the
same current limit in the power electronics inverter. For this reason, brushless dc
machines have found use in industrial machine tools and some traction applications.
In Figure 5.10(c) the tapered magnet geometry is shown that tends to a more
trapezoidal back-emf. This breadloaf style of magnet is typical of tapered designs
for which the gradual magnetization, through gradual increase in the magnet
thickness, yields a smooth shape for the reluctance torque. Reluctance torque in
brushless machines of either variety is a serious noise issue, particularly for high-
energy rare earth magnets. A motor design with NdFeB can produce three times the
commutating torque than a ferrite ceramic design. The NdFeB design therefore has
far more reluctance, or cogging, torque. The motor cogging torque gives the feeling
of detents as the rotor is turned. The spectrum of reluctance torque effects is line-
arly decreasing for parallel magnetization (sinusoidal back-emf) designs with har-
monic number. For a gradual magnetization, the effect is a similar linear decrease
with harmonic number, but the initial value of reluctance is some 30% higher. For
radial magnetization (trapezoidal back-emf), the reluctance torque increases with
harmonic number, peaks for the second and third harmonics and then decreases
linearly with higher harmonics. This harmonic flux is a serious issue with brushless
dc machines: the trapezoidal back-emf causes very significant detent torque and
consequent vibration. For traction applications, the inertia of the driveline may or
may not swamp out the reluctance torque induced vibrations.
There have been many techniques proposed for minimizing reluctance torque
production in brushless dc machines, such as skewing the magnets along the rotor
axis length, and careful design of the magnet pole arc and interpolar gap. The
magnet pole arc can be visualized as the circumferential span of the magnet in
Figure 5.10 versus the pole arc (in the 1-pole case shown this would be
p
-radians).
It is most common to have magnet pole arcs of 0.7-0.8 times the pole span in order
to minimize harmonic production. One of the more effective means to reduce
detent torque in a brushless dc machine has been the implementation of stator pole
notching. The effect is to have the magnet edges pass evenly spaced discontinuities
