SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
•
MILWAUKEE, WI
•
800.334.5214
•
acmetransformer.com
12
THREE PHASE LOADS
1. Determine electrical load
A.
Voltage required by load.
B.
Amperes or kVA required by load.
C.
Frequency in Hz (cycles per second).
D.
Verify load is designed to operate on three phase.
All the above information is standard data normally obtained from equipment
nameplates or instruction manuals.
2. Determine supply voltage
A.
Voltage of supply (source).
B.
Frequency in Hz (cycles per second).
The frequency of the line supply and electrical load must be the same. A three
phase transformer is selected which is designed to operate at this frequency
having a primary (input) equal to the supply voltage and a secondary (output)
equal to the voltage required by the load.
3. If the load nameplate expresses a rating in kVA, a transformer can be
directly selected from the charts. Choose from the group of transformers
with primary and secondary voltages matching that which you have
just determined.
A.
Select a transformer with a standard kVA capacity
equal to or greater than
that needed to operate the load.
B.
Primary taps are available on most models to compensate for line voltage
variations. (Refer to question #2 in the Transformer Questions and Answers
Section on page 6.)
C.
When load ratings are given only in amperes, tables 3 and 4 or the following
formulasmay be used to determine proper kVA size for the required transformer.
(1)
To determine three phase
kVA
when volts and amperes are known:
Three Phase kVA =
Volts x Amps x 1.73
1000
(2)
To determine
Amperes
when kVA and volts are known:
Amps =
3 Phase kVA x 1000
Volts x 1.73
Three Phase Example
Question:
Select a transformer to fulfill the following conditions. Load is a three phase
induction motor, 25 horsepower @ 240 volts, 60 Hz and a heater load of 4 kilowatts @
240 volts single phase. The supply voltage is 480Y/277, three phase, 4 wire.
Answer:
Compute the kVA required.
Motor
—From table 4 the current is 68 amps.
240 volts x 68 amps x 1.73 = 28.2 kVA
1000
(The kVA can also be obtained from table 4).
Heater
— 4 kVA
A three phase transformer must be selected so that any one phase is not overloaded.
Each phase should have the additional 4 kVA rating required by the heater even though
the heater will operate on one phase only. So, the transformer should have a minimum
kVA rating of 28.2 + 4 + 4 + 4 or 40.2 kVA. Refer to the appropriate selection chart.
A 480 delta primary—240 delta secondary transformer may be used on a 4 wire,
480Y/277 volt supply. The fourth wire (neutral) is not connected to the transformer. To
not overload the transformer, a 45 kVA transformer should be selected.
NOTE:
Any two wires of the 240 volts, 3 phase developed by the secondary of the
transformer may be used to supply the heater. Any 2 wires of a 3 phase system is
single phase.
TABLE 3
Full Load Current in Amperes–
Three Phase Circuits
TABLE 4
Full Load Amperes
Three Phase A.C. Motors
①
①
When motor service factor is greater than 1,
increase full load amps proportionally.
Example:
If service factor is 1.15, increase
above amp values by 15%.
3 Phase kVA = Volts x Amps x 1.73
1000
NOTE:
If motors are started more than once per
hour, increase minimum transformer kVA by 20%.
Min.
Horse- 208 V 230 V 460 V 575 V Trans-
power
former
kVA
1
/
2
2.2 2.0 1.0 0.8 0.9
3
/
4
3.1 2.8 1.4 1.1 1.2
1 4.0 3.6 1.8 1.4 1.5
2 7.5 6.8 3.4 2.7 2.7
3 10.7 9.6 4.8 3.9 3.8
5 16.7 15.2 7.6 6.1 6.3
10
31 28
14 11 11.2
15
46 42
21 17 16.6
20
59 54
27 22 21.6
25
75 68
34 27 26.6
30
88 80
40 32 32.4
40 114 104
52 41 43.2
50 143 130
65 52 52
60 170 154
77 62 64
75 211 192
96 77 80
100 273 248 124 99 103
125 342 312 156 125 130
150 396 360 180 144 150
200 528 480 240 192 200
kVA 208 V 240 V 380 V 440 V 480 V 600 V
3 8.3 7.2 4.6
3.9 3.6 2.9
4.5 12.5 10.8 6.8
5.9 5.4 4.3
6 16.6 14.4 9.1
7.8 7.2 5.8
9 25 21.6 13.7
11.8 10.8 8.6
15 41 36 22.8
19.6 18.0 14.4
22.5 62 54 34.2
29 27 21.6
30 83 72 45.6
39 36 28
45 124 108 68.4
59 54 43
75 208 180 114
98 90 72
112.5 312 270 171 147 135 108
150 416 360 228 196 180 144
225 624 541 342 294 270 216
300 832 721 456 392 360 288
500 1387 1202 760 655 601 481
750 2081 1804 1139 984 902 721
1000 2775 2405 1519 1312 1202 962
