Load Contactor; Current Transformers; Rectifiers - Hobart 140GT17P Operation And Maintenance Manual With Illustrated Parts List

(1) Load contactor

Two load contactors are used in this unit. One contactor (8) serves output circuit No. 1, and
the other contactor (9) serves output circuit No. 2.
Each load contactor is a sealed unit which contains a magnetic operating coil and four sets of
contacts. The three larger contacts conduct three-phase AC generator output. A smaller con-
tact set is connected in the protective monitor circuit and supplies 12VDC power used by
sensing relays to signal the protective monitor when a fault occurs. Three-phase, 400-Hz gen-
erator output power is conducted to and through the load contactor by cables which also pass
through four sets of current transformers (3, 5, 16, and 18) .
NOTE : This contactor may be replaced by the old style contactor used in previous 60-KVA machines.
This note is for the benefit of those users who may have old style contactors Part number 75GH-566
(Hartman No. A-874C) in stock.

(2) Current transformers

(a) Ammeter current transformers
• Three current transformers (16) lower the output load current to a lesser value of definite ra-
tio (500-A to 5-A) which will operate the ammeter movement without damage. The amme-
ter dial scale is graduated and numbered so that the ammeter pointer will indicate the true
load current value rather than the meter movement current.
(b) Line-drop current transformers
• The three line-drop current transformers (18), in conjunction with burden resistors (17), de-
tect the magnitude and power factor of current flowing from generator to load. They feed a
signal to the voltage regulator which interprets the signal and alters the exciter field current
as required to maintain a constant predetermined voltage at the load (see Voltage Regula-
tor Manuals No. TM-759 for a Mag-Amp type or OM-2020 for a Solid-State type regulator).
(c) Overload current transformers
• Each output circuit has three overload current transformers, (item 5 for No. 1 output, or
item 3 for No. 2 output) which, in conjunction with burden resistors monitor output load cur-
rent in each of the three output phases, and supply a reduced value current signal to the
overload module which serves the output circuit.

(3) Rectifiers

A diode-bridge rectifier (6 and 7) receives 400-Hz AC from phase C of the generator output
an converts it to a pulsating, direct current for energization of the load contactor holding coil
only. This DC coil-holding circuit is controlled indirectly be controlling the 400-Hz AC to the
rectifier. The ground circuit for the rectifier's AC supply must pass through the relay contacts
in the protective monitor module to ground cable N. Therefore, any time a protective device
functions to open the protective monitor relay, the rectifier's AC circuit is opened. No DC is
then available for the load contactor holding coil, hence, the load contactor opens.
September 30/89 Revised
OM-2029
1-1
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