Figure 1-17.-Reversing dc controller.

contacts

are electrically interlocked open when the

Reversing

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

Dc Contactor

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,

contractors.

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,

Blowout Coils

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

and

in the

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

circuit contacts

and the line contactor circuit

is started. If the arc continues long enough, it will either

and opens the normally closed contacts

contacts

melt the contacts or weld them together. Magnetic

of the reverse contactor circuit. The normally closed

Figure 1-17.-Reversing dc controller.

1-13