Hobart Brothers 6910, Operation And Maintenance Manual With Illustrated Parts List

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(2) Protective relay tray (see Figure 8)
The bottom tray in the control box is identified as the protective relay tray and contains electrical
and safety devices designed to protect the aircraft electrical system against damage which could
result from overvoltage, undervoltage, overfrequency, 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 gener-
ator output leads between the generator and load contactor. These solid-state modules
sense any abnormal condition of voltage or frequency and signal the solid-state circuitry of
the memory and time delay module
(4) to trip open the load contactor holding circuit and dis-
connect output to the aircraft. A solid-state overload signaling device (Ref. 7, Fig 11) is also
connected to the protective monitor module and performs a function similar to the voltage and
frequency sensing modules. Trip values for these sensing modules are as follow:
RELAY TRIP POINT
Overvoltage 130 V to 134 V
Undervoltage 102 V or below
Overfrequency 415 Hz to 425 Hz
Underfrequency 390 Hz to 395 Hz
Overload Any value over 125% rated load capacity.
See Para 6,H, (3) for more specific and detailed information regarding
the overload device.
For generator set operation wherein it is not desired that the underfrequency relay should
trip instantly, this relay is available with an optional time delay circuit. This relay can be
ordered with a timer on its PC board which times the trip to occur 5 seconds after an underfre-
quency condition occurs.
For generator set operation wherein it is not desired that the overfrequency should trip in-
stantly, the memory and time delay PC module
(described in the following paragraph)
can be
ordered with a 5 to 6 second time delay.
For generator set operation wherein it is not desired that the overvoltage relay should trip in-
stantly, the memory and time delay PC module can be ordered with a 0.25 to 0.75 second
time delay.
(b) Memory and time delay module
The memory and time delay module
(4)
is sometimes called the protective monitor module. It
is a solid-state device with a hermetically-sealed, reed-type relay. The printed circuit board or
“card” includes five memory circuits and a time delay circuit. Each circuit is connected to a
corresponding sensing circuit in the sensing modules
(5 and 6) . All memory circuits are con-
nected to the module relay coil, and any one of the circuits can energize the coil to open the
relay contacts. Thus, when a sensing device energizes any one of the module circuits, the
module relay is also energized to break the load contactor holding circuit and allow the load
contactor to open. Unless a generator set’s protective circuitry is provided with a mem-
ory/time-delay board having optional timer circuits, such as are described in the preceding
paragraph, all circuits - except the undervoltage circuit - function immediately to open the
load contactor. A time delay system is designed into the undervoltage circuit to prevent nui-
sance opening of the contactor under conditions of momentary undervoltage in the generator
output. However, an undervoltage condition which continues uninterrupted for a period of 4
to 12 seconds
(adjustable)
will cause the time delay circuit to open the load contactor.
OM-2028
May 1/91 Revised 1-1
Page 15