Pulse Generation And Phase Control Circuits - Marconi LORAIN RHM800E50 Manual

Section 4387
common. causing the output of amplifier Al7 to go posi-
at the sfkler of potentiometer R12 becomes poslive with
respect to common, causing the output of amplifier Al7
to go negative. Diode CR1 at
Al7 is used in conjunction with 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 CRI. At this
nected in an OR type configuration. Whichever regula-
tor output is more positiie controls the pulse and phase
control circuls. The output of this OR circuft is con-
nected to an inverting voltage follower A18, wbii gen-
erates a second etmr signal equal in magnlude but of
me
to signal generated by A17. The
positiie and negative error signals thus generated by
Al7 and Al8 are applied to the pulse generation circuits
me
where error signal inputs are used to generate the
thyristor gate pulses.
8.11 This circuit senses the output current of the racti-
fier and changes the operation of the rectifier from vok-
age regulation to current regulation when the output
current reaches a predetermined value. The positive
output of the rectifier is passed
meter shunt R25. The voltage drop across the shunt
deflects the output ammeter and afso provides a control
signal is applied to the input of shunt amplffier A8
through resistors R21 and R28. Shunt amplifier A6 pro-
output
vides an voltage to
and toad sharing circul as well as the current regulator
circuit which limits and regulates rectifiir output. The
current limit setting of the rectifier is determined by a
series string of resistors connected at the output of
shunt ampllier A6. These resistors include CURRENT
LIMIT potentiometer R23 and resistors R17. Rl9, and
6.12 Resistor A17 is associated wfth the temperature
sensing and fan fail circuitry. Resistor R17 is normally
shorted out by closed contacts of relay K3 which are
connected at J4 pins 11 and 12. When the temperature
of the rectifier heat sinks exceeds +85"C (+185'F) or if
air flow through the cabmet ceases, resistor RI7 is
inserted into the circuit to decrease the rectifier output
current. Refer to the description of the temperature
sensing circuit for details.
Page 18
6.13 Resistor R64 is connected to the cathode of
current
output of ampfifier
tion. When output current exceeds the level set by
potentiometer R23. the voltage at the junction of potenti-
ometer R23 and resistor Al5 (34) causes amplifier A5
to generate an emor signal. The current limit error sig-
nal output of currant control amplaier A5 is coupled to
the voltage control circuit mmugh CR4. This signal
causes the rectifier to operate in the current regulation
mode.

pulse Generation and Phase Control Circuits

6.14
cuits, one for each AC phase. Each pulse circuit gener-
ates the firing pulse used to gate two thyrfston in each
phase. The pulse circuits control the phaseback angle
of the thyrfstors in response to a signal from the voltage
regulator described previously. The firing circuit for
each thy&or consists of a driver transistor which is
controlled by two comparators. a reference comparator
and a synchmnbmg comparator. The reference com-
parator receives an en-or signal from the regulator circuit
from the power transformer Tl secondary winding. The
reference comparator utiiies this information to deter-
mine when to turn on the driver transistor. The synchro-
nizing comparator pmvides damping actiin to prevent
firing of the thyristor when the Utyristor is reverse
6.15 The control of phaseback angle is based on a
comparison of a cosine reference voltage, which leads
m.9
current
tor circuit. Since the circuit card contains three identical
phase control circuits, the operation ot the one may be
taken as typical. The cosine voltage is generated by an
A4. resistors A9 (S-9), Al0 (6-9). and capacitor Cll.
Resistors A9 (5-10) and Al0 (5-10) and capacitor Cl4
me output of A4 is applied to the inputs of two compara-
tors of A3. Since the voltages on the power thyristors
are 160' out of phase, it folbws that their corresponding
firing pulses must be 180' out of phase. To develop two
signals wfth the same phaseback angle but 180' out of
phase with each omer, the cosine voltage is applied to
me non-inverting fnput of one mmpamtor of A3. and to
me inverting input of the second comparator of A3. At
this point, the positive and negatfve error signals gener-
ated by the regulator circuit are applied to the pulse
the current limit settmg of potentiometer
This circuit card contains three identical pulse cir-
line by 90". against a sfgnaf from the voltage regula-
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