shunt field winding (SH) is connected across the line
and establishes normal shunt field strength. Contacts
LC3 close and prepare the circuit for the accelerating
contactor coil (AC). Contacts LC4 close the holding
circuit for the line contactor coil (LC).
The motor armature current flowing through the
series relay coil causes its armature to pull in, opening
the normally closed contacts (SR). As the motor speed
picks up, the armature current drawn from the line
decreases. At approximately 110 percent of normal
running current, the series relay current is not strong
enough to hold the armature in; therefore, it drops out
and closes its contacts (SR). These contacts are in series
with the accelerating relay coil (AC), and cause it to pick
up its armature, closing contacts AC1 and AC2.
Auxiliary contacts (AC1) on the accelerating relay
Figure 6-22.--AND symbol and circuit.
keep the circuit to the relay coil closed while the main
contacts (AC2) short out the starting resistor and the
AND and OR logic circuits are used in logic
series relay coil. The motor is then connected directly
controllers. Their use is discussed in this section of the
across the line, and the connection is maintained until
the STOP button is pressed.
One common application of logic control that is
If the motor becomes overloaded, the excessive
being incorporated on newer ships is the elevator
current through the overload coil (OL) (at the top right
system. Since this system is large and consists of many
of fig. 6-21) will open the overload contacts (OL) (at the
symbols, only a small portion of this system is
bottom of fig. 6-21), disconnecting the motor from the
Assume that the elevator platform is on the third
If the main contactor drops out because of an
deck and that you require it on the main deck. Refer to
excessive drop in line voltage or a power failure, the
motor will remain disconnected from the line until an
operator restarts it with the START push button. This
prevents automatic restarting of equipment when
normal power is restored.
The basic concept of logic circuits is shown in
figures 6-22 and 6-23. As you read this section, refer to
In figure 6-22, view A, an AND symbol is shown.
The AND symbol can be compared to the electrical
circuit in figure 6-22, view B. (NOTE: Both switches
A AND B must be closed to energize the lamp.)
In figure 6-23, view A, an OR symbol is shown. The
OR symbol can be compared to the electrical circuit in
figure 6-23, view B. (NOTE: Either switch A OR B
Figure 6-23.--OR symbol and circuit.
needs to be closed to energize the lamp.)