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PN524 (08-01-20)
CURRENT UNBALANCE EXAMPLE AND WORKSHEET
INSTALLATION
3-Phase Current Unbalance - Example
Here is an example of current readings at maximum pump loads
on each leg of a three wire hookup. Make cal cu lations for all three
possible hookups.
A. For each hookup, add the readings for the three legs:
Ex.: Hookup #1
Hookup #2
Hookup #3
L1 = 51Amps
L1 = 50 Amps
L1 = 50 Amps
L2 = 46 Amps
L2 = 48 Amps
L2 = 49 Amps
L3 = 53 Amps
L3 = 52 Amps
L3 = 51 Amps
Total 150 AmpsTotal 150 AmpsTotal 150 Amps
B. Divide each total by three to get average amps:
Example: 150/3 = 50
Example: 150/3 = 50
Example: 150/3 = 50
C. For each hookup, find current value farthest from aver age
(Calculate the greatest current difference from the average).
Ex. #1
Ex. #2
Ex. #3
50 Amps
50 Amps
50 Amps
–46 Amps
–48 Amps
–49 Amps
= 4 Amps
= 2 Amps
= 1 Amps
D. Divide this difference by the average and multiply by 100 to obtain
the percentage of unbalance. Example:
Ex. 1: 4/50 = .08 x 100 = 8%
Ex. 2: 2/50 = .04 x 100 = 4%
Ex. 3: 1/50 = .02 x 100 = 2%
Use smallest percentage unbalance, in this case Ex. 3.
3-Phase Current Unbalance - Worksheet
Use this worksheet to calculate curent unbalance for your
installation.
A. Add the readings for the three legs:
Ex.: Hookup #1Hookup #2:Hookup #3
L1 =
AmpsL1 =
AmpsL1 =
Amps
L2 =
AmpsL2 =
AmpsL2 =
Amps
L3 =
AmpsL3 =
AmpsL3 =
Amps
Total
Amps Total
AmpsTotal
Amps
B. Divide each total by three to get average amps:
Hookup #1: /3 =
Hookup #2: /3 =
Hookup #3: /3 =
C. For each hookup, find current value farthest from aver age
(Calculate the greatest current difference from the average).
Hookup #1 Hookup #2
Hookup #3
Amps
Amps
Amps
Amps
Amps
Amps
Amps
Amps
Amps
D. Divide this difference by the average to obtain the
percentage of unbalance:
Hookup #1:
/
=
x100 =
%
Hookup #2:
/
=
x100 =
%
Hookup #3:
/
=
x100 =
%
Use hookup with smallest percentage unbalance.
Figure 3: Voltage Test
VOLTAGE TEST (FIGURE 3)
Low or high voltages can cause motor failure. While pump is
operating, check voltage across each pair of leads at motor
controller. Readings more than 10% above or below rated
nameplate voltage can damage pump; correct before placing
pump in service. Test as follows:
1. Disconnect main power supply and open controller.
2. Connect power and start pump. For 3-phase motors, read
voltage across three pairs of leads (L1 – L3, L3 – L2, L2 – L1)
while pump is operating. For single phase motors, read
voltage across L1 and L2 while pump is operating. Voltage
should be within ±10% of motor nameplate rated voltage. If
not, consult power company.
LOAD CURRENT TEST (FIGURE 4)
Load current should be obtained on each motor lead at the
controller. Partially close pump dis charge valve (keep pressure
and flow within specified operating range) until maximum
amp reading has been obtained. Compare reading with motor
nameplate rating. If reading is 15% or more over rated load,
check for incorrect voltage in supply line or overload due to
abrasives in pump. Find and correct problem before putting
pump in service.
Measure electrical resistance between motor leads and well
casing when motor is cold.