Description Of Operation; Control Module; Filter Section; Troubleshooting Procedure - HindlePower SCR series Operating Instructions Manual

9. DESCRIPTION OF OPERATION

a. There are four major sections of the SCR single-phase charger which work together to produce
stable, regulated, filtered output. The functions of these four sections may be described as follows.
1. The Power Transformer T1. This section includes T1 and its associated input protection. Its
purpose is basically to supply an AC voltage of the proper magnitude and capacity to the rectifier section.
It also supplies various other voltages used by the control unit and accessories. It is connected to an AC
source by means of a circuit breaker.
2. The Rectifier Section. This section consists mainly of the voltage regulating silicon controlled
rectifiers and the power rectifier diodes. It accepts the AC voltage from the transformer, rectifies this
voltage to DC, and controls the voltage's magnitude so that the charger output is regulated at all times.
The firing angle of the SCR's is controlled by the action of the control module. Both the SCR's and the
diodes are protected from AC and DC surge voltages by means of the metal-oxide varistor surge
suppressors.
3. The Control Module. This printed circuit board generates the single-phase phase-fired gate
signals that turn on the SCR diodes in response to the charging requirements of the battery load. The
output voltage of the charger is monitored by the voltage feedback circuit and advances or retards the
phase angle of the trigger pulses so that the output voltage is maintained essentially constant. This is
accomplished by comparing a small portion of the output voltage to a stable voltage reference and creating
an error signal proportional to the differential voltage. This error signal is then used to alter the phase
angle of the SCR gate trigger pulses in order to correct the output voltage. The load current is also
monitored by the circuit so that when its value exceeds an arbitrary value (110% rated current) the system
is "phased back" to limit the output current to no more than 110% of its rated value.
4. The Filter Section
(a). Depending on the application, the charger may be unfiltered. In this case, one filter choke,
L1, is utilized not for filtering but for phase correction of the highly leading current-voltage condition
created by the batteries during the charging pulses. The batteries represent a very large capacitor in shunt
with a resistive load. This creates out-of-phase current problems for the SCR diodes causing non-uniform
triggering problems particularly at low load currents. The single filter choke corrects this condition and
also aids the ratio of average current to RMS current flowing in the circuit.
(b). For filtered units, the objective is to remove the charging ripple at the battery terminals. To
accomplish this a "T" or "double-L" section filter consisting of inductors L1 and L2 and capacitors C1 and
C2 are used. C1 and C2 may consist of one or more individual capacitors. The degree of filtering required
dictates whether the "T" or "double-L" configuration is used. The "double-L" section filter is normally
used to reduce the ripple to 0.06% of nominal output voltage when the charger is operated as a filtered
eliminator.
CAUTION: Before trouble shooting, always isolate and deenergize the charger by opening the AC circuit
breaker, CB1 and the DC circuit breaker, CB2. This avoids the possibility of high short circuit current
damaging the charger, tools, test equipment, or injuring personnel. Note — Circuit breaker terminals and
terminal boards TB1, TB2, TB3, TB5 and TB15 have dangerous voltage across them even when breakers
are open. After isolating charger, check that the voltage across filter capacitors is zero, because once
charged they will take several minutes to discharge if bleeder resistor R1 is open. If CB2 is not provided,
remove the DC fuses F1 and F2 in order to isolate the charger from the battery. AFTER LOCATING THE
CHARGER PROBLEM, ALWAYS DE-ENERGIZE ALL AC AND DC CHARGER INPUTS AT THE
POWER SOURCE BEFORE SERVICING.
SECTION III
4