Environmental Engineering Reference
In-Depth Information
Both types of electronically commutated dc motors require electronic controls.
A question that should immediately come to mind is how does the flux generate
trapezoidal versus sinusoidal voltage? The answer is the rotor magnet design and
magnetization orientation will determine the character of the voltage, and to some
extent the slot design.
Permanent magnets used in electric machines are invariably parallel magne-
tized with the magnetic field intensity lines oriented across the magnet length,
which is generally of the order of 8-15 mm for ceramic magnets and 4-7 mm for
rare earth magnets. Figure 5.10 illustrates the trend in back-emf as magnetization
orientation proceeds from parallel
to radial,
that
is sinusoidal
to trapezoidal
waveform.
(a)
(b)
(c)
(d)
More sinusoidal
<--->
<--->
More trapezoidal
Figure 5.10 Illustration of permanent magnet magnetization orientation:
(a) Halbach, (b) parallel, (c) tapered (breadloaf) and (d) radial
Some comments on Figure 5.10 are necessary to explain the magnet config-
urations. In Figure 5.10(a) the SPM is made up of individually magnetized seg-
ments, each having a slightly different magnetization orientation so that the
resulting flux is more dense at the centre and tapered towards the magnet ends. The
length of the magnet is in the direction from the rotor back iron core (shown
hashed) along a radius line to the rotor OD defined as the surface of the permanent
magnet facing the airgap. The Halbach array, as this orientation is known, comes
from nuclear physics focusing magnet arrays wherein the flux on the inside of the
array extends to the centre, but on the outside of the annular magnet array, the flux
is zero. Halbach arrays are self-shielding and require no back iron or minimal back
iron on the self-shielding side. Electric machines have been fabricated with Hal-
bach array techniques in an attempt to minimize rotor mass and inertia.
The magnet orientation in Figure 5.10(b) is the conventional parallel magne-
tized magnet arc segment. This is the most common magnetization orientation
found in dc brushed motors and brushless ac motors having sinusoidal back-emf.
Ceramic magnet brushless ac motors are magnetized in situ by placing either the
rotor or the entire motor in the proper orientation into a magnetizing fixture and
applying a very high magnetizing intensity pulse. Rare earth permanent magnets
such as SmCo of NdFeB have such high intrinsic coercivity that in situ magneti-
zation is not possible and individual magnet segments must be pre-magnetized.
One reason for this is that magnetizing fixtures are unable to supply the intense
