Figure 4.64: Spectrum of the MMF shown in Figure 4.62.
The number of magnetic pole-pairs of the motor is increased if the 2 nd
order harmonic fi eld can be selected to match the pole-pair of the PM ring.
Therefore, the motor with 9 slots can have 6 pole-pairs. In the same way,
the spindle motors with 6 slots and 12 slots can have 4 and 8 pole-pairs, re-
spectively. The 3 rd harmonic does not contribute to energy conversion (see
section 4.2.2). The 4 th and higher harmonics contribute to energy conversion,
buts their amplitudes are too small and they should not be taken into consid-
eration. Otherwise, it results in very poor power density and eﬃciency of the
motor. Only the fundamental and the second harmonic are used to determine
the number of pole-pairs while designing the spindle motor. So far it explained
that every three slots produce one, or two, magnetic pole-pairs. However, for
a stator core with 9 slots, a special winding can realize 4 magnetic pole-pairs,
as shown in Figure 4.63. The intervals between the centers of these three
phase windings are 120 ◦ (mechanical). Therefore, they still form a symmetri-
cal winding set in the range of 360
mechanical degree. The MMF waveform
generated by one phase winding for this structure is shown in Figure 4.66, and
the spectrum of the MMF is shown in Figure 4.67.
It can be concluded from the spectrum that the winding shown in Fig-
ure 4.65 is a special case as its fundamental harmonic is very low, but the
fourth harmonic is the strongest. Therefore, it is reasonable to use the fourth
order harmonic as the motor pole-pair. In this way, 4 pole-pairs can be realized
with the 9-slot structure. A signi fi cant advantage of this structure of motor
lies in the fact that the cogging torque is very small , . Unfortunately,
however, this structure introduces unbalanced magnetic pull problem , .