Three Phase Power Unbalance; Insulation Resistance Readings - Xylem Bell & Gossett Explosion Proof 3XWC Directives D'installation, D'utilisation Et D'entretien

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 mo-
tor 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 read-
ings 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.
To calculate percent of current unbalance:
A. Add the three line amp values together.
Starter Terminals L1
Motor Leads T3
Example:
T3 = 51 amps
T1 = 46 amps
T2 = 53 amps
Total = 150 amps
÷ 3 = 50 amps
– 46 = 4 amps
4 ÷ 50 = .08 or 8%

INSULATION RESISTANCE READINGS

Normal Ohm and Megohm Values between all leads and ground
Condition of Motor and Leads
A new motor (without drop cable).
A used motor which can be reinstalled in well.
Motor in well. Readings are for drop cable plus motor.
New motor.
Motor in good condition.
Insulation damage, locate and repair.
Insulation resistance varies very little with rating. Motors of all HP, voltage and phase ratings have similar values
of insulation resistance.
Insulation resistance values above are based on readings taken with a megohmmeter with a 500V DC output.
Readings may vary using a lower voltage ohmmeter, consult factory if readings are in question.
Hookup 1
L2 L3
T1 T2
T2 = 50 amps
T3 = 48 amps
T1 = 52 amps
Total = 150 amps
÷ 3 = 50 amps
– 48 = 2 amps
2 ÷ 50 = .04 or 4%
B. Divide the sum by three, yielding 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% 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 com-
ing from the power source.
Contact your local power company to resolve the imbal-
ance.
Hookup 2
L1 L2 L3
T2 T3 T1
T1 = 50 amps
T2 = 49 amps
T3 = 51 amps
Total = 150 amps
÷ 3 = 50 amps
– 49 = 1 amps
1 ÷ 50 = .02 or 2%
Ohm Value
20,000,000 (or more)
10,000,000 (or more)
2,000,000 (or more)
500,000 - 2,000,000
Less than 500,000
Hookup 3
L1 L2 L3
T1 T2 T3
Megohm Value
20 (or more)
10 (or more)
2 (or more)
.5 - 2
Less than .5
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