Onan MJA Series Service Manual page 96

94
REVOLVING FIELD GENERATOR
To aid servicing and repair, all output leads from the generator and
connections to the exciter are marked with metal tags. The lead and
terminal markings are noted on the plant wiring diagram, see Section
10.
Magneciteris the trade name of ONAN'S static exciter system. As the
name implies, it has no moving parts but uses magnetic amplifiers and
rectifiers to supply direct current to the alternator's revolving field
and regulate the ac output.
Because
it
is a relatively new development, a brief description of the
.1
Magneciter's operation is given here to aid understanding and repair;
The Magneciter functions as a power supply for the revolving field and
a voltage regulator. By regulating the amount of current to the field,
it
controls the ac output of the generator. Here is how it works.
/ RECTIFIERS
FIELD " \ 120
v
120
v:
A.C
/I.C­
TERMINALS
TERM­
WINDING
INALS
'-RECrtFURS/
t..FIEL[)RECTlF'lERS
WINDING
RE.CTIFIERS
FIG 4. C - POWER SUPPLY
FIG 4.D- POWER SUPPLY WITH
GATE REACTORS
The circuit shown in Fig. 4. C is the power supply. It's full wave rec ­
tifier made up of 2 half wave rectifiers and supplies direct current to
the field. In order to regulate the generator output voltage~ some form
of control over the current flowing in the field is necessary. Two gate
reactors provide this control (Fig. 4. D).
Each gate reactor is a metal doughnut shaped core with 2 windings, an
output or gate wjnding and a control winding. The amount of current
"
the reactor allows to flow in the gate winding is dependent on the amount
of magnetism in the core. The current flow increases - (the gate
open~­
when there is more magnetism in the core, until finally when the core
is saturated, (the gate is all the way open) the reactor then does not op ­
pose current flow. Since the rectifiers allow current in the gate winding
to flow in only 1 direction, it can act only to magnetize
~he
core. If
the magnetism in the core were decreased this would requce the cur ­
rent flow through the gate winding. That is the purpose of the control
windings.