Page 184 - Power Quality in Electrical Systems

166 Chapter Eleven

Three-phase source
voltage to tool
Main AC Box
On Emergency EMO
button off button relay

Step-down
transformer
EMO relay EMO relay
Main
24 V AC contact 2 contact 1 contactor

Main
contactor
contacts
To tool, subsystems,
and other EMO circuits
Figure 11.13 Typical main contactor control circuit. Emergency OFF button [11.10].

system. For example, in Figure 11.13, the “On” button and EMO Relay
Contact 2 “order” the EMO Relay and the Main Contactor to close their
contacts, to start and maintain power to the motor. A sag or interrup-
tion in line voltage can cause either or both the relay and contactor to
“drop out,” open their contacts, and shut down the motor. The contac-
tor or relay has “failed” from the voltage disturbance.
The elementary magnetic circuit for the contactor or relay is shown
in Figure 11.14 [11.13]. The device is a DC relay, activated by a DC coil
current. The current produces a magnetic field B in the air gap. The
m
force in the armature is proportional to B m 2 . At a given coil current, the

Iron core Air gap Spring

Bm
+
Source
–
Iron armature

Figure 11.14 Elementary diagram for an electromagnetic relay or
contactor [11.13].