Selection Of Cool Time Constants; Example - GE 469 Instruction Manual

APPENDIX A
A.2 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 def-
inition of thermal limits mean different things to different manufacturers and quite often, information is not available. There-
fore, 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 469 thermal model 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.
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%.
Attempt to determine a conservative value from available data on the motor. See the following example for details.
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.
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 (1 hot / 2 cold starts) in as the hot/
cold ratio.
Programming Start Inhibit 'On' makes a restart possible as soon as 62.5% (50 × 1.25) 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 Section e): Motor Cooling on page 4–39 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.
GE Multilin
Courtesy of NationalSwitchgear.com
A.2.1 SELECTION OF COOL TIME CONSTANTS
469 Motor Management Relay
A.2 COOL TIME CONSTANTS
A.2.2 EXAMPLE
A
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