7.1.2 Check for current imbalance
Current imbalance causes the motor to have reduced starting
torque, overload tripping, excessive vibration and poor performance
which can result in early motor failure. It is very important that
current imbalance be checked in all three-phase systems.
Make sure that the current imbalance between the phases
do not exceed 5 % under normal operating conditions.
Determine if the supply power service is a two-transformer or three-
transformer system. If two transformers are present, the system is
an "open" delta or wye. If three transformers are present, the
system is true three-phase.
Make sure the transformer ratings in kilovolt amps (KVA) is
sufficient for the motor load. See section Transformer capacity
required for three-phase submersible motors.
The percentage of current imbalance can be calculated with the
following formulas and procedures:
Average current =
Total of current values measured on each
leg
3
% current
imbalance
=
Greatest amp difference from the
average
x 100
Average current
To determine the percentage of current imbalance, proceed as
follows:
1. Measure and record current readings in amps for each leg
(hookup 1). Disconnect the power.
2. Shift or roll the motor leads from left to right so the submersible
drop cable lead that was on terminal 1 is now on 2, lead on 2 is
now on 3, and lead on 3 is now on 1 (hookup 2). Rolling the
motor leads this way does not reverse the motor rotation. Start
the pump, measure and record current reading on each leg.
Disconnect the power.
3. Shift submersible drop cable leads from left to right so the lead
on terminal 1 goes to 2, 2 to 3 and 3 to 1 (hookup 3). Start the
pump, measure and record current reading on each leg.
Disconnect the power.
4. Add the values for each hookup.
5. Divide the total by 3 to obtain the average.
6. Compare each single leg reading from the average to obtain the
greatest amp difference.
7. Divide this difference by the average to obtain the percentage of
imbalance.
Use the wiring hookup which provides the lowest percentage of
imbalance. See section Correcting for three-phase current
imbalance for a specific example of correcting for three-phase
current imbalance.
Related information
10.3 Transformer capacity required for three-phase submersible
motors
10.6.3 Correcting for three-phase current imbalance
7.1.3 Developing the well
After proper rotation and current imbalance are checked, start the
pump and let it operate until the water runs clear of sand, silt and
other impurities.
Slowly open the valve in small increments as the water clears until
the desired flow rate is reached. Do not operate the pump beyond
its maximum flow rating.
Do not stop the pump until the water runs clear.
If the water is clean and clear when the pump is first started, open
the valve slowly until the desired flow rate is reached. As the valve
is being opened, check the drawdown to ensure that the pump is
always submerged.
Make sure that the dynamic water level is always more
than 3 feet (0.9 m) above the suction interconnector of the
pump.
Disconnect the temporary piping arrangements and complete the
final piping connections.
Do not operate the pump with the outlet valve closed. This
can result in damage to the motor and the pump due to
overheating. Install a properly sized relief valve at the well
head to prevent the pump from running against a closed
valve.
Start the pump and test the system. Check and record the voltage
and current draw on each motor lead.
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