Figure 4.71: Power fl ow in spindle motor.
4.4 Spindle Motor Drive System
Figure 4.72: Three phase winding connections.
As mentioned in section 4.3, spindle motors used in HDD are generally
PM AC motors with surface-mounted permanent magnets. There are usually
three phases with the phase windings are arranged either Y-connected, or
∆-connected (Figure 4.72). The Y-connected winding is the most prevailing
choice because of its low cost. In the Y-connected winding, besides the winding
terminals A, B and C, the neutral point may also be used. The back-EMFs
induced in the armature windings of the motor are normally symmetrically
sinusoidal. Unlike the conventional method of driving PM AC motors with
sinusoidal back-EMF, the spindle motor is usually driven as a BLDC (Brushless
DC ) motor. An introduction to the BLDC drive system is given, and the
performance of a motor is explained for different BLDC drive modes. Since the
motor drive is linked directly to electrical quantities, to make the explanation
concise, the degree mentioned in the following is electrical degree (de fi ned in
section 4.2.6), unless stated otherwise.