PEC MACC, Manual, Page: 15 / 26

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B. Peak Limit Control
In the event of excessive instantaneous line current due to either an internal or external fault, the
current is electronically interrupted after a single half cycle (worst case) of fault current. The
control will then automatically "ease" the converter back on while continuing to monitor the line
current. The control will limit the peak input current to a safe preset level and after 5 seconds, if
the fault is not cleared, will turn the converter off. In case of a short on the output of the converter,
the normal current limit control will take over and maintain the converter average output current
at rated value.
C. Thermal Overload
Normally-closed thermal overload are used in the converter. Mounted on the diode heat sink they
will open and shut the converter off if the stud temperature is excessive (approximately 230 ° F).
3.2 Cooling System
The MACC converter is air cooled, requiring very little maintenance.
The air enters the cabinet through a screened opening in the bottom. The heated air generated from
the power components rises and exits through the top of the cabinet. It is important therefore, to
keep a minimum clearance of 12" between walls, etc.
Air at ambient or room temperature is used as the cooling medium. Therefore, the ambient air
temperature should not exceed 104 ° F (40° C) for proper operation of the converter.
3.2.1 Control Operation
A. AC1 Control Board
Continuously monitors the current transformer (1,2,3 CT) output while the converter is operating.
In the event of an external or internal fault, the peak limit circuitry will shut the converter off. The
board also accepts the feedback signals from the DC bus and shunt to provide reference levels to
control the converter output. By controlling the firing of the line SCR’s, the control board varies
the converter output voltage and current. Using the feedback signals, the converter output can be
controlled and held constant at: voltage with current limiting, average current density with current
limiting or current with voltage limiting. The board provides the references controlling voltage and
accepts the control input from the potentiometers.
3.2.2 Control Function
A. Automatic Voltage Control with Current Limiting
The function of this control is to maintain the converter output voltage at a value preset by the
operator's control knob. The preset value of output voltage will be held constant within -+ 1% of
the converter's full rated output voltage over a range of 10% to 100% of output current under
varying load conditions. For example, with a 1000 Amp @ 12 Volt unit, the output voltage will be
held constant -+ 0.12 Volts over range of operation from 10 Amp to 1000 Amp. The voltage will
remain constant throughout the operational cycle and will eliminate the costly burning or over-
plating which can result from widely varying load voltages.