APPENDIX C APPLICATION NOTES
C-1
Selection of Cool Time Constants
Thermal limits are not a black and white science and there is some art to setting a protective relay thermal model. The definition of
thermal limits mean different things to different manufacturers and quite often, information is not available. Therefore, it is important to
remember what the goal of the motor protection thermal modeling is: to thermally protect the motor (rotor and stator) without impeding
the normal and expected operating conditions that the motor will be subject to.
The thermal model of the SR469 provides integrated rotor and stator heating protection. If cooling time constants are supplied with the
motor data they should be used. Since the rotor and stator heating and cooling is integrated into a single model, use the longer of the
cooling time constants (rotor or stator).
If however, no cooling time constants are provided, settings will have to be determined. Before determining the cool time constant
settings, the duty cycle of the motor should be considered. If the motor is typically started up and run continuously for very long periods
of time with no overload duty requirements, the cooling time constants can be large. This would make the thermal model conservative.
If the normal duty cycle of the motor involves frequent starts and stops with a periodic overload duty requirement, the cooling time
constants will need to be shorter and closer to the actual
thermal limit
of the motor.
Normally motors are rotor limited during starting. Thus RTDs in the stator do not provide the best method of determining cool times.
Determination of reasonable settings for the running and stopped cool time constants can be accomplished in one of the following
manners listed in order of preference.
1. The motor running and stopped cool times or constants may be provided on the motor data sheets or by the manufacturer if
requested. Remember that the cooling is exponential and the time constants are one fifth the total time to go from 100% thermal
capacity used to 0%.
2. Attempt to determine a conservative value from available data on the motor. See the following example for details.
3. If no data is available an educated guess must be made. Perhaps the motor data could be estimated from other motors of a similar
size or use. Note that conservative protection is better as a first choice until a better understanding of the motor requirements is
developed. Remember that the goal is to protect the motor without impeding the operating duty that is desired.
Example:
1) Motor data sheets state that the starting sequence allowed is 2 cold or 1 hot after which you must wait 5 hours before
attempting another start.
• This implies that under a normal start condition the motor is using between 34 and 50% thermal capacity. Hence,
two consecutive starts are allowed, but not three.
• If the hot and cold curves or a hot/cold safe stall ratio are not available program 0.5 (1hot/2cold starts) in as the
hot/cold ratio.
• Programming
Start Inhibit
‘On’ makes a restart possible as soon as 62.5% (50h1.25) thermal capacity is available.
• After 2 cold or 1 hot start, close to 100% thermal capacity will be used. Thermal capacity used decays exponentially
(see SR469 manual section on motor cooling for calculation). There will be only 37% thermal capacity used after 1
time constant which means there is enough thermal capacity available for another start. Program 300 minutes (5
hours) as the
stopped cool time constant
. Thus after 2 cold or 1 hot start, a stopped motor will be blocked from
starting for 5 hours.
• Since the rotor cools faster when the motor is running, a reasonable setting for the running cool time constant might
be half the stopped cool time constant or 150 minutes.
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