Dokorder 7700 - Page 33

v
v
v
v
ln
the
motor drive circuit
above, the
PLL
logic output
operates
transistors
Or and
Oz to control the
DC
motor. At
the
same
time, there
is a
bridge circuit composed
of
resistors Rr,
Rz,
R: and Ro
(DC
motor).
Resistor
Rs is used to sense
the
motor current, while
Rr and Rz are
the
motor voltage sensing
resistors. The
balance
condition of the
bridge,
where R1, R3
= Rz, Ro,
produces
across
o
and
B
a
counter
electomotive
force
proportional
to the
motor
speed.
By supplying this
electromotive
force to the
base-emitter
of
transistor
01
,
a
feedback
is
provided
to
maintain the
motor speed
against
load variations
and
power
voltage
fluctuations.
lf
+Bz
increases, the
voltage
drop
across Oz
will
increase
to keep the
motor
voltage constant,
while the
pulse
width
will also be
kept constant
(Figs.8
15 and 8-16).
The
motor is also
driven to the
rated speed against
variations
in
the
load torque,
so
that sharp
rising charac-
teristics
have been obtained
when the
pinch
roller is
pressed,
as
shown
in
Fig.8-l7.
R.:
DC motor current sensing
resistor
(in
a negative feedback
loop)
Fl
,,
Rr: DC motor
terminal
voltage sensing
resistor
(in
a
positive
feedback
loop)
E(V)
Fis.815
F
iq.
8-1 6
v
30