Adaptive pulse width modulated motor control

A motor control method and apparatus for spindle motor control and for voice coil motor (VCM) control in a direct access storage device (DASD). Characteristics of the pulse width modulation (PWM) waveform are adapted to allow tradeoffs to be made between lower noise emission, greater power efficiency, and current ripple in the motor in order to optimize the driver operation. The transition rate of the PWM waveform is modified by changing the "off-time", which is maintained as a constant in a control register, in response to a change in the condition or mode of operation of the DASD. The "slew rate" of the PWM waveform is also controlled by a changing the value of a constant stored within the control register and can be modified in response to changes in the condition or operational mode of the DASD. In addition, in accordance with the present invention, the current that flows through the spindle motor windings during commutations is controlled by pulsing the power amplifier drive circuit at a relatively high rate in order to maintain a relatively constant total current through the windings of the motor, thus reducing acoustic noise. Thus, mechanical vibrations which result in acoustic noise are prevented or suppressed.