(C) Plug-Interlock Relays; (D) Test-Bank Switches; (E) Resistor; (F) Fuse-Interlock Relay - Hobart 140GT17P Operation And Maintenance Manual With Illustrated Parts List

(c) Plug-interlock relays

Each ouput of the generator has a plug interlock relay. For either output, the function of the
plug interlock relay is to cause its output load contactor to open in the event the cable plug
connector becomes accidentally disconnected from the aircraft during power delivery, or if an
attempt is made to deliver power when the output cable is not connected to the aircraft.
Twenty-eight-volt, direct current for operation of the relay is supplied from the aircraft either
through an on-board transformer-rectifier, or from a twenty-eight-volt, electrical system. Con-
nection from the aircraft to the interlock relay is made through terminals E and F on the output
cable plug connector.

(d) Test-bank switches

Each ouput of the generator has a test-bank switch. For either output, the function of this sin-
gle-pole-single-throw switch is to provide a means of by-passing the output's interlock relay
when supplying power to a load bank or to an aircraft not equipped with a plug interlock sys-
tem.

(e) Resistor

Each output of the generator has a 100-ohm, 25-watt resistor connected in series with the
plug interlock relay to protect the relay in the event that phase C contacts in the load contac-
tor should fail to close when the generator ON switch is operated. For outputs No. 1 and No.
2, the resistors are items 22 and 20, respectively.
A variable 20-ohm, 100-watt, ballast resistor (21) is connected in series in the generator ex-
citer DC field and thus determine the voltage range through which the regulator can control
generator output voltage.

(f) Fuse-interlock relay

The function of the fuse-interlock relay (6) is to interrupt the load contactor holding coil circuit
and remove the load in case of a tripped circuit breaker in the protective relay coil circuit.
(g) Auxiliary underfrequency relay
The function of the auxiliary underfrequency relay (7) is to automatically open the excitation-
deenergization relay and disconnect the voltage regulator anytime generator frequency drops
to 380 Hz or below. This protects the voltage regulator and ballast resistor (24) against over-
load which could be caused by very high voltage regulator output in its attempt to maintain
voltage when the generator is operating at a speed which cannot produce normal voltage out-
put.
NOTE: If the auxiliary underfrequency relay is tripped, it will be necessary to momentarily place engine
operating mode switch in BUILD-UP-VOLTAGE position to restore generator voltage.
(h) Regulated-diagnostic switch
When the regulated-diagnostic switch (26) is in the REGULATED position, generator output
voltage is regulated by the voltage regulator or 115/200 VAC output to an aircraft. When this
switch is placed in the DIAGNOSTIC position, battery voltage (12VDC) is applied to the gen-
erator exciter with the engine running at rated RPM, in order to check the operation of the
generator set. By applying this DC voltage to the exciter and observing generator output volt-
age, it can be determined if a particular power output malfunction is caused by a defective
generator or by a defective voltage regulator.
(j) Excitation-deenergization relay
The purpose of this relay (5) is to allow automatic excitation to be connected to the exciter
field only when the engine speed is being controlled by the electric governor.
July 27/90 Revised
OM-2029
1-1
Page 15