GE 469 Instruction Manual

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Contents

A–4 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUALA: APPENDIXA.2 Cool Time ConstantsA.2.1 Selection of Cool Time ConstantsThermal limits are not a black and white science and there is some art to setting aprotective relay thermal model. The definition of thermal limits mean different things todifferent manufacturers, and information is often not available. Therefore, it is important toremember the goal of the motor protection thermal modeling: to thermally protect themotor (rotor and stator) without impeding the normal and expected operating conditionsof the motor.The 469 thermal model provides integrated rotor and stator heating protection. If suppliedwith the motor, the cooling time constants recommended by the manufacturer should beused. Since rotor and stator heating and cooling is integrated into a single model, use ofthe longest cooling time constants (rotor or stator) is recommended.If no cooling time constants are provided by the motor manufacturer, settings will have tobe determined. Before determining the cool time constant settings, the motor duty cyclemust be considered. If the motor is typically started up and run continuously for very longperiods of time with no overload duty requirements, the cooling time constants can belarge, making the thermal model conservative. If the normal duty cycle of the motorinvolves frequent starts and stops with a periodic overload duty requirement, the coolingtime 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 providethe best method of determining cool times. Determination of reasonable settings for therunning and stopped cool time constants can be accomplished in one of the followingmanners listed in order of preference.1. The motor running and stopped cool times or constants may be provided onthe motor data sheets or requested from the manufacturer. Remember thatthe cooling is exponential and the time constants are one fifth of the totaltime interval from 100% to 0% thermal capacity used.2. Attempt to determine a conservative value from the available motor data. Seethe following example for details.3. If no motor data is available, an educated guess must be made. Perhaps themotor data could be estimated from other motors of a similar size or use. Notethat conservative protection is the best first choice until a betterunderstanding of the motor requirements is developed. Remember that thegoal is to protect the motor without impeding the operating duty that isdesired.ExampleMotor 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 and50% 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.