Figure 4.86: Simpli fi ed drive circuit at low speed.
18.104.22.168 Review of Different Methods for Sensorless Starting
The spindle motor can be started from standstill using one of the following
1. Detection of inductance variation and
2. Frequency-skewed or voltage-skewed drive signal.
As the permanent magnet is mounted on the surface of the rotor of spindle
motors used in HDD, the motor inductance and variation of inductance are
very small. Therefore the 1 st method mentioned above is diﬃcult to realize in
HDD spindle motor, and the second one is the most commonly used method
in the HDD industry. In this approach, a six-step signal is applied open-loop
with gradual increase in frequency to spin up the motor from standstill.
22.214.171.124 Starting of Spindle Motor using Skew Frequency
Since the starting procedure works only at low-speed range, the effect of the
inductance can be neglected. So an inverter connected to a star-connected 3-
phase resistive circuit can be used to illustrate the starting sequence, as shown
in Figure 4.86.
The six-step starting method are widely used for starting of the PMSM.
With this method, the inverter gate is switched every 60 ◦ . The gate signals
of the MOSFETs are illustrated in Figure 4.87, in which each gate signal
remains HIGH for 120 ◦ . With this pattern of gate signal, waveforms of the
phase voltages (V AN ,V BN and V CN of Figure 4.86) are as shown in Figure 4.88
with their amplitudes equal to |V DC /2|. These two pictures are used just to
show the basic principles of this method; the phenomenon of frequency skew
or voltage skew is not shown here. It is clear that the phase voltages have
sudden jumps, which will cause jumps in the drive currents as well. As a
consequence, oscillation is induced in the rotor by the jumps in the current
waveform, making it diﬃcult to start the motor smoothly.