Example Using Flc Thermal Capacity Reduction Setpoint - GE 169 Instruction Manual

6 APPLICATION EXAMPLES

6.3 Example Using FLC Thermal Capacity Reduction Setpoint

The purpose of the FLC Thermal Capacity Reduction Setpoint is to accurately reflect the reduction of thermal
capacity available (increase the thermal capacity used) in a motor that is running normally (100% of FLC or less).
This setpoint allows the user to define the amount of thermal capacity used by their motor running at 1 FLC. A motor
that is running at 10% of FLC will obviously use less thermal capacity than a motor at 100% FLC.
For example, if the FLC Thermal Capacity Reduction Setpoint is set at 30%, then with the motor running at 1 FLC,
the thermal capacity used will settle at 30%. Using the same example, with the motor running at 50% FLC, the
thermal capacity used will settle at 15% (50% of 30%). A practical example of implementation of this setpoint to
coordinate hot/cold damage curves is illustrated below.
Assume the motor manufacturer has provided the following information:
1. Maximum permissible locked rotor time (hot motor) = 15.4 seconds.
2. Maximum permissible locked rotor time (cold motor) = 22 seconds.
3. Recommended thermal limit curves are as shown in Figure 6-1.
Note: Hot motor is defined as a motor that has been running at 1 FLC, but not in an overload, for a period of time
such that the temperature remains constant (typical 90 C). Cold motor is defined as a motor which has been
stopped for a period of time such that the temperature remains constant (ambient temperature is defined by NEMA
standard as 40 C).
The hot motor locked rotor time is 30% less than the cold motor locked rotor time. Therefore the FLC Thermal
Capacity Reduction Setpoint should be set to 30%. The overload curve selected should lie below the cold thermal
damage curve. Once the motor has been running for a period of time at 1 FLC the thermal capacity used will remain
constant at 30%. The time to trip at any overload value will correspondingly be 30% less.
Once a motor comes out of an overload condition, the thermal capacity used will discharge at the correct rate which
is approximately exponential and settle at a value defined by the FLC Thermal Capacity Reduction Setpoint and the
present current value. Using the example above if the motor came out of an overload and the present current value
was 50% FLC the thermal capacity used would discharge to a value of 15% (50% of 30%).
Figure 6-1 Thermal Limit Curves
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