Undercurrent - GE 339 Instruction Manual

S3 PROTECTION

Undercurrent

6–78
When the motor is in the running state, an alarm will occur if the magnitude of any phase
current falls below the undercurrent alarm pickup level for the time specified by the
undercurrent alarm delay. Furthermore, a trip will occur if the magnitude of any phase
current falls below the undercurrent trip pickup level for the time specified by the
undercurrent trip delay. The alarm and trip pickup levels should be set lower than the
lowest motor loading during normal operations.
For example, if a pump is cooled by the liquid it pumps, loss of load may mean that the
pump overheats. In this case, enable the undercurrent feature. If the motor loading
should never fall below 75% FLA, even for short durations, the Undercurrent Trip Pickup
could be set to "70% FLA" and the Undercurrent Alarm Pickup be set to "75% FLA". The
Undercurrent Trip Delay and Undercurrent Alarm Delay settings are typically set as quick
as possible, i.e. 1.00 second.
The Undercurrent element is active only when the motor is running and is blocked upon
the initiation of a motor start for a period of time defined by the setting BLK U/C ON START.
This block may be used to allow pumps to build up head before the undercurrent element
trips or alarms.
A second independent Undercurrent Protection element is provided for High Speed. If two-
speed functionality is enabled, the 339 relay relies on the motor speed indication to switch
the undercurrent settings as per the motor running speed, so the main Undercurrent
Protection element is only active when the motor is running at low speed, and the High
Speed Undercurrent Protection element is only active when the motor is running at high
speed. If two-speed functionality is not deployed, only the main Undercurrent is active, and
the High Speed Undercurrent is disabled.
PATH:
SETPOINTS > S3 PROTECTION > UNDERCURRENT
U/CURR ALARM FUNC
Range: Disabled, Enabled
Default: Disabled
This setting enables the Undercurrent alarm functionality.
BLK U/C ON START
Range: 0 to 600 s in steps of 1 s
Default: 0 s
This setting specifies the length of time to block the undercurrent function when the
motor is starting. The undercurrent element is active only when the motor is running and
is blocked for a period of time specified by this setting, upon the initiation of a motor
start. A value of 0 s specifies that the feature is not blocked from start.
U/CURR ALARM PKP
Range: 1% to 100% FLA in steps of 1% FLA
Default: 70% FLA
This setting specifies a pickup threshold for the alarm stage. The alarm pickup threshold
should be less than the motor load current during normal operation.
U/CURR ALARM DELAY
Range: 1.00 to 60.00 s in steps of 0.01 s
Default: 1.00 s
This setting specifies a time delay for the alarm stage. The time delay should be long
enough to overcome any short lowering of the current (e.g. during system faults).
U/CURR TRIP FUNC
Range: Disabled, Enabled
Default: Disabled
This setting enables the Undercurrent trip functionality.
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS