Exide Technologies Section 93.30 - Manual (Page 9)

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9. DESCRIPTION OF OPERATION
There are four major sections of the SCR three-phase charger, which work together to produce
stable, regulated, filtered output. The functions of these four sections may be described as follows.
a. 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.
b. 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 SCRs is controlled by the action of the control module. Both the SCRs and the diodes
are protected from AC and DC surge voltages by means of the metal-oxide varistor surge suppressors.
c. The Control Module: This printed circuit board generates the three-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. An error signal is created
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.
d. The Filter Section:
(1). 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.
(2). 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.