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

20. 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.
Current unbalance should not exceed 5 % at service
factor load or 10 % at rated input load. If the
unbalance cannot be corrected by rolling the 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. 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 if the cause can
be a damaged cable, an untight cable splice, a poor
connection, or a faulty motor winding.
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
Explanation and examples
Blank worksheet
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
TOTAL = 150 amps
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
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