Bell & Gossett IM194R03 Grinder Pumps, Single and Three Phase, 1-1⁄4″ Discharge - Manual (Page 8)

8
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
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.
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
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 coming
from the power source.
Contact your local power company to resolve the
imbalance.
Hookup 1 Hookup 2 Hookup 3
Starter Terminals L1 L2 L3 L1 L2 L3 L1 L2 L3
Motor Leads R B W W R B B W R
T3 T1 T2 T2 T3 T1 T1 T2 T3
Example:
T3-R = 51 amps T2-W = 50 amps T1-B = 50 amps
T1-B = 46 amps T3-R = 48 amps T2-W = 49 amps
T2-W = 53 amps T1-B = 52 amps T3-R = 51 amps
Total = 150 amps Total = 150 amps Total = 150 amps
÷ 3 = 50 amps ÷ 3 = 50 amps ÷ 3 = 50 amps
— 46 = 4 amps — 48 = 2 amps — 49 = 1 amps
4 ÷ 50 = .08 or 8% 2 ÷ 50 = .04 or 4% 1 ÷ 50 = .02 or 2%
INSULATION RESISTANCE READINGS
Normal Ohm and Megohm Values between all leads and ground
Condition of Motor and Leads Ohm Value Megohm Value
A new motor (without drop cable). 20,000,000 (or more) 20 (or more)
A used motor which can be reinstalled in well. 10,000,000 (or more) 10 (or more)
Motor in well. Readings are for drop cable plus motor.
New motor. 2,000,000 (or more) 2 (or more)
Motor in good condition. 500,000 - 2,000,000 .5 - 2
Insulation damage, locate and repair. Less than 500,000 Less than .5
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.
This table was reprinted through the courtesy of Franklin Electric.
INSULATION RESISTANCE READINGS
THREE PHASE POWER UNBALANCE