Marconi LORAIN RHM800E50, Manual

Page: 25 / 74

Section 4387
common. causing the output of to go
If rectifier output voltage is too low, the voltage
at the of potentiometer becomes with
respect to common, causing the output of amplifier Al7
to go negative. CR1 at output of ampfifier
Al7 is used in conjunction diode CR4 of the current
sense circuit to provide isolation of control between the
voltage and current regulating circuits. The current
sensing and current regulation circuit is connected into
the regulation circuit at the cathode of At this
the voltage and current regulator circuits are con-
nected in an OR type configuration. Whichever regula-
tor output is more positiie controls the pulse and phase
control
The of this OR is con-
nected to an inverting voltage follower gen-
erates a second equal in but of
to
me
signal generated by A17. The
positiie and negative error signals thus generated by
Al7 and Al8 are applied to the pulse generation circuits
where
me
error signal inputs are used to generate the
thyristor gate pulses.
and Current
8.11 This circuit senses the output current of the
fier and changes the operation of the from
age regulation to current regulation when the output
current reaches a predetermined value. The
output of the rectifier is passed
meter shunt R25. The voltage across the shunt
deflects the output ammeter and provides a control
to the output current. This control
signal is applied to the input of shunt A8
through resistors and R28. Shunt amplifier A6 pro-
vides
output
voltage to
me
current
and toad as well as the current regulator
circuit which and regulates output. The
current limit of the rectifier is determined by a
series string of resistors connected at the output of
shunt These resistors include CURRENT
LIMIT potentiometer and resistors R17. and
6.12 Resistor wfth the temperature
sensing and fan fail circuitry. Resistor R17 is
shorted out by
contacts of K3 are
connected at J4 11 and 12. When the temperature
of the rectifier heat sinks exceeds or if
air through the ceases, resistor is
inserted into the to decrease the output
current. Refer to the description of the temperature
sensing circuit for details.
Page 18
6.13 Resistor is connected to the cathode of
diode CR3. When the rectifier is supplying less
current the current limit of potentiometer
the current performs no regulation func-
tion. When output current exceeds the level set by
potentiometer
the voltage at the junction of potenti-
ometer and resistor Al5 (34) causes amplifier A5
to generate an signal. The current error sig-
nal output of currant control A5 is coupled to
the voltage control CR4. This signal
causes rectifier to operate in the current regulation
mode.
pulse Generation and Phase Control Circuits
6.14 This circuit card contains three identical pulse cir-
cuits, one for AC phase. Each pulse circuit gener-
ates the firing pulse used to gate two in each
phase. The pulse circuits control the phaseback angle
of the in response to a signal from the voltage
regulator described previously. The firing circuit for
each consists of a driver transistor which is
controlled by two comparators. a reference comparator
and a
comparator. The reference com-
parator receives an en-or signal from the regulator circuit
is compared to an integrated sine wave derived
from the power transformer Tl secondary winding. The
reference comparator utiiies this information to deter-
mine when to turn on the transistor. The synchro-
nizing comparator damping to prevent
firing of the thyristor when is reverse
6.15 The control of angle based on a
comparison of a cosine reference which leads
m.9
line by against a from the voltage regula-
tor circuit. the circuit contains three identical
phase control circuits, the operation ot the one may be
taken as typical.
cosine voltage is generated by an
integrator consisting of an operational amplifier
A4. resistors A9 Al0 and capacitor
Resistors A9 (5-10) and Al0 (5-10) and capacitor Cl4
the to amplifier A4. The cosine voltage at
me output of A4 is applied to the inputs of two compara-
tors of the voltages on the power
are out of phase, it that their corresponding
firing pulses must be 180’ out of phase. To develop two
signals wfth the same phaseback angle but
out of
phase with each the cosine voltage is applied to
me non-inverting of one of and to
me inverting input of the second comparator of At
this the and error signals gener-
ated by the regulator circuit are applied to the pulse
circuit. These error signals are equal in
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