Three Phase Power Unbalance - Xylem GOULDS M30430 Manuel D'utilisation

THREE PHASE POWER UNBALANCE

THREE PHASE POWER UNBALANCE
A full three phase supply consisting of three individual
transformers or one three phase transformer is recom-
mended. "Open" delta or wye connections using only two
transformers can be used, but are more likely to cause poor
performance, overload tripping or early motor failure due to
current unbalance.
Check the current in each of the three motor leads and
calculate the current unbalance as explained below.
If the current unbalance is 2% or less, leave the leads as
connected.
If the current unbalance is more than 2%, current readings
should be checked on each leg using each of the three pos-
sible hook-ups. Roll the motor leads across the starter in the
same direction to prevent motor reversal.
Starter Terminals
Motor Leads
Example:
To calculate percent of current unbalance:
A. Add the three line amp values together.
B. Divide the sum by three, yielding average current.
C. Pick the amp value which is furthest from the average
D. Determine the difference between this amp value
E. Divide the difference by the average.
Current unbalance should not exceed 5% at service factor
load or 10% at rated input load. If the unbalance cannot be
corrected by rolling leads, the source of the unbalance must
be located and corrected. If, on the three possible hookups,
the leg farthest from the average stays on the same power
lead, most of the unbalance is coming from the power
source.
Contact your local power company to resolve the imbalance.
Hookup 1 Hookup 2
L1 L2 L3 L1
R B Y Y
T3 T1 T2 T2
T3-R = 51 amps T2-Y = 50 amps
T1-B = 46 amps T3-R = 48 amps
T2-Y = 53 amps T1-B = 52 amps
Total = 150 amps Total = 150 amps
÷ 3 = 50 amps ÷ 3 = 50 amps
— 46 = 4 amps — 48 = 2 amps
4 ÷ 50 = .08 or 8% 2 ÷ 50 = .04 or 4%
current (either high or low).
(furthest from average) and the average.
Multiply the result by 100 to determine percent of
unbalance.
Hookup 3
L2 L3 L1 L2
R B B Y
T3 T1 T1 T2
T1-B = 50 amps
T2-Y = 49 amps
T3-R = 51 amps
Total = 150 amps
÷ 3 = 50 amps
— 49 = 1 amps
1 ÷ 50 = .02 or 2%
L3
R
T3
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