Fujitsu MPA3017AT Product Manual page 61

e) The MPU is waiting for a PHASE signal. When no phase signal is sent for a specific period,
the MPU resets the SVC and starts from the beginning. When a PHASE signal is sent, the
SVC enters the acceleration mode.
(2) Acceleration mode
In this mode, the MPU stops to send the phase switching signal to the SVC. The SVC starts a
phase switching by itself based on the counter electromotive force. Then, rotation of the spindle
motor accelerates. The MPU calculates a rotational speed of the spindle motor based on the
PHASE signal from the SVC, and accelerates till the rotational speed reaches 5,400 rpm. When
the rotational speed reaches 5,400 rpm, the SVC enters the stable rotation mode.
(3) Stable rotation mode
The MPU calculates a time for one revolution of the spindle motor based on the PHASE signal
from the SVC. The MPU takes a difference between the current time and a time for one revolution
at 5,400 rpm that the MPU already recognized. Then, the MPU keeps the rotational speed to
5,400 rpm by charging or discharging the charge pump for the different time. For example, when
the actual rotational speed is 5,600 rpm, the time for one revolution is 10.714 ms. And, the time
for one revolution at 5,400 rpm is 11.111 ms. Therefore, the MPU discharges the charge pump for
0.397 ms k (k: constant value). This makes the flowed current into the motor lower and the
rotational speed down. When the actual rotational speed is later than 5,400 rpm, the MPU charges
the pump the other way. This control (charging/discharging) is performed every 1/6 revolution.
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