GE 239 Instruction Manual page 64

4 PROGRAMMING
Under normal balanced conditions the stator current in each of the three motor phases is equal
and the rotor current is just sufficient to provide the turning torque. When the stator currents are
unbalanced, a much higher current is induced in the rotor because it has a lower impedance to
the negative sequence current component present under unbalanced conditions. This current is
normally at about twice the power supply frequency and produces a torque in the opposite direc-
tion to the desired motor output. Usually the increase in stator current is small (125 to 200%) so
that timed overcurrent protection takes a long time to trip. However the much higher induced
rotor current can cause extensive rotor damage in a short period of time. Motors can tolerate dif-
ferent levels of current unbalance depending on the rotor design and heat dissipation character-
istics.
• UNBALANCE TRIP PICKUP: Unbalance protection is recommended at all times. Motor data is
rarely provided and direct measurement of rotor temperature is impractical so setting the unbal-
ance level is empirical. For a known balanced situation, a pickup level of 10% and time delay of 5
seconds is recommended as a starting point. The pickup level can be decreased until nuisance
tripping occurs. Similarly the time delay may be increased if necessary.
To prevent nuisance trips/alarms on lightly loaded motors when a much larger unbalance level
4
will not damage the rotor, the single phase detection will automatically be defeated if the average
motor current is less than 30% of the full load current ( I
≥
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
FLC
• UNBALANCE ALARM: The operation of this feature is identical to the operation of the unbal-
ance trip feature.
• UNBALANCE ALARM PICKUP: The operation of this feature is identical to the operation of the
unbalance trip pickup feature.
UNBALANCE DELAY: If phase current unbalance increases above 81%$/$1&( $/$50 3,&.83 or
•
81%$/$1&( 75,3 3,&.83 setpoint value and remains this way for the time delay programmed in this
setpoint, the respective relay will activate and the respective warning message will be displayed.
g) HOT MOTOR
• THERMAL CAPACITY USED: This feature is used to signal a warning when the thermal capac-
ity has exceeded a level set in this setpoint. Once the set level is exceed the alarm relay will acti-
vate immediately and the "THERMAL CAPACITY USED ALARM" message will be displayed.
h) BREAKER FAILURE
• BREAKER FAILURE FUNCTION: This feature is used to activate the selected relay, if the cur-
rent continues to flow after a trip has occurred. If the feature is assigned to $/$50 or $/$50
the "BREAKER FAILURE ALARM" message will be displayed and the assigned output relay will
be active. If the function is assigned to $8;,/,$5< , the auxiliary output relay will be active but, no
message will be displayed.
4-26
Courtesy of NationalSwitchgear.com
I I
–
m av
×
-------------------- -
: 100%
I
av
I – I
m av
×
-------------------- -
: 100%
I
FLC
= motor full load current setting
239 Motor Protection Relay
4 PROGRAMMING
) setting. Unbalance is calculated as:
FLC
av
$8; ,
GE Multilin