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S3 PROTECTION
6–42
If the RTD Biased Thermal Capacity Used value is higher than the Thermal Overload
Thermal Capacity Used, then that value is used to replace the Thermal Overload Thermal
Capacity Used. If the RTD Biased Thermal Capacity is lower than the Thermal Overload
Thermal Capacity Used, the Thermal Overload curve does not need to be biased by stator
RTD temperature.
Typically, the RTD BIAS MINIMUM is set as 40
rated motor running temperature, and the RTD BIAS MAXIMUM is set at the stator
insulation rating or slightly higher.
where:
TCU
– the value of the Thermal Capacity Used register when the hottest stator
Center
RTD temperature is equal to the setpoint, RTD Bias – Center T.
T
– the temperature in degrees Celsius of the hottest RTD that is neither
HottestStator
open nor short and is declared to be a stator RTD. If there is no such RTD, use a value
of zero.
T
– the value of the setpoint, RTD Bias – Minimum T.
Min
T
– the value of the setpoint, RTD Bias – Center T.
Center
T
– the value of the setpoint, RTD Bias – Maximum T.
Max
TCU
– the RTD Biased Thermal Capacity determined by the hottest stator RTD
RTDbias
temperature.
HCR – the setting HOT/COLD SAFE STALL RATIO
Note that the RTD bias feature alone cannot create a trip. If the RTD bias forces Thermal
Capacity Used to 100%, the motor current must be above the overload pickup before an
overload trip occurs.
COOLING RATE
The Thermal Capacity Used value decreases exponentially when the motor equivalent
current (I
) is less than the Thermal Overload Pickup setting. This reduction simulates
eq
motor cooling. As a stopped motor normally cools significantly slower than a running
motor, the relay has two cooling time constant setpoints, one is used when the motor is not
in service (stopped, tripped, locked out, etc.), the other is used when the motor is in service
(starting, running). The time constant is, in each case, the time in minutes for the motor's
temperature to cool by 63% of the difference between the initial temperature and ambient
temperature.
Motor cooling is calculated as:
where:
o
C, the RTD BIAS CENTER POINT is set at the
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 12
Eq. 13
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