GE 239 Instruction Manual page 49

4 PROGRAMMING
• USE OVERLOAD PICKUP INHIBIT ON: This setpoint allows the overload pickup inhibit to be
applied during a START, RUN, or START & RUN condition.
• LOCKED ROTOR CURRENT AND SAFE STALL TIME COLD: During starting the locked rotor
current and safe stall time are used to determine how fast the thermal memory fills up. Timed
overload curves are disabled. The start time allowed depends on the actual start current.
Start Time Allowed
For example, assuming the normal inrush current is 6 × FLC. If the actual current inrush current
was only 5 × FLC on a start and the 6$)( 67$// 7,0( &2/' has been set to 20 seconds, the actual
maximum start time allowed would be:
Start Time Allowed
If the 6$)( 67$// 7,0( and /2&.(' 52725 &855(17 settings cannot be determined from the motor
nameplate, then use the above formula to determine the allowed start time. A good rule of thumb
is to set the /2&.(' 52725 &855(17 to 6 × FLC and 6$)( 67$// 7,0( to the trip time for the speci-
fied timed overload curve at 6 × FLC.
• HOT/COLD CURVE RATIO: This feature determines thermal capacity used when the motor is
running at or below the full load current setpoint. The +27&2/' &859( 5$7,2 setpoint is deter-
mined from the motor data using the Locked Rotor Time Hot and Locked Rotor Time Cold speci-
fications as shown below.
where:
LRT Hot = Locked Rotor Time Hot, is defined as the locked rotor time when the motor has
been running at FLC for a time sufficient for the motor temperature to reach a steady state
value.
LRT Cold = Locked Rotor Time Cold, is defined as the locked rotor time when the motor has
been stopped for a time sufficient for the motor temperature to reach ambient.
LRT Hot and LRT Cold are usually determined from the motor specifications. If this information is
not known, enter a typical value of 85% for the +27&2/' &859( 5$7,2 .
The +27&2/' &859( 5$7,2 setpoint is used by the 239 to thermally model the motor when the
average phase current is at or below the FLC setpoint. When the motor is cold (motor tempera-
ture at ambient) the thermal capacity used will be 0%. When the motor is hot (motor running at
FLC for a time sufficient to reach a steady state temperature) the thermal capacity used will be
calculated as 100% – +27&2/' &859( 5$7,2 , or 100 – 85 = 15% using the example value given
GE Multilin
Courtesy of NationalSwitchgear.com
= SAFE STALL TIME COLD
= SAFE STALL TIME COLD
2 2
× ( 6 ( ) ⁄ 5 ( ) )
20
=
28.8 seconds
=
HOT/COLD CURVE RATIO
239 Motor Protection Relay
4 PROGRAMMING
(
LOCKED ROTOR CURRENT
×
------------------------------------------------------------------------------------- -
(
Actual Start Current
(
LOCKED ROTOR CURRENT
×
------------------------------------------------------------------------------------- -
(
Actual Start Current
LRT Hot
×
------------------------ - 100
=
LRT Cold
2
)
2
)
2
)
2
)
4
4-11