ARMY TM 9-6115-673-13&P
AIR FORCE TO 35C2-3-512-1
1.21.
GENERATOR VOLTAGE REGULATION AND OUTPUT SUPPLY (MEP-531A).
1.21.1. The generator voltage regulation and output supply system (Figures 1-7 and 1-8) senses the load being drawn
at the load terminals and adjusts the alternator output accordingly. The system also monitors and adjusts generator
set performance and provides power to a GFCI convenience receptacle.
1.21.2 The system consists of the AC alternator (G2), voltage regulator (A1), ON-OFF load circuit breaker (CB1),
output load terminals (L and N), GFCI convenience receptacle (J3), AC VOLTS meter (M2), % LOAD meter (M1),
HERTZ frequency meter (M4), VOLTAGE ADJ. potentiometer (R2), and fuse (F1). Power produced by the AC
alternator is supplied to load terminals on the load terminal board and the GFCI convenience receptacle.
1.21.3. Voltage Regulation. (Figure 1-7)
1.21.3.1 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 AC
generator (G2) exciter windings through the positive (+) field brushes at A1 pin 3. The load sensing circuit monitors the
current being drawn by the load at A1 pin E1. 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 F-.
1.21.3.2. Starting the diesel engine automatically field flashes the AC alternator (G2) with residual magnetism stored in
the rotor. The residual magnetism induces voltage in the power excitation windings at AC alternator (G2) pins + and 2.
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 3. Whenever the voltage in the load sensing
circuit matches the set point, the current entering the voltage regulator (A1) at pin 3 is allowed to pass through the
power input circuit. Whenever 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 F- to the AC alternator rotor brushes at G2 pins + and -. The application of direct
current to the rotor increases the field magnetism between the AC 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 matches the set point at
which point, the alternator output stabilizes and the A1 DC output circuit stops drawing current from the power input
circuit.
1.21.3.3. The voltage regulator (A1) set point can be changed by adjusting the VOLTAGE ADJ. potentiometer (R1).
The set point can be changed from 114 to 126 VAC 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 AC meter (M2).
