C.1.1 Description - GE MOTOR MANAGEMENT RELAY 469 Instruction Manual

APPENDIX C
APPENDIX C APPENDIX CC.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, informa-
tion is not available. Therefore, it is important to remember what the goal of the motor protection thermal mod-
eling 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 con-
stants 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 4.6.5: MOTOR COOLING on page 4–43 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 con-
stant might be half the stopped cool time constant or 150 minutes.
GE Power Management
C.1 SELECTION OF COOL TIME CONSTANTS
469 Motor Management Relay

C.1.1 DESCRIPTION

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C.1.2 EXAMPLE
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