Hobart 120CU20, Operation And Maintenance Manual With Illustrated Parts List

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OM-2228 / Operation and Maintenance Manual
120CU20 / Series 500120 / 400 Hz. Generator Set
October 15, 2013 Chapter 1-1
Page 20
This voltage regulator PC board is designed to provide voltage regulation for a three-phase, four-
wire, 115/200-volt, 400-Hz brushless alternator. This regulator provides field excitation power as
required to meet varying alternator load conditions to hold the alternator voltage constant. In
addition, the voltage regulator PC board circuitry provides line drop compensation. Any deviation
of the alternator voltage from its set, regulated level is sensed at the voltage regulator PC board.
The sensing signal is compared to a reference signal, and, with associated circuitry, varies the
field power supplied to the rotary exciter.
a When the machine is started and the engine is at rated speed, the rotary exciter is excited
from alternator residual magnetism through the half-wave rectifier-bridge, located on the
voltage regulator PC board assembly. As the rotary exciter voltage increases, alternator
excitation increases and the alternator voltage builds up. The sensing circuit of the voltage
regulator PC board then compares the input voltage to a reference voltage and adjusts the
field power of the rotary exciter to bring the voltage into regulation limits.
b When the alternator is loaded, its terminal voltage decreases, lowering the rectified three-
phase voltage of the voltage sensing circuit. The sensing voltage is low in respect to its
reference voltage, causing the voltage regulator PC circuitry to increase the power to the field
of the rotary exciter. The alternator voltage increases until the voltage returns to its regulated
value.
c When a load is removed from the alternator, the alternator voltage rises. The rectified three-
phase voltage-sensing signal increases, causing this signal to be higher than the reference
signal. The associated voltage regulator circuitry causes the field power of the rotary exciter
to decrease, lowering the alternator voltage until the voltage returns to regulated value. The
line drop voltage compensation circuit consists of a current transformer on each phase of the
load circuit, and fixed resistance in parallel with each current transformer. The current
transformers detects the magnitude of current flowing through the power cables from the
alternator to its load and feeds a signal into the voltage regulator PC board. The PC board
processes this signal to change the output voltage proportional to the current draw. The
regulator output increases slightly so that the alternator output voltage is equal to the
regulated voltage plus the voltage drop in the lines. The line drop compensation
potentiometer may be adjusted to match exactly the voltage drop of the power cables
carrying the load current.
The under/over frequency protection, EF signal, and lost neutral detection are also monitored by
the REG and will signal the CTL when a fault has occurred. The CTL issues the appropriate
command that corresponds to the fault.
(8) Transformer-Rectifier PC Board [TRB] (A404) {optional}.
The TRB PC Board is only used when the optional 28.5 VDC transformer-rectifier assembly is
installed. The TR monitors the output voltage, output current, controls the input and output
contactors, and monitors all fault events associated with the DC output. When a fault event does
occur, the TRB relays this information to the CTL. The CTL will issue the command to the system
that fits the fault event.
(9) +5, -12 VDC Power Source (PS1.)
Supplies the internal power distribution of +5 VDC and –12 VDC into the control system.