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S3 PROTECTION
6–64
HOT/COLD BIASING
When the motor is running with a constant load below the overload level, the motor will
eventually reach a steady state temperature, which corresponds to a particular steady
state Thermal Capacity Used. As some thermal capacity is used, there is less thermal
capacity left in the motor to cover transient overloads than is available when the motor is
cold. Typically, the extent of this effect is calculated by taking the ratio of the motor's rated
Safe Stall Time Hot to its rated Safe Stall Time Cold. Safe Stall Time (also known as Locked
Rotor Time) is the time taken with the rotor not turning, for the motor to heat, at an
unacceptable rate, to a temperature beyond which motor damage occurs. "Cold" refers to
starting off with the motor at ambient temperature, "Hot" refers to starting off with the
motor at the temperature reached when running at rated load. The method used by the
thermal overload curve to account for the pre-overload state, is thus known as hot/cold
biasing.
The Hot/Cold Ratio setpoint is determined by the equation shown below:
where: HCR is the value of the Hot/Cold Ratio setpoint expressed as a fraction of 1.00.
The steady state Thermal Capacity Used is calculated according to the equation:
where: TCU
is the steady state Thermal Capacity Used expressed as a percentage. I
ss
the equivalent motor heating current in per-unit on an FLA base, which was discussed in
the unbalance biasing section above.
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.
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 9
Eq. 10
is
eq
Eq. 11
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