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1–28
•
Short Circuit Trip
The short circuit trip should be set above the maximum locked rotor current but below
the short circuit current of the fuses. The data sheets indicate a maximum locked rotor
current of 630% FLC or 6.3 × FLC. A setting of 7 × FLC with a instantaneous time delay
will be ideal but nuisance tripping may result due to the asymmetrical starting
currents and DC offset. If asymmetrical starting currents limits the starting capability,
set the S/C level higher to a maximum of 11 × FLC to override this condition
(1.7 × 6.3 = 11.7 where 1.7 is the maximum DC offset for an asymmetrical current).
•
Ground Fault
Unfortunately, there is not enough information to determine a ground fault setting.
These settings depend on the following information:
1.
The ground fault current available.
2.
System grounding; for example, high resistive grounding or solidly grounded
3.
Ground fault CT used.
4.
Ground fault connection; for example, zero-sequence or residual connection
For the purpose of this example, assume a fault current of 10 Amps or 10/50 = 0.2 x
CT, no intentional time delay.
•
Unbalance Alarm and Trip
The unbalance settings are determined by examining the motor application and
motor design. The heating effect of unbalance will be protected by enabling
unbalance input to thermal memory; described in details in Chapter 5, Thermal Model.
A setting of 10% for the unbalance alarm with a delay of 10 seconds would be
appropriate and the trip can be set to 25% with a delay of 5 seconds.
•
Stopped and Running Cool Times
The motor manufacturer usually supplies this information as either cooling times, or
cooling time constants not provided in the data sheet issued with this motor. Since
RTDs are present and wired to the relay, biasing of the thermal model will be used so it
is not critical to have these cooling times from the manufacturer. The default values of
motor cooling time constants are 15 and 30 minutes, and can be used for the running
and stopped cool times respectively. If the manufacturer provides cooling times
instead, the approximate values of the cooling time constants is 1/5th the cooling
times provided by the manufacturer.
•
Acceleration Trip
This settings should be set higher than the maximum starting time to avoid nuisance
tripping when the voltage is lower or for varying loads during acceleration. If reduced
voltage starting is used, according to the acceleration curves, a setting of 18 seconds
would be appropriate, or if across the line starting is used, a setting of 13 seconds
would be appropriate.
•
Enable Start Inhibit
This function will limit starts when the motor is already hot. The relay learns the
amount of thermal capacity used at start. If the motor is hot, thus having some
thermal capacity used, the relay will not allow a start if the available thermal capacity
is less than the required thermal capacity for a start.
CHAPTER 1: GETTING STARTED
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
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