4.15
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
Basic Motor Protection
Set the undercurrent trip and warn levels greater than the expected motor no
load current, but less than the minimum current expected when the motor is
operating normally. These settings are entered in per unit of the full load amps
(FLA) setting.
If you expect the motor to normally operate at no load, disable the trip and
warn elements by setting LLTPU and LLAPU equal to OFF. The relay
automatically hides the associated time delay settings.
Current Imbalance
Elements
Imbalanced motor terminal voltages cause imbalanced stator currents to flow
in the motor. The negative-sequence current component of the imbalance
current causes significant rotor heating. While the SEL-749M motor thermal
element models the heating effect of the negative-sequence current, you may
desire the additional imbalance and single-phasing protection offered by a
current imbalance element.
The SEL-749M calculates percent imbalance current in one of two ways,
depending on the magnitude of the average current. When the average current,
Iav, is greater than the motor-rated full load current, the relay calculates the
percent imbalance:
Equation 4.5
When the average current is less than the motor-rated full load current, the
relay calculates the percent imbalance:
Equation 4.6
Table 4.12 Current Imbalance Settings
Setting Prompt Setting Range Setting Name := Factory Default
CI TRIP LEVEL Off, 5–80% 46UBT := OFF
CI TRIP DELAY 0–240 sec 46UBTD := 5
CI WARN LEVEL Off, 5–80% 46UBA := 10
CI WARN DELAY 0–240 sec 46UBAD := 10
where
UB% = Current imbalance percentage
Im = Maximum deviation of Iav from highest and lowest
magnitudes of the phase currents
Iav = Magnitude of the average phase current
FLA = Motor-rated full load current
UB% 100
Im
Iav
-------
• =
UB% 100
Im
FLA
-----------
• =
Courtesy of NationalSwitchgear.com
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