Worksheet For Three-Phase Motors - Grundfos CR Installation And Operating Instruction Manual

16. Worksheet for three-phase motors

Below is a worksheet for calculating current unbalance on a
three-phase hookup. Use the calculations below as a guide.
Note: 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
Here is an example of current readings at maximum pump loads on each leg of a three-wire hookup. You
must make calculations for all three hookups. To begin, add up all three readings for hookup numbers 1, 2,
and 3.
Divide the total by three to obtain the average.
Calculate the greatest current difference from the average.
Divide this difference by the average to obtain the percentage of the unbalance.
In this case, the current unbalance for Hookup 1 is 8%.
Hookup 1
L to T
= ___ amps
1 1
L to T
= ___ amps
2 2
L to T
= ___ amps
3 3
TOTAL = ___ amps
Hookup 1
___ amps
3 ___ amps
Hookup 1
___ amps
___ amps
___ amps
Hookup 1
___ or ___ %
___ ___ amps
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farthest from the average stays on the same power lead, most of
the unbalance is coming from the power source. However, if the
reading farthest from the averages moves with the same motor
lead, the primary source of unbalance is on the "motor side" of
the starter. In this instance, consider a damaged cable, leaking
splice, poor connection, or faulty motor winding.
Explanation and examples
Figure here
Hookup 2
L to T
= ___ amps
1 3
L to T
= ___ amps
2 1
L to T
= ___ amps
3 2
TOTAL = ___ amps
Hookup 2
___ amps
3 ___ amps
Hookup 2
___ amps
___ amps
___ amps
Hookup 2
___ or ___ %
___ ___ amps
Hookup 1
T1 = 51 amps
T2 = 46 amps
T3 = 53 amps
150
TOTAL =
Hookup 1
50 amps
3 150 amps
Hookup 1
50 amps
— 46 amps
4 amps
Hookup 1
.08 or 8%
50 4.00 amps
Hookup 3
L to T
= ___ amps
1 2
L to T
= ___ amps
2 3
L to T
= ___ amps
3 1
TOTAL = ___ amps
Hookup 3
___ amps
3 ___ amps
Hookup 3
___ amps
___ amps
___ amps
Hookup 3
___ or ___ %
___ ___ amps