GE Power Management
469 Motor Management Relay
4-31
4 SETPOINT PROGRAMMING
4.6 S5 THERMAL MODEL
4
a) FUNCTION
The overload curve accounts for motor heating during stall, acceleration, and running in both the stator and the
rotor. The
OVERLOAD PICKUP LEVEL
setpoint dictates where the running overload curve begins as the motor
enters an overload condition. This is useful for service factor motors as it allows the pickup level to be defined.
The curve is effectively cut off at current values below this pickup.
Motor thermal limits consist of three distinct parts based on the three conditions of operation: locked rotor or
stall, acceleration, and running overload. Each of these curves may be provided for a hot and a cold motor. A
hot motor is defined as one that has been running for a period of time at full load such that the stator and rotor
temperatures have settled at their rated temperature. A cold motor is defined as a motor that has been stopped
for a period of time such that the stator and rotor temperatures have settled at ambient temperature. For most
motors, the distinct characteristics of the motor thermal limits are formed into a smooth homogeneous curve.
Sometimes only a safe stall time is provided. This is acceptable if the motor has been designed conservatively
and can easily perform its required duty without infringing on the thermal limit. In this case, the protection can
be conservative and process integrity is not compromised. If a motor has been designed very close to its ther-
mal limits when operated as required, then the distinct characteristics of the thermal limits become important.
The 469 overload curve can take one of three formats: Standard, Custom Curve, or Voltage Dependent.
Regardless of the selected curve style, thermal memory is retained in the
A1 STATUS\MOTOR STATUS\THERMAL
CAPACITY USED
register. This register is updated every 100 ms using the following equation:
where: time_to_trip = time taken from the overload curve at
I
eq
as a function of FLA.
The overload protection curve should always be set slightly lower than the thermal limits provided by the man-
ufacturer. this will ensure that the motor is tripped before the thermal limit is reached.
If the motor starting times are well within the safe stall times, it is recommended that the 469 Standard Over-
load Curve be used. The standard overload curves are a series of 15 curves with a common curve shape
based on typical motor thermal limit curves (see Figure 4–8: 469 STANDARD OVERLOAD CURVES and Table
4–1: 469 STANDARD OVERLOAD CURVE MULTIPLIERS on the following pages).
The standard overload curves equation is:
b) CUSTOM OVERLOAD CURVE
If the motor starting current begins to infringe on the thermal damage curves, it may become necessary to use
a custom curve to tailor the motor protection so that successful starting may occur without compromising pro-
tection. Furthermore, the characteristics of the starting thermal damage curve (locked rotor and acceleration)
and the running thermal damage curves may not fit together very smoothly. In this instance, a custom curve
may be necessary to tailor motor protection to the motor thermal limits so it may be started successfully and be
utilized to its full potential without compromising protection. The distinct parts of the thermal limit curves now
become more critical. For these conditions, it is recommended that the 469 custom curve thermal model be
used. The custom overload curve feature allows the user to program their own curve by entering trip times for
30 pre-determined current levels.
It can be seen in Figure 4–9: CUSTOM CURVE EXAMPLE on page 4–34 that if the running overload thermal
limit curve were smoothed into one curve with the locked rotor overload curve, the motor could not start at 80%
line voltage. A custom curve is required.
TC
used t
TC
used t
100ms
–
100 ms
time to trip
----------------------------
100%
×
+
=
Time to Trip
Curve_Multiplier
2.2116623
×
0.02530337
Pickup
1
–
(
)
2
0.05054758
Pickup
1
–
(
)
×
+
×
-----------------------------------------------------------------------------------------------------------------------------------------------------
=
