GE 239 Instruction Manual page 59

4 PROGRAMMING
a) OVERLOAD
• OVERLOAD CURVE: One of 15 different time/overload curves can be selected with the Phase
Overload Curve number setpoint to closely match the thermal characteristics of the motor. Over
lay motor curve data, if available, on the time overcurrent curves of Figure 4–6: PHASE TIMED
OVERLOAD CURVES on page 4–22 and choose the curve that falls just below the motor dam-
age curve.
Each of the curves represents an I
this setpoint can be set using the locked rotor time from the motor nameplate. Plot the point cor-
responding to the rated locked rotor or stall time (vertical axis) at the rated locked rotor current
(horizontal axis). For example, choose the point at 9 seconds and 6 × FLC for a motor with a
locked rotor time of 9 seconds and a locked rotor current of 6 × FLC. If the stall time is specified
at some other inrush current, the point can be plotted on the time/overload curves of Figure 4–6:
PHASE TIMED OVERLOAD CURVES on page 4–22 and the next lowest curve selected. Curve
points are also shown in tabular form in Table 4–2: 239 PHASE OVERLOAD TRIP TIMES (SEC-
ONDS) on page 4–23. Points for a selected curve can be plotted directly on curves for associ-
ated equipment to facilitate a coordination study. These points can also be entered into a
computer co-ordination program to assist in curve selection.
The phase timed overload curve will come into effect when the motor current in any phase goes
over the overload pickup × FLC level. During overload motor thermal capacity will increase
accordingly until the trip relay is activated when 100% of the available thermal capacity has been
reached. After a trip, the thermal memory locks out a reset until the motor has cooled sufficiently
(TC < 15%) to allow restarting.
• OVERLOAD TRIP TIME CALCULATION: This feature acts as a built-in calculator for a quick
check of the expected trip time at all the selectable overload values. Using the
keys, scroll through the trip levels. As the trip level is being changed the trip time will
VALUE
automatically be updated to correspond with the currently displayed value. When the
key is pressed the currently displayed trip level is kept in the memory for future reference. The
resolution of the displayed trip time is as shown in the table below.
Table 4–1: OVERLOAD TRIP TIME CALCULATION
TRIP TIME RANGE
trip time < 100 seconds
trip time ≥ 100 seconds and < 600 seconds
trip time ≥ 600 seconds and < 6000 seconds
trip time ≥ 6000 seconds
• OVERLOAD LOCKOUT TIME: The motor cooling rate is controlled by this setpoint. Enter a typi-
cal time of 30 minutes to allow sufficient cooling. If process criteria requires shorter cooling peri-
ods, particularly for small motors, a different time can be entered. Care should be exercised in
selecting short lockout times since operators may restart a hot motor resulting in damage if too
short a lockout time is chosen. Timed overload is not active during motor start. The locked rotor
current and safe stall time are used to model thermal capacity effect during starting.
• AUTO RESET O/L TRIPS: When enabled, this feature will automatically reset overload trips
once the thermal capacity (TC) decreases to 15% or less. All other types of trips are not affected
by this feature.
GE Multilin
Courtesy of NationalSwitchgear.com
2
t characteristic of a motor. If no motor curve data is available,
DISPLAY RESOLUTION
0.01 x seconds
0.1 x seconds
1.0 x seconds
1.0 x minutes
239 Motor Protection Relay
4 PROGRAMMING
/
VALUE
STORE
4-21
4