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GM COACH MAINTENANCE MANUAL
Sec. 7
Page 127
resistance and reducing the voltage or current,
depending on which regulator is called upon to
function.
Under a steady condition of speed and
load, the regulator assumes the position required
to furnish the proper resistance and remains static
as long as this condition persists. This character-
istic tends to prolong the useful life of the carbon
pile regulator.
The very small change in length of the carbon
pile in operation is necessary to prevent uneven
operation which might otherwise result from over-
control.
It is also necessary to use a special
spring which closely matches the pull of the elec-
tromagnet over the whole range of movement. The
effect of surges is partially nullified and stability
is improved by the incorporation of a small oil-
filled dash pot which acts much like a shock ab-
sorber on the movements of the armature of the
electromagnet.
SYSTEM VOLTAGE RHEOSTAT
In addition to the current and voltage regula-
tors, which operate as described, a small rheostat
is included in the control portion of the regulator.
This rheostat, called the system voltage rheostat,
is connected in series with the coil of the elec-
tromagnet in the voltage regulator and a pair of
paralleled fixed resistors (fig. 20). By changing
this variable resistance, the voltage setting of the
regulator can be raised or lowered within a small
range (approximately 3 volts). The system volt-
age rheostat is mounted on the wall of the box so
that it can be adjusted from the outside. This of-
fers a ready means of correcting the system volt-
age (within limits) without removing the cover of
the regulator or changing mechanical adjustments
of the units.
ACTUATING, CIRCUIT BREAKER,
AND OVERLOAD RELAYS
The remainder of the regulator is made up of
the actuating and circuit breaker relays and the
special overload relay.
Each of these relays per-
forms a different function and it is important that
their oper::i.tion be understood. Generally speaking,
the actuating and circuit breaker relays act to-
gether to open and close the battery circuit in
accordance with generator voltage, while the over-
load relay prevents excessive reverse currents.
Details of these operations are covered in the
following paragraphs.
The actuating relay contains two windings and
one set of contact points. One of the windings is
a series winding of a few turns of heavy wire
(fig. 20) which is part of the charging circuit.
The second winding is a shunt winding consisting
of many turns of fine wire (shown in dashed lines)
which, in series with a resistor, is shunted across
the generator. When the generator is not operating,
REGULATOR
the actuating relay contact points are held open
by the tension of a spiral spring.
The circuit
breaker relay (fig. 19) contains a shunt winding
on a core (shown in dashed lines in fig. 20), above
which is an armature with two heavy contact points.
Beneath these points are two stationary contact
points, one of which is connected through the ac-
tuating relay and current regulator series wind-
ings to the "ARMATURE" terminal and thence to
the insulated terminal of the generator. The other
is connected directly to the regulator "BATTERY"
terminal and thence to the battery through the
wiring circuit. The · contact points are held open
by the tension of a spiral spring when the gener-
ator is not operating.
When the generator begins to operate, a mag-
netic field is built up by the actuating relay shunt
winding. When generator voltage reaches the value
for which the relay is adjusted, the magnetic field
is sufficiently strong to pull the armature down
toward the core, causing the contact points to
close.
Closing of the contact points connects the
circuit breaker relay shunt winding across the
generator.
This creates a strong magnetic field
which pulls the circuit breaker relay armature
down so that the contact points close and complete
the circuit between the generator and battery.
Charging current then flows in the series winding
of the actuating relay, building up an additional
magnetic field which assists the field of the shunt
winding in holding the armature down.
When the generator voltage drops below bat-
tery voltage, current flows from the battery to
the generator. This reverses the flow of current
through the actuating relay series winding. As a
result, the magnetic fields of the series winding
and the shunt winding no longer assist each other,
but become magnetically opposed. The resultant
magnetic field becomes too weak to hold the ac-
tuating relay armature down and the contact points
are opened by spring tension.
This interrupts the
flow of current to the circuit breaker relay shunt
winding, causing a sudden loss of magnetic field
strength which releases the armature and allows
the contact points to open. The sudden interrup-
tion of current flow in the circuit breaker shunt
winding causes a surge of induced voltage which
is partly dissipated by a resistor connected in
parallel with the winding (fig. 20). This resistor
action reduces the energy dissipated through arc-
ing at the contacts of the actuating relay.
The overload relay contains two windings and
one set of contact points. One of the windings is
a series winding consisting of one turn of heavy
wire (fig. 20) which is a part of the charging cir-
cuit. The second winding is a shunt winding con-
sisting of many turns of fine wire (shown in dashed
lines) which is shunted across the generator. The
series and shunt windings are wound so that their
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