84-38 9424200996
Settings Calculation Examples BE1-11m
Figure 84-47. Protection, Current, Instantaneous Overcurrent (50-2) Screen
Instantaneous Overcurrent (50-3) - Unbalance Protection Trip
The NEMA MG-1 standard states that the existence of unbalanced voltage feed results in four to 10 times
the current unbalance in a synchronous motor stator. Small voltage unbalance causes a large current
unbalance, so current is used to measure the unbalance. The unbalanced voltage feed to the motor
should not exceed 5 percent. The BE1-11m must monitor and react quickly for a current unbalance
condition. Because the BE1-11m uses negative-sequence current in the 49TC thermal model, additional
unbalance (and single-phasing) protection can be added using the Average mode.
Low current levels must be scaled to prevent nuisance pickups. This scaling is in reference to motor Full
Load Amperes. This element has a block output when the average current is less than 25 percent of rated
FLA.
The Average unbalance calculation has two states. When the average current, I
Average
, is greater than the
motor FLA, the BE1-11m uses Equation 84-28.
(%) =
Equation 84-28. Unbalance Calculation, State 1
When the average current is less than motor FLA, then the BE1-11m uses Equation 84-29.
(%) =
Equation 84-29. Unbalance Calculation, State 2
Refer to the Instantaneous Overcurrent (50) Protection chapter for more information on unbalance
calculations.
The pickup setting calculation is shown in Equation 84-30.
=
(
5%
)
(
4
)
= 5% 4 = 20%
Equation 84-30. 50-3 Pickup
Select Unbalanced mode, set the Pickup setting to 20%, and set the Time Delay to 5 seconds as shown
in Figure 84-48.
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