Current; Motor Capacity - GE 239 Instruction Manual

5.3.1

Current

5.3.2

Motor Capacity

5–64
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• A: B: C: CURRENT: Current in each phase corresponding to the A, B and C phase inputs
is displayed. Current will only be measured correctly if
the installed CT primary and the CT secondary is wired to match the 1 or 5 A input. If
the displayed current does not match the actual current, check this setpoint and
wiring. During starting, the display will automatically 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
≤ 50A. The resolution is 1 A if
PRIMARY
• 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 correct
connecting a clamp-on ammeter around all 3 phases. If the ground current appears
incorrect, check the ground CT settings in
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. Excessive unbalance can be caused by loose terminal connections, faulty
utility supply, a blown fuse or a faulty contactor.
• 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
the average 3 phase current. I
setpoint
S2: SYSTEM SETUP\MOTOR DATA
overloaded motor that will eventually trip on timed overload. 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 gradually decrease to a steady state value, determined
as described in
S2: SYSTEM SETUP\MOTOR DATA\HOT/COLD CURVE RATIO
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.
CT PRIMARY
CT PRIMARY > 50A.
CT PRIMARY value is entered. Verify ground current by
S2: SYSTEM SETUP\CT INPUTS
I I
–
m av
-------------------- - × 100%
I
av
I I
–
m av
-------------------- -
× 100%
I
FLC
is the rated motor full load current entered in
FLC
. A value greater than 100% indicates an
239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
CHAPTER 5: MONITORING
is entered to match
CT
and verify the
av
/ I . I is
av FLC av
, to simulate