10
Use this worksheet to calculate current unbalance for our
installation.
Table 4-1: Electrical Current Unbalance Example
EXAMPLE
Arrangement 1
Amps
Arrangement 2
Amps
Arrangement 3
Amps
L1–T1=17
L2–T2=15.3
L3–T3=17.7
L1–T3=16.7
L2–T1=16.3
L3–T2=17
L1–T2=16.7
L2–T3=16
L3–T1=17.3
Total Amps 50 50 50
Average Amps 50 ÷ 3 = 16.7 50 ÷ 3 = 16.7 50 ÷ 3 =16.7
From Average Amps
Deviation L1
Deviation L2
Deviation L3
0.3
1.4
1.0
0.0
0.4
0.3
0.0
0.7
0.6
% Current Unbalance
Largest Deviation
1.4 ÷ 16.7 0.4 ÷ 16.7 0.7 ÷ 16.7
% Unbalance + 8.4% 2.4% 4.2%
4.6 Transformer Sizing
A full three-phase power supply is recommended
for all three-phase motors and may consist of three
individual transformers or one three-phase transformer.
“Open” delta or wye connections which use only two
transformers can be used, but are more likely to cause
unbalanced current problems. Transformer ratings
should be no smaller than listed in Table 4-2 for supply
power to the motor alone.
Transformers are rated by KVA capacity. This must be
high enough capacity for the motor being installed. If the
transformer capacity is too small, the motor will receive
reduced voltage and may be damaged.
Any other loads in the system would be in addition to the
motor alone.
Refer to Table 4-2. Note that the open delta configuration
can only use 87% of the rated power of the two
transformers.
SECTION 4: Electrical Power
T1 T2 T3
Wye or Open Delta 3-Phase
T1
T2
T3
Full 3-Phase (Delta)
Table 4-2: Transformer Capacity
HP kW
KVA Rating (smallest) For Each Transformer
Required KVA
Open WYE or D
2Transformers
WYE or D
3Transformers
1/2 0.37 1.5 1.0 0.5
3/4 0.55 1.5 1.0 0.5
1 0.75 2.0 1.5 0.75
1-1/2 1.1 3.0 2.0 1.0
2 1.5 4.0 2.0 1.5
3 2.2 5.0 3.0 2.0
5 3.7 7.5 5.0 3.0
7.5 5.5 10.0 7.5 5.0
10 7.5 15.0 10.0 5.0
15 11.0 20.0 15.0 7.5
20 15.0 25.0 15.0 10.0
25 18.5 30.0 20.0 10.0
30 22.0 40.0 25.0 15.0
40 30.0 50.0 30.0 20.0
50 37.0 60.0 35.0 20.0
60 45.0 75.0 40.0 25.0
Figure 4-12: Three Phase Power
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