Figure 4.43: Two typical back-EMF waveforms of PMACM a: sinusoidal, b:
The number of cycles in the back-EMF waveform for one revolution is
equal to p, the number of pole-pairs in the motor. For an ideal spindle motor
operating at constant speed, and when the rotor position is referred to using
mechanical degree, its back-EMF can show the following performance,
θ+ m360 ◦
where m is an integer number.
From equation 4.71, the interval between two successive zero-crossing po-
sitions (ZCP) of back-EMF should be 180
/p (as illustrated in Figure 4.44).
However, like any other component, the motor is neither perfect in its me-
chanical dimensions nor magnetically homogenous. So the intervals between
the ZCPs are not exactly 180 ◦ /p, which is illustrated in Figure 4.44. These
errors are called pole-jitter in the nomenclature used for analysis of motor.
The zero-crossings of the back-EMF waveform are widely used to detect the
rotor position and to measure the speed of the spindle motor in HDD products.
This issue is explained in details in section 4.4. Any jitter in the locations of
zero-crossings introduces error in determining the rotor position as well as in
sensing the speed of the motor. In the application of HDD, the accuracy and
precision of the spindle motor's angular speed is very important. Therefore,
the existence of pole-jitters is not a desirable feature for the spindle motors.