GE 869, Instruction Manual

Page: 275 / 606

CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–143
Mechanical Jam
(50LR)
A motor load can become constrained (mechanical jam) during starting or running. The
starting current magnitude alone cannot provide a definitive indication of a mechanical
jam; however, the running current magnitude can. Therefore, the Mechanical Jam element
is specially designed to operate for running load jams. Starting load jams are detected by
monitoring acceleration time and speed.
After a motor has started and reached the running state, a trip or alarm occurs if the
magnitude of any phase current exceeds the setting Pickup for a period of time specified
by the setting Pickup Delay.
The thermal element also operates during mechanical jams but after a delay when the
thermal capacity reaches 100%. Not only does the Mechanical Jam protect the motor by
tripping it quicker than the thermal protection, it can also prevent or limit damage to the
driven equipment in the event of a locked rotor during running.
The Mechanical Jam is armed as long as the motor status is not “Starting”; this includes
“Running”, “Overload”, and “Stopped”. As soon as any phase current exceeds the user-
selectable threshold, the element picks up and operates after the programmed time delay.
The element uses currents configured under
Setpoints > System > Current > Sensing
and
motor status asserted by the thermal model element. Both the signal source and thermal
protection must be configured properly in order for the mechanical jam protection to
operate.
When the 2-Speed Motor Protection functionality is employed, the 869 will block
Mechanical Jam Protection during the acceleration time from Speed 1 to Speed 2 until the
motor current has dropped below overload pickup level (OL x FLA) or the
ACCEL TIME Fr.
SPD 1-2
has expired. At a point in time when the motor has reached the Speed 2 running
stage, the Mechanical Jam will be re-enabled using the setpoint
Speed2 Motor FLA
set
under Setpoints > System > Motor > Setup . The Pickup level must be set higher than the
motor loading during normal operation, but lower than the motor stall level for both
speeds. Normally the delay is set to the minimum time delay or set so that no nuisance
trips occur due to momentary load fluctuations.
Path: Setpoints > Protection > Group 1(6) > Motor > Mechanical Jam
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
The setting enables the Mechanical Jam functionality.
PICKUP
Range: 1.00 to 10.00 x FLA in steps of 0.01
Default: 2.00 x FLA
The setting defines the excessive current condition that identifies a mechanical jam. As
the element is not armed during start conditions, this threshold can be set below the
starting current. Since the element is armed during overload conditions, this setting
must be higher than the maximum overload current. The setting is entered in multiplies
of FLA (programmed under
Setpoints > System > Motor > Setup menu).
PICKUP DELAY
Range: 0.10 to 180.00 s in steps of 0.01 s
Default: 1.00 s
The setting specifies the pickup delay of the element. In the case of large motors that
could feed close-in feeder faults, this setting can coordinate with feeder protection to
avoid false tripping due to excessive fault currents fed by the motor.
DROPOUT DELAY
Range: 0.00 to 180.00 s in steps of 0.01 s
Default: 1.00 s
The setting defines the reset delay of the element. Typical application includes time
seal-in of the tripping command