KEPCO JQE 150-1.5MVPY-26956 Half Rack - Manual (Page 41)

JQE SPECIAL SVC 081111 5-1
SECTION 5 - THEORY OF OPERATION
5.1 SIMPLIFIED DIAGRAM DISCUSSION
Referring to Figure 5-1, the main power transformer converts the line voltage to the required lev-
els needed to produce the d-c operating voltages for the Main and Auxiliary Supplies. The main
power supply is in series with the pass elements and the current sensing RESISTOR and pro-
duces the output voltage.The main pass elements are changing their series resistance to keep
the output voltage constant. The necessary drive for this change is produced by the driver circuit
which, in turn, receives its command signals either from current error amplifier A or B or the volt-
age error amplifier.
The output voltage is constantly compared to the voltage reference source, while the output cur-
rent, monitored by the current sensing resistor, is compared to the current reference source.
Any change, in either output voltage or current, is amplified by the associated amplifier and
transferred to the diode-gate which feeds directly into the driver circuit which produces the
needed drive signal for the main pass elements, thus either keeping the output voltage constant,
or, if the signal from the current limit amplifier is dominant, limiting the output current to the pre-
adjusted value.
The Overvoltage Crowbar is connected across the output terminals of the power supply. A por-
tion of the output voltage is continuously compared to an internal reference. If an overvoltage
occurs; the SCR "crowbar" will short-circuit the output immediately. Recycling takes place auto-
matically, since the a-c input power is removed by the tripping of the circuit breaker.
5.2 CIRCUIT DESCRIPTION
For the purpose of analysis, the circuitry of the JQE special power supplies may be divided into
several sections which are individually described below. The overall schematic diagram (Figure
7-8) and the detailed schematic diagrams (Figures 7-9 through 7-11) Illustrate the circuitry
described in this section.
5.2.1 A-C INPUT CIRCUIT.
A-C line power is introduced through the a-c input barrier-strip or a heavy-duty three-wire line
cord with safety plug. The use of a grounded a-c power outlet will automatically ground the
power supply, since the third wire of the line cord is directly connected to the metal chassis and
case. Once the a-c power circuit breaker (CB101) is closed, the primary of the main transformer
(T201), the fan (B201) and the a-c pilot light (DS101) will be energized. The two primary wind-
ings of the main transformer are either connected in parallel (for 115V a-c nominal line voltage)
or in series (for 230V a-c nominal line voltage). The a-c power circuit breaker (CB101), aside
from switching the a-c primary power, has the function of disconnecting the power supply from
the a-c line in case of input overloads, short circuits, heatsink overtemperature or output over-
voltage. CB101 has two sensing coils, one of which is always in series with one of the primary
windings on T201 (T2-T5), whether the main transformer is wired for 115V or for 230V a-c. The
second sensing coil of CB101 is in series with the secondary winding on T201 which generates
operating potential for the overvoltage protection circuit (T201-T6, T7). If an overload occurs,
due to either internal component failure or crowbar action of the voltage protector, the coil is
energized sufficiently to activate the trip mechanism of CB101, thus disconnecting the power
supply from the a-c line.