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8 TESTING
Table 8–3: PHASE UNBALANCE ALARM TEST
INJECTED
CURRENT (A)
3. Unbalance is calculated as follows:
For average currents ( I
For average currents less than motor full load current:
where:
I
I
+
+
a
b
I
=
------------------------ -
av
3
I
= RMS current in any phase with maximum deviation from the average current ( I
m
I
= motor full load current
FLC
I
= phase A current
a
I
= phase B current
b
I
= phase C current
c
a) EXAMPLE: CALCULATING THE PERCENT OF UNBALANCE
Find % unbalance given the following information:
PRIMARY
SECONDARY (5A)
I
= 73 A
a
I
= 100 A
b
I
= 100 A
c
8
The average of the three phase currents is:
I
I
+
+
a
b
I
=
------------------------ -
av
3
Now, since I
< I
av
FLC
I
–
m
-------------------- -
%UB
=
I
FLC
Therefore, the % unbalance in this case is 18%.
8-4
Courtesy of NationalSwitchgear.com
ACTUAL DISPLAY READING (A)
PHASE A
PHASE B
) greater than or equal to the motor full load current ( I
av
I
–
m
-------------------- -
%UB
=
I
I
–
m
-------------------- -
%UB
=
I
FLC
I
c
average of three phase currents
=
3.65 A
5 A
5 A
I
73
100
100
+
+
c
-------------------------------------- - A
=
3
, we have % unbalance given by:
I
73 91
–
av
×
--------------------- -
100%
=
100
239 Motor Protection Relay
PHASE C
I
av
×
≥
for I
100%
av
av
I
av
×
<
for I
100%
av
273
--------- - A
=
=
91 A
3
×
100%
18%
=
8 TESTING
):
FLC
I
FLC
I
FLC
)
av
GE Multilin
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