Unbalance Examples - GE Multilin MM3 Motor Manager 3 Instruction Manual

FUNCTIONAL TESTS
7.2.2 Unbalance
Examples
7–4
For average currents less than the motor full load:
I I
–
n av
>
15% (alarm)
----------------
I
fl
where: I = RMS current in any phase with maximum deviation from the average current,
n
I = average of the phase currents, I
av
I = phase B current, and I
b c
EXAMPLE #1:
Find the percentage unbalance given the following information:
Value Primary
Ia 73 A
Ib 100 A
Ic 100 A
We have I given by:
av
I I
+
a b
I
= ------------------------
av
3
Since I < I , the following formula is used:
av fl
% unbalance
Since the unbalance is greater than 15%, an unbalance alarm will occur if this condition
persists for longer than 5 seconds and auxiliary relay 1 will energize.
EXAMPLE #2:
Find the percentage of unbalance given the following information:
Value Primary
Ia 100 A
Ib 80 A
Ic 150 A
We have I given by:
av
I I
+
a b
I
= ------------------------
av
3
Since I > I , the following formula is used:
av fl
% unbalance
=
Since unbalance is greater than 30%, a single phase trip will occur if this condition persists
for longer than 5 seconds and auxiliary relay 2 will energize.
30% trip , with
= motor full load current, I
fl
= phase C current.
Secondary (5 A)
3.65 A
5 A
5 A
I
+ 73 100 100 A
c + +
= ------------------------------------------ -
3
I I
–
73 91
n av –
× 100
------------------- -------------------- -
= =
I 100
fl
Secondary (5 A)
5 A
4 A
7.5 A
I
+ 100 80 150 A
c + +
= ------------------------------------------ - =
3
I I
–
150 110
n av –
× 100
------------------- --------------------------- -
=
I 110
av
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL
CHAPTER 7: TESTING
I I I
+ +
a b c
I
= ------------------------
av
3
= phase A current,
a
273
-------- - A = 91 A
=
3
× 100 18%
=
330
-------- - A = 110 A
3
× 100 36.4 %
=
(EQ 7.2)
(EQ 7.3)
(EQ 7.4)
(EQ 7.5)
(EQ 7.6)