ARMY TM 9-6115-673-13&P
AIR FORCE TO 35C2-3-512-1
1.22.
GENERATOR VOLTAGE REGULATION AND OUTPUT SUPPLY (MEP-501A).
1.22.1. The generator voltage regulation and output supply system (Figures 1-9 and 1-10) senses the load being
drawn at the load terminals and adjusts the alternator output accordingly. The system also monitors and adjusts
generator set performance.
1.22.2 The system consists of the alternator (G2), voltage regulator (A1), ON-OFF load circuit breaker (CB1), output
load terminals (+ and -), DC VOLTS meter (M2), % LOAD meter (M1), and VOLTAGE ADJ. potentiometer (R2). Power
produced by the alternator supplies load terminals on the load terminal board.
1.22.3.1 The generator set output voltage is controlled by the voltage regulator (A1). The voltage regulator
continuously senses the alternator (G2) output voltage. The voltage regulator (A1) reacts to voltage variations by
manipulating the alternator field current to maintain the output voltage. The field current controlled by the voltage
regulator is supplied by the alternator excitation windings. The voltage regulator (A1) performs this function using three
interactive sub circuits: power input, load sensing, and DC output. The power input circuit draws current from the
generator (G2) exciter windings through the positive (+) field brushes at A1 pin F. The load sensing circuit monitors the
current being drawn by the load across A1 pins S and A. As demand increases, the DC output circuit draws current
from the power input circuit, rectifies it to direct current, and reapplies it to the AC alternator (G2) field via A1 pins F and
L.
1.22.3.2. Starting the diesel engine provides the current necessary to field flash the alternator (G2). This current is
supplied from the dynamo (G1) to the generator control unit (A2) where it is rectified to direct current. The direct
current exits A2 at pin F and is applied to the alternator (G2) field windings. As soon as the generator control unit (A2)
detects current at pin VINDC, it disables the flow of current exiting A2 at pin F.
1.22.3.3. The initial field flash current induces voltage in the power excitation windings at alternator (G2). As the diesel
engine speed begins to increase toward its governed no load speed of 3750 rpm, the induced voltage in the power
excitation windings increases. The voltage regulator (A1) power input circuit receives current from the power excitation
windings via the positive (+) field brushes at A1 pin F. When the voltage in the load sensing matches the set point, the
current entering the voltage regulator (A1) at pin F is allowed to pass through the power input circuit. When the voltage
entering the sensing circuit is lower than the set point (indicating a load increase) the DC output circuit reacts by
drawing current from the power input circuit. This current is rectified to DC and reapplied via A1 pins F and L to the
alternator rotor brushes. The application of direct current to the rotor increases the field magnetism between the
alternator stator and rotor which in turn, increases the current measured across the alternator power windings. The
current measured at both the excitation and power windings will increase until the voltage entering the voltage
regulator (A1) load sensing circuit at A1 pins S and A match the set point at which point, the alternator output stabilizes
and the A1 DC output circuit stops drawing current from the alternator excitation windings.
1.22.3.4. The voltage regulator (A1) set point can be changed by adjusting the VOLTAGE ADJ. potentiometer (R1).
The set point can be changed from 26.6 to 32.2 VDC by loosening the locknut and turning the adjustment screw
counterclockwise to lower the set point or clockwise to increase the set point. The set point adjustment can be
checked with the diesel engine running by observing the needle on VOLTS DC meter (M2).
1.22.3.5. The generator set performance can be monitored by observing the %LOAD meter (M1) and the VOLTS DC
meter (M2). The %LOAD meter (M1) measures the current being drawn by the load at the load terminals (+ and -) and
displays the value as a percent (0 to 125) of generator set capacity. The %LOAD meter (M1) operates on current
provided by shunt resister (R3). The VOLTS DC meter (M2) measures the voltage across the power windings of the
alternator (G2) and displays the value in VDC from 0 to 40. Line fuse (F1) protects the VOLTS DC meter from a
possible over current condition.