7. Acoustic noise and vibration must be kept at low level. HDDs are now
used in many consumer electronic products, e.g., high-end video record-
ing and PDA phone. Any component used in these products must have
low acoustic noise and vibration.
8. Low Contamination: Modern HDD products are using Magnetoresis-
tive (MR), Giant Magnetoresistive (GMR), Tunnelling Magnetoresistive
(TMR), and Current Perpendicular to Planes (CPP) materials as the
read head sensor. The head performance is thus very sensitive to the
contamination and particles from the components used. The major con-
tamination from the spindle motor is from the leakage of bearing lu-
brication oil. How to prevent the leakage effectively is an important
consideration in the HDD industry.
9. For obvious reasons, eﬃciency of spindle motor must be high. The ef-
fi ciency refers to not only low power consumption but also low heat
production. This is important for guaranteeing the life and reliability of
10. Low cost: This is self explanatory. The trends of HDD products are
driven by the market. Therefore, low cost is an essential requirement for
such a widely used product.
Only a few motor structures can meet the above mentioned requirements.
From the 80s, this role has always been played by the permanent magnet BLDC
motor. The analysis in the following sections explains in detail the importance
held by the PM BLDC motor as the choice for spindle motor in HDD.
4.3.2 Back-EMF in Spindle Motor
Permanent magnet AC motors (PMACM) are available with two kinds of back-
EMF waveforms, (i) sinusoidal back-EMF and (ii) trapezoidal back-EMF with
120 ◦ fl attop as shown in Figure 4.43. A PMACM with sinusoidal back-EMF is
traditionally known as permanent magnet synchronous motors (PMSM), and
that with trapezoidal back-EMF is called permanent magnet brushless DC
(BLDC) motors. These two kinds of PMACMs should be driven with different
drive modes. These drive modes are explained in section 4.4.
It is also discussed later that the spindle motor must use very limited stator
slots to realize multiple magnetic pole-pairs. It is diﬃcult to realize trapezoidal
back-EMF using such EM structure. As a result, sinusoidal back-EMF is more
commonly used in most of the spindle motors. These motors are normally re-
quired to be low in their back-EMF harmonics to simplify the quality control
procedure, which is very important in mass production. The following discus-
sion is restricted to the analysis of spindle motors with sinusoidal back-EMF.