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5 MONITORING
matically switch to a bar graph showing multiples of full load current. Once the current drops
below the motor full load setting, the display will revert to the three phase currents. The current
resolution is 0.1 A if &7 35,0$5< ≤ 50A. The resolution is 1 A if &7 35,0$5< > 50A.
•
GROUND CURRENT: Presence of ground current indicates some undesirable current to ground
leakage. The ground current reading will only be correct if the CT is wired correctly and the cor-
rect &7 35,0$5< value is entered. Verify ground current by connecting a clamp-on ammeter
around all 3 phases. If the ground current appears incorrect, check the ground CT settings in 6
6<67(0 6(783?&7 ,13876 and verify the CT wiring.
•
CURRENT UNBALANCE: Current unbalance causes rotor heating. It is calculated as:
≥
I
I
:
av
FLC
<
I
I
:
av
FLC
where:
I
= average phase current
av
I
= current in a phase with maximum deviation from I
m
I
= motor full load current setting
FLC
These formulas allow larger levels of unbalance to be tolerated by lightly loaded motors. Exces-
sive unbalance can be caused by loose terminal connections, faulty utility supply, a blown fuse or
a faulty contactor.
b) MOTOR CAPACITY
•
MOTOR LOAD: In order to gauge how closely the motor is running to its maximum capacity, the
motor load is calculated and displayed as: Motor Load = I
is the rated motor full load current entered in setpoint 6 6<67(0 6(783?02725 '$7$ . A
rent. I
FLC
value greater than 100% indicates an overloaded motor that will eventually trip on timed over-
load. Values less than 100% indicate that the motor is operating normally.
•
THERMAL CAPACITY USED: The heating effect of starts and overloads is integrated and
stored in a thermal memory that models the heat buildup within the motor. When the thermal
capacity used equals 100%, the 239 trips the motor since the motor is considered to be running
at its maximum temperature. With no overloads present, the thermal capacity used will gradu-
ally decrease to a steady state value, determined as described in 6 6<67(0 6(783?02725
'$7$?+27&2/' &859( 5$7,2 , to simulate motor cooling. When thermal capacity used is close to
100%, attempting to restart a stopped motor may result in a trip due to the rapid increase in the
thermal memory used under a start condition.
GE Multilin
Courtesy of NationalSwitchgear.com
I
I
–
m
av
×
-------------------- -
100%
I
av
I
–
I
m
av
×
-------------------- -
100%
I
FLC
239 Motor Protection Relay
5 MONITORING
av
/ I
. I
is the average 3 phase cur-
av
FLC
av
5
5-7
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