are electrically interlocked open when the
forward contactor (F) coil is energized.
In certain applications, the direction in which a dc
After the line contactor is energized, acceleration is
motor turns is reversed by reversing the connections of
accomplished in the manner described previously.
the armature with respect to the field. The reversal of
connections can be done in the motor controller by
A dc contactor is composed of an operating magnet
adding two electrically and mechanically interlocked
energized by either switches or relays, fixed contacts,
A dc motor reversing connection is shown in fig-
and moving contacts. It maybe used to handle the load
of an entire bus, or a single circuit or device. Larger
ure 1-17. Note there are two start buttons-one marked
START-EMERG FORWARD and the other marked
contacts must be used when heavy currents are to be
interrupted. These contacts must snap open or closed to
START-EMERG REVERSE. These buttons serve as
master switches, and the desired motor rotation is
reduce contact arcing and burning. In addition to these,
obtained by pressing the proper switch.
other arc-quenching means are used.
Assuming that the forward button has been pressed,
the line voltage will be applied through the button to the
When a circuit carrying a high current is interrupted,
forward contactor coil (F). This pulls in the armature and
the collapse of the flux linking the circuit will induce a
closes the normally open contacts
voltage, which will cause an arc. If the spacing between
motor armature circuit, the forward contactor holding
the open contacts is small, the arc will continue once it
and the line contactor circuit
is started. If the arc continues long enough, it will either
and opens the normally closed contacts
melt the contacts or weld them together. Magnetic
of the reverse contactor circuit. The normally closed
Figure 1-17.-Reversing dc controller.