ITT M05430 Directives D'installation Et D'utilisation page 11

4.6 Three Phase Power Unbalance
A full three phase supply consist-
ing of three individual transformers
or one three phase transformer is
recommended. "Open" delta or wye
connections using only two trans-
formers 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 possible hook-ups. Roll
the motor leads across the starter in
the same direction to prevent motor
reversal.
S tarter Terminals
Motor Leads
Example:
T3-R = 51 amps
T1-B = 46 amps
T2-Y = 53 amps
Total = 150 amps
÷ 3 = 50 amps
– 46 = 4 amps
4 ÷ 50 = .08 or 8%
Hookup 1
L1 L2 L3
R B Y
T3 T1 T2
T2-Y = 50 amps
T3-R = 48 amps
T1-B = 52 amps
Total = 150 amps
÷ 3 = 50 amps
– 48 = 2 amps
2 ÷ 50 = .04 or 4%
To calculate percent of current
unbalance:
A. Add the three line amp values
together.
B. Divide the sum by three, yield-
ing average current.
C. Pick the amp value which is
furthest from the average current
(either high or low).
D. Determine the difference
between this amp value (furthest
from average) and the average.
E. Divide the difference by the
average.
Multiply the result by 100 to
determine percent of
unbalance.
Current unbalance should not exceed
5%. If the unbalance cannot be cor-
rected 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 2
L1 L2 L3
Y R B
T2 T3 T1
Total = 150 amps
1 ÷ 50 = .02 or 2%
Hookup 3
L1 L2
B Y
T1 T2
T1-B = 50 amps
T2-Y = 49 amps
T3-R = 51 amps
÷ 3 = 50 amps
– 49 = 1 amps
L3
R
T3
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