combination of the motor capacitors is connected to the series combination of start
winding number 1 and start winding number 2 and
this
series network is operated directly
from the line voltage and is not switched by the triac. Thus, it
can
be
seen that current
flows in the start winding at
all times when power is applied via the
ON/OFF
switch
on
the
operator panel.
Torque
will not
be
developed
by
the motor to
an
extent which will allow starting rotation of
the disks unless the main winding is energized for a sufficient amount of time to provide
the net rotating magnetic field. Likewise, sufficient torque to maintain rotation will
be
available only
if
the main winding is energized sufficiently often that the available field
from a combination
of
the main and the start winding can provide the necessary torque
to
the load. Torque available from the drive motor is therefore provided by switching on and
off the current in the main winding. The ability to control the speed of the drive motor and
the torque that it supplies to the load is therefore contingent upon switching the triac on
the Motor Control PCBA at the correct times and allowing current to flow in the main
winding
as
required.
The current amplification necessary to provide the high current drive for the gate of the
triac is provided
by
transistor current switches
on
the Motor Control PCBA.
Since the main winding of the drive motor is
an
inductive load, there can exist a phase
shift
between the current through the motor and the applied voltage. This means that at
the time the triac current falls below the holding current value and the triac ceases to
conduct, there will exist a certain voltage across the triac.
If
this
voltage appears too
rapidly, the triac will resume conduction and control
will
be
lost.
In
order
to
achieve
control with certain inductive loads, such
as
the drive motor, the rate
of
rise in voltage
must
be
limited
by
a series
R-C
network across the triac. The capacitor
limits
the rate of
change of voltage across the triac with respect to time; the resistor
limits
the surge of
current from the capacitor discharging when the triac first conducts.
It is also used
to
damp the ringing of the capacitance with the drive motor inductance and the inductance of
a series inductor mounted on the Motor Control PCBA. This additional inductor is required
to reduce transients caused by the triac switching into conduction.
4.9
POWER
SUPPLY
Figure
4-13
is a block diagram
of
the disk power supply which is
in
two parts. The first
part, the power supply module mounted on the power supply chassis, is fastened to the
base casting and contains the power transformer, rectifiers, capacitors, fuses, and power
resistors. Three unregulated dc supplies are generated at nominal voltages of
±
20v
and
± 10v dc. Three
ac
supplies are generated at nominal voltages
of
ac
line voltage,
8v
ac
(rms) and
21
v ac (peak).
The
second part of the power supply consists of the ± 1
Ov
and ±
5v
voltage regulators
which are located
on
the Servo PCBA. Interconnection between the two parts is provided
by
a harness from the power supply chassis which plugs into the Servo PCBA via a 12-pin
connector. Interconnection for
ac
line voltage,
ac
common, and
8v
ac
(rms)
to
the Motor
Control PCBA is provided via a 6-pin connector.
The transformer primary connections are shown in Figure 4-14
for
several line voltages.
Line voltage is connected to the transformer via the
ON
/OFF switch. The
ac
line voltage is
also used directly to power the disk drive motor. Also,
8v
ac
(rms) and
21
v
ac
(peak) are
used for drive motor speed control circuits and to power the brush motor and associated
circuits. Unregulated dc (at a nominal of
+
20v
under load) is used to provide power to the
4-31
630D
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