GE Digital Energy Multilin 239, Instruction Manual

Page: 71 / 160

CHAPTER 4: PROGRAMMING
239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL 4–33
4.5.1 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 4–34 and choose the curve that
falls just below the motor damage curve.
Each of the curves represents an I
2
t characteristic of a motor. If no motor curve data is
available, this setpoint can be set using the locked rotor time from the motor
nameplate. Plot the point corresponding 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 4–34 and the next lowest curve selected. Curve points
are also shown in tabular form in Table 4–2: 239 Phase Overload Trip Times (Seconds)
on page 4–34. Points for a selected curve can be plotted directly on curves for
associated 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 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.
• OVERLOAD LOCKOUT TIME: The motor cooling rate is controlled by this setpoint.
Enter a typical time of 30 minutes to allow sufficient cooling. If process criteria
requires shorter cooling periods, 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.
VALUE  VALUE 
STORE
Table 4–1: Overload Trip Time Calculation
TRIP TIME RANGE DISPLAY RESOLUTION
trip time
<
100 seconds 0.01 x seconds
trip time
≥
100 seconds and
<
600 seconds 0.1 x seconds
trip time
≥
600 seconds and
<
6000 seconds 1.0 x seconds
trip time
≥
6000 seconds 1.0 x minutes