Undercurrent; Unbalance - GE 239 Instruction Manual

CHAPTER 4: PROGRAMMING
4.5.5

Undercurrent

4.5.6

Unbalance

239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
• 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
will never operate during a start.
• 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 broken 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 whenever 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 process 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 create 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
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
would always sense an undercurrent condition with the auxiliary relay energized.
Bearing wear could cause the current to increase above 95 A causing the
undercurrent 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
UNDERCURRENT PICKUP
programmed in this setpoint, the alarm relay will activate and the "UNDERCURRENT
ALARM" message will be displayed if the setpoint
CURRENT\UNDERCURRENT\UNDERCURRENT FUNCTION
setpoint S4: PROTECTION\PHASE CURRENT\UNDERCURRENT\UNDERCURRENT
FUNCTION is set to AUXILIARY
displayed after the delay expires.
• 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
UNLIMITED
UNDERCURRENT PICKUP
UNDERCURRENT FUNCTION
setpoint value and remains this way for the time delay
S4: PROTECTION\PHASE
, the auxiliary relay will activate and no message will be
, the mechanical jam function
to be above the normal
setpoint, the 239
is set to . If the
ALARM
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