Goulds Pumps VS - IOM - Manual (Page 25)

17
3. CURRENT UNBALANCE TEST:
For three phase units, after correct rotation has
been established, check the current in each of the
three motor leads and calculate the current unbal-
ance as explained below. If the current unbalance
is 2% or less, leave the leads as connected. If the
current unbalance is over 2%, current readings
should be checked on each leg using one of three
possible hook-ups indicated in the Table below.
Roll the motor leads across the starter in the same
direction to prevent motor rotation reversal. This
procedure is commonly known as “rolling the
leads”. THE HOOKUP THAT RESULTS IN THE
LOWEST PERCENT CURRENT UNBALANCE
SHOULD BE USED FOR THE FINAL CONNEC-
TION OF THE POWER LEADS.
b. Current unbalance is determined by measur-
ing the amperage of each of the three legs
and then calculating the percent current
unbalance using the formula below. This cal-
culation must be performed using each of the
three hookups shown.
Percent Current Unbalance =
Maximum current difference in
any leg from average current
Average current
x
100
Sample Calculation
ƒMeasure current in
each leg.
ƒAdd leg currents to
determine total
current.
ƒCalculate average leg
current.
T1 L1 51 amps
T2 L2 46 amps
T3 L3 53 amps
=======
150 amps
÷ 3
=======
50 amps
T1 L1 ________
T2 L2 ________
T3 L3 ________
÷3
=======
T3 L1 ________
T1 L2 ________
T2 L3 ________
÷3
=======
T2 L1 ________
T3 L2 ________
T1 L3 ________
÷3
=======
ƒDetermine maximum
difference of any one
leg from the average.
51 – 50 = 1
50 – 46 = 4 max
53 – 50 = 3 ___ - ___ = ___
___ - ___ = ___
___ - ___ = ___ ___ - ___ = ___
___ - ___ = ___
___ - ___ = ___ ___ - ___ = ___
___ - ___ = ___
___ - ___ = ___
ƒCalculate percent
unbalance using
formula above.
4 x 100 = 8%
50
___ x 100 = %
___ x 100 = %
___ x 100 = %