GE 339 Instruction Manual page 175

CHAPTER 6: SETPOINTS
NOTE:
NOTE
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
For example, a motor with a Safe Stall Time Hot of 7 seconds, and a Safe Stall Time Cold of
10 seconds would typically have the Hot/Cold Ratio set to 7/10= 0.70. If the motor current
is 0.8 pu, the steady state Thermal Capacity Used is:
If a Hot/Cold Ratio value of 1 is entered, hot/cold biasing is defeated, and unless RTD
biasing is deployed, the thermal overload curve will operate as if the motor was cold pre-
overload.
RTD BIASING
The thermal overload curves can operate based solely on measured current and the
assumption of rated ambient and normal motor cooling, as described above. However, if
the ambient temperature is unusually high, or motor cooling is blocked, the motor will
have an un-modelled temperature increase. The RTD biasing feature can correct for this by
forcing the Thermal Capacity Used register up to the value appropriate to the temperature
of the hottest stator RTD. Since RTDs are relatively slow, the rest of the thermal overload is
still required during starting and heavy overload conditions when motor heating is
relatively fast. Thus the RTD bias feature does not prevent the Thermal Capacity Used
value from rising above the value appropriate to the RTD temperature.
The value of the Thermal Capacity Used register appropriate to the RTD temperature is
determined by the straight line segmented curve shown in the figure below. This curve is
characterized by minimum, center and maximum temperature setpoints, and by the hot/
cold ratio setpoint.
Figure 6-17: RTD bias curve
80%
60%
40%
20%
TCU
Center
0
0
RDT Bias Minimum, RTD Bias Center, RTD Bias Maximum and HCR are setpoints.
If the maximum stator RTD temperature is below the RTD BIAS MINIMUM setting, no biasing
occurs. If the maximum stator RTD temperature is above the RTD BIAS MAXIMUM, then the
thermal memory is fully biased and THERMAL CAPACITY USED is forced to 100%. At values
between the maximum and minimum, the THERMAL CAPACITY USED created by the
overload curve is compared to the RTD Bias Thermal Capacity Used determined by the
hottest stator RTD temperature.
(1-HCR)x100%
50 100
S3 PROTECTION
Eq. 11
150
896816.cdr
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