Example - GE 469 Instruction Manual

Cool Time Constants

Example

GE Multilin
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.
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
• Programming the
START INHIBIT
soon as 62.5% (50 × 1.25) thermal capacity is available.
• After two (2) cold or one (1) hot start, the thermal capacity used will approach
100%. The thermal capacity used decays exponentially (see Motor Cooling on
page 5–46 for details). As such, the thermal capacity used after 1 time constant
will be 37%, meaning there is enough thermal capacity available for another
start. Program 300 minutes (5 hours) as the
setpoint. Thus, after two (2) cold or one (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.
http://www.GEindustrial.com/multilin
Motor Management Relay
HOT/COLD SAFE STALL RATIO
setpoint to "On" makes a restart possible as
COOL TIME CONSTANT STOPPED
469
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8–3