Differential-Voltage-and-Reverse-Current Relay.

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TM 9-8000
b. Generator. The generator is a shunt generator with
(which is called the differential-voltage winding). As this
current flows, a magnetic field is produced that pulls the
a maximum output of 150 amperes, and is used with a
relay armature down and causes relay contacts to close.
24-volt battery set. The generator contains an additional
When the relay contacts close, the line-switch winding is
field winding of a few turns of very heavy wire through
connected across the generator so that it closes, thereby
which the entire generator output passes.  This is a
directly connecting the generator to the battery.
series winding.  It is wound so that its magnetic field
opposes the magnetic field from the shunt windings.
(2) Opening. With the relay and line switch closed so
This acts as a current-limiting device because the higher
the generator charges the battery, current flows through
the output, the greater the opposition from the series-
the battery and back to the generator by means of
winding magnetic field. When the output reaches the
ground wire and the series winding in the generator field.
rated maximum, the series-winding field is so strong that
Because the current is flowing through the series
it effectively prevents any further increase in output.
winding, there is a voltage difference between the two
More increases would strengthen the series-winding
ends of this winding. This voltage varies with the rate of
field, thus further opposing the shunt- winding field and
current. With a high generator output, a high current is
causing a decreased total field and, consequently, a
flowing and the voltage across the series winding is
drop-off of output.  This series-field winding in the
greater. This voltage is applied to  the  differential-
generator also plays an important part in the operation of
voltage-and-reverse-current relay. When the generator
the regulators and certain relays in the control system
is charging and the relay is closed, this voltage is
(para c and d, below).
c.  Pilot Relay.  There is a pilot relay for each
imposed across a second winding in the relay (the
reverse-current winding) in such a direction as to help
generator.  Because both operate the same, the one
the differential-voltage winding hold the relay contacts
used with the main generator will be discussed.  It
closed. But when the generator voltage falls below the
contains a shunt winding that is connected across the
battery voltage, the battery begins to discharge through
main generator terminals. In addition, it has two sets of
the generator. In other words, the current reverses. This
contacts: an upper set and a lower set. The upper set is
means that the current in the series-field winding, and
used in conjunction with the paralleling system, so
thus the voltage across the series-field winding, also
consider the lower set first. This lower set is open when
reverses. The resultant reverse voltage, which is applied
the generator is not operating. But when the generator
to the reverse-current winding, causes the magnetic field
begins to run and its voltage increases sufficiently to
of this winding to reverse. This winding then no longer
charge the battery, then the lower set closes. The shunt
helps the differential-voltage winding, but opposes it. As
winding in the relay produces this action because, with
a result, the total magnetic field is so weakened that the
sufficient voltage, it has a strong enough magnetic pull to
relay armature is pulled up by its spring tension and the
pull down the relay armature and close the lower
contacts open. This then opens the line switch winding
contacts. When the lower contacts close, this causes
circuit so the line switch opens. This disconnects the
the
differential-voltage-and-reverse-current relay to
generator from the battery.
operate.
e. Line Switch. The line switch is a simple magnetic
d. Differential-Voltage-and-Reverse-Current Relay.
switch. When its winding is electrically energized, it pulls
the armature down so the switch is closed. When the
(1) Closing. When the pilot relay closes its points, one
winding is disconnected, the spring pressure under the
of the windings in the differential- voltage-and-reverse-
armature moves the armature up so the switch opens.
current
relay
becomes connected between the
insulated battery terminal and the installed generator
f. Paralleling Relays.
terminal.  If the generator voltage is greater than the
battery voltage, the difference (or differential) between
(1) Connections.  In operation, the contacts of the
the two causes current to flow in the winding
paralleling relays are connected in
13-20