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OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets
b. Generator output monitors (meters)
The generator output is monitored by three instruments; a frequency meter (10, Fig. 9) a
voltmeter (9) , and an ammeter (7) . The frequency meter is a resonant-reed type which
indicates the frequency of the generator output alternating current in the range of 380 to 420
Hz (cycles per second) . The voltmeter indicates the generator output voltage in each
phase-to-neutral (A-N, B-N and C-N) or phase-to-phase (A-B, B-C and C-A) as selected by
the meter selector switch (6) and the line selector switch (5) . The voltmeter scale is
graduated 0 to 300 V and the ammeter is graduated 0 to 500 A on 90-kVA units. The
amperage value in each of the three phases may be read on the ammeter by selecting the
desired phase with switch (6) . Three ammeter current transformers, (Ref. 4, Fig. 14), lower
the output load current to a lesser value of definite ratio, which will operate the ammeter
movement without damage. The ammeter dial scale is graduated and numbered so that the
pointer will indicate the true load current value rather than the meter movement current.
c. Meter and line switches
These switches provide a means of selecting and determining which phase of voltage and
current is indicated on the voltmeter and ammeter and whether the voltage is line-to-neutral
or lineto-line. The meter switch (6, Fig. 9) is a four-position, rotary type. An instruction plate
(4) , located under the switch knob is marked and lettered to indicate the three functional
positions of the meter switch. (When the knob is pointing straight down, the switch is OFF) .
The line switch (5) is a two-position, toggle switch used to select either line-to-neutral or
line-to-line voltage to the voltmeter. The instruction plate is also marked to indicate the
position of this switch.
d. Receptacle connector
An "Amphenol" connector (12, Fig. 9) provides a means of quickly disconnecting all wires to
control box components.
e. Tray
The tray (15) slides in and out on nylon rollers. It is secured in place by a twist-lock, screw
fastener (8) .
f. Manual voltage control
When the automatic-manual switch (11) is placed in MANUAL position, AC power is supplied
to the rectifier (14) rather than to the automatic voltage regulator. The DC output of the
rectifier is routed to the exciter field through a rheostat (13) . The generator output voltage is
manually controlled by adjustment of the rheostat.
g. Excitation-deenergization relay
The purpose of this relay (3) is to allow automatic excitation to be connected to the exciter
field only when the engine speed is being controlled by the electric governor.
(2) Protective relay tray (Figure 10)
In the bottom of the control box is the protective relay tray which contains electrical and
electronic safety devices. These devices are designed to protect the aircraft electrical system
against damage resulting from over or undervoltage, over or underfrequency. The tray also
contains devices for the protection and control of the generator output electrical system.
a. Sensing modules
The voltage sensing module (5) and frequency sensing module (6) are connected to the
generator output leads between the generator and load contactor. These are the solid-state
circuitry of the memory and time delay module (4) which open the load contactor and
disconnect the generator output to the aircraft. A solid-state overload signaling device (Ref.
7, Fig. 14) is also connected to the protective monitor module and performs a function similar
to the sensing relays. Trip values for the protective circuits are as follows:
Overvoltage - at 130 V to 134 V Undervoltage - at 102 V or below Overfrequency - at 415 Hz
to 425 Hz Underfrequency - at 390 Hz to 395 Hz Undervoltage time delay (adjustable) 4 to 12
seconds Overload circuit trips at any value over 125% rated load capacity.
Chapter 1-1
Page 12
February 28/94
Revised
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