Cool Time Constants - GE Multilin 469 Instruction Manual

A.2

Cool Time Constants

A.2.1 Selection of Cool Time Constants
A–4
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 information is often not available. Therefore, it is important to
remember the goal of the motor protection thermal modeling: to thermally protect the
motor (rotor and stator) without impeding the normal and expected operating conditions
of the motor.
The 469 thermal model provides integrated rotor and stator heating protection. If supplied
with the motor, the cooling time constants recommended by the manufacturer should be
used. Since rotor and stator heating and cooling is integrated into a single model, use of
the longest cooling time constants (rotor or stator) is recommended.
If no cooling time constants are provided by the motor manufacturer, settings will have to
be determined. Before determining the cool time constant settings, the motor duty cycle
must 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, making 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 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 requested from the manufacturer. Remember that
the cooling is exponential and the time constants are one fifth of the total
time interval from 100% to 0% thermal capacity used.
2. Attempt to determine a conservative value from the available motor data. See
the following example for details.
3. If no motor 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 the best 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
Motor data sheets state that the starting sequence allowed is two (2) cold or one (1) hot,
after which you must wait five (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) as the
HOT/COLD SAFE STALL RATIO
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
A: APPENDIX
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