GE 239, Instruction Manual

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GE Multilin 239 Motor Protection Relay 4-25
4 PROGRAMMING 4 PROGRAMMING
4
• INHIBIT ON START FOR: If all other conditions are met for a mechanical jam feature to activate
and the motor is starting, the function will operate when the delay set in this setpoint has elapsed.
If this setpoint is set to
81/,0,7('
, the mechanical jam function will never operate during a start.
e) UNDERCURRENT
• UNDERCURRENT FUNCTION: Typical uses for undercurrent include protection of pumps from
loss of suction, fans from loss of airflow due to a closed damper or conveyor systems from a bro-
ken belt. Undercurrent can either be disabled, used as an alarm, a trip or as a process control.
Set this setpoint to off if the feature is not required. Selecting alarm relay will cause the alarm
relay to activate and display an alarm message whenever an undercurrent condition exists.
Selecting trip relay will cause the trip relay to activate and display a cause of trip message when-
ever an undercurrent condition occurs. Selecting auxiliary relay will cause the auxiliary relay to
activate for an undercurrent condition but no message will be displayed. This is intended for pro-
cess control.
For example, if the motor full load current (FLC) is set to 100 A for a pump motor, setting the
undercurrent pickup to 60% and selecting the alarm relay will cause the relay to activate and cre-
ate an alarm message when the average phase current drops below 60 A while running which
might represent loss of suction.
• UNDERCURRENT PICKUP: A further use of this feature is as a pre-overload warning. This is
accomplished by setting the
81'(5&855(173,&.83
to be above the normal operating current of
the motor but below the rated full load current. Suppose a fan normally draws 90 A and the full
load current (FLC) was set to 100 A, which was the maximum rating for the fan motor. If the
undercurrent pickup was set at 95% and the auxiliary relay was selected with the
81'(5&855(17
)81&7,21
setpoint, the 239 would always sense an undercurrent condition with the auxiliary relay
energized. Bearing wear could cause the current to increase above 95 A causing the undercur-
rent condition to disappear. If an external alarm was wired across the normally closed auxiliary
relay contacts, the alarm would sound above the normal current but before an overload occurred
signaling an abnormal condition prior to actual shut down. Alternatively, the output could be wired
to a process controller input to take automatic corrective action. The undercurrent feature works
as long as the average phase current is
≥
5% of full load current.
• UNDERCURRENT DELAY: If the average phase current drops below the
81'(5&855(173,&.83
setpoint value and remains this way for the time delay programmed in this setpoint, the alarm
relay will activate and the “UNDERCURRENT ALARM” message will be displayed if the setpoint
63527(&7,21?3+$6(&855(17?81'(5&855(17?81'(5&855(17)81&7,21
is set to
$/$50
. If the set-
point
63527(&7,21?3+$6(&855(17?81'(5&855(17?81'(5&855(17)81&7,21
is set to
$8;,/,$5<
,
the auxiliary relay will activate and no message will be displayed after the delay expires.
f) UNBALANCE
• UNBALANCE TRIP: Unbalanced three phase supply voltages are a major cause of induction
motor thermal damage. Unbalance can be caused by a variety of factors and is common in
industrial environments. Causes can include increased resistance in one phase due to a pitted or
faulty contactor, loose connections, unequal tap settings in a transformer or non-uniformly distrib-
uted three phase loads. The incoming supply to a plant may be balanced but varying single
phase loads within the plant can cause a voltage unbalance at the motor terminals. The most
serious case of unbalance is single phasing which is the complete loss of one phase of the
incoming supply. This can be caused by a utility supply problem or by a blown fuse in one phase
and can seriously damage a three phase motor.