Electrical Distribution Systems
When a low voltage condition
Figure 13 LVR Controller
occurs, the M coil will drop out at
some pre-determined value of
voltage, and the M and Ma
contacts will open. The START
button must then be pushed to
restart the motor.
Depressing the STOP button de-
energizes the M coil, which then
opens the M and Ma contacts.
The purpose of the LVR controller is to
de-energize the motor in a low voltage
condition and restart the motor when
normal voltage is restored. This type of
controller (Figure 13) is used primarily on
small and/or critical loads (e.g., cooling water pumps required for safety-related equipment).
LVR Controller Operation:
Place the START switch in Run which energizes coil M, closing the M contacts and
starting the motor.
When a low voltage condition occurs, the M coil drops out, opening the M contacts and
de-energizing the motor. When normal voltage is restored, the M coil is again energized,
closing the M contacts and restarting the motor.
The LVRE controller maintains
Figure 14 LVRE Controller
the motor across the line at all
times. This type of controller
is of the manual variety and is
found mostly on small loads
that must start automatically
upon restoration of voltage
controller may or may not
overloads are used, they will
be placed in the lines to the
The motor controllers that have been discussed are very basic. There are many automatic control
functions that can be incorporated into these types of controllers, but they are beyond the scope
of this text.