630D
The
circuit consists
of
R212,
R213, R214,
C54,
C55
and the transistor switch U26-A. This
combination generates a sawtooth waveform at
120
Hz.
When
the signal
at
U26-A
pin 1 and
2 is high, the transistor in the output state is cut
off
and capacitors
C54
and
C55
charge
through
R212,
R213,
and
R214
to
generate a ramp output at
TP27.
Conversely, when the
signal at U26-A pin 1
and
2 goes low, the transistor in the output stage is turned on and
discharges capacitors
C54
and
C55
through
R214.
Since the value of
R214
is much smaller
than the sum of
R212
and
R213,
the discharge time constant is much smaller than the
charge time constant. Thus, the
120
Hz
sawtooth waveform is generated and
can
be
monitored at
TP27.
Potentiometer
R212
(zone
C2)
Is
used
to adjust the peak-to-peak
amplitude of the sawtooth waveform which is
fed
to pin 1 of comparator U25-A (zone
C3)
along with the output
of
operational amplifier
U24
(zone F5).
The network associated with the operational amplifier
U24
(zone
F5)
is
an
integrator
circuit. When the
Increase Motor
Speed
signal (NLlMS1) is high, the output at
TP24
is a
ramp function whose time constant is dependent upon the
value of
R215, R216,
R217,
C56,
C57,
C58,
and
C59.
Diodes
CR51
,
CR52, CR53,
and
CR54
limit the maximum positive
voltage to approximately
2.0v. Diodes
CR55,
CR56,
and
resistor
R218,
limit
the lower
excursion of the voltage
to
approximately
Ov.
Thus, when
NLIMS1
is low, the steady state
value of the output voltage at
TP24
is approximately
Ov.
The
dc voltage at
TP24,
which is proportional to the NLlMS1 pulses, and the reference
120
Hz
sawtooth waveform from U26-A (zone
C3)
are compared in comparator
U25-A
(zone
E3).
Thus, the signal at
TP22
is either high or low, depending on the error in voltage between
these two signals.
The
operation of the
AC
Motor
Speed
Control circuit during start-up, constant
speed
control,
and
brake cycle
can
be
summarized briefly
as
follows. During start-up, NLlMS1 is
low, NLBCEG is high,
and
NLDMEG is low at
J202
(zone
F6).
The
output of the integrator
at
TP24
Is
approximately
Ov
which, when compared with the reference
120
Hz
sawtooth
waveform, makes
TP22
low.
The
low output of
U25-A
at
TP22
triggers the triac
on
the
Motor
Control
PCBA
every
cycle, with practically full cycle power to the motor, allowing
the motor to come
up
to the speed.
When the motor comes to correct speed, depending
on
the error
In
speed, NLlMS1 is
either high or low
and
the voltage at
TP24
is such that
when
compared with the reference
sawtooth waveform at
TP27,
it will generate a correction pulse at
TP22.
This correction
pulse triggers the triac
on
the Motor Control
PCBA
at a specific part
of
the
ac
waveform so
that power is
on
for only part of the cycle.
In
other words, firing the triac at a specific time
in one cycle of
an
ac
waveform achieves the phase angle control of the
ac
waveform,
thereby controlling the power to the motor.
5-10
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