The BE1-47N has the ability to detect negative
sequence voltage levels of this magnitude.
Undervoltage and voltage balance relays have tradition
ally been applied for the protection of induction motors
from operating with one phase open. These relays may
not reliably detect this condition due to the back emf of
the motor on the open phase. However, an induction
motor, with a starting current of 6 per unit, will generate
a negative sequence voltage of 16% if fully loaded when
a fuse blows. The negative sequence voltage will be
somewhat reduced if the motor is not fully loaded. The
sensitivity of the BE1-47N and its insensitivity to fre
quency will provide reliable protection for motors.
Motor losses and current unbalance increase with the
negative sequence voltage level. The increased loss,
due to negative sequence voltage, is not a function of
motor load but is independent. A machine operating
with 3.5% negative sequence voltage will have its stated
losses increased by 25%. A blown fuse on a power factor
correction capacitor bank can result in a negative se
quence voltage increase. The increase in motor losses
can be costly.
The BE1-47N should be applied to protect motor buses
from a open phase. It can also be applied to protect
critical individual motors, static and non-rotating loads
from the effects of negative sequence voltage.
A choice of time delay characteristics allows the relay to
respond in the desired manner for a wide variety of
transient and fault conditions.
The BE1-47N may also be used to provide over/
undervoltage protection. A separate time delay is uti
lized to prevent shutdown of equipment for minor volt
age dips and to permit sequential operation when the
relay is being used in a supervisory capacity.