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Transformers

Alternating voltage and induced electro magnetic field - e.m.f. - in a transformer

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A transformer can transfer electrical energy from one electric circuit to an another through inductively coupled conductors.

When current in the primary1 circuit changes, a changing magnetic field will induce a changing voltage in the secondary2 circuit.

For an ideal transformer the induced voltage and current can be expressed as

U1 / U2 = N1 / N2 = I2 / I1                     (1)

where

U = electrical potential difference (Voltage)

N = number of turns of wire

I = current (Amps)

Example - Secondary Voltage in a Transformer

The secondary voltage in a transformer with 500 primary turns, 3000 secondary turns and 230 V alternating primary current can be calculated as

U2 = U1 N2N1  

    = (230 V) 3000 / 500 

    = 1380

    = 1.38 kV

Single Phase Transformer Full Load Current (ampere)

Transformer Rating
Apparent Power
(kVA)
Full Load Current (A)
120 V240 V480 V
0.05 0.42 0.21 0.10
0.075 0.63 0.31 0.16
0.1 0.83 0.42 0.21
0.15 1.25 0.63 0.31
0.25 2.08 1.04 0.52
0.5 4.17 2.08 1.04
0.75 6.25 3.13 1.56
1 8.33 4.17 2.08
1.5 12.5 6.25 3.13
2 16.7 8.33 4.17
3 25 12.5 6.3
5 41.7 20.8 10.4
7.5 62.5 31.3 15.6
10 83.3 41.7 20.8
15 125 62.5 31.3
25 208 104.2 52.1
37.5 313 156.3 78.1
50 417 208.3 104.2
75 625 313 156
100 833 417 208
167 1392 696 348
200 1667 833 417
250 2083 1042 521

Three-Phase Transformer Full Load Current (ampere)

Transformer Rating
Apparent Power
(kVA)
Full Load Current (A)
240 V480 V
3 7.2 3.6
6 14.4 7.2
9 21.7 10.8
15 36.1 18.0
22 53.0 26.5
30 72.2 36.1
45 108 54.1
75 180 90.2
112.5 271 135
150 361 180
225 541 271
300 722 361
500 1203 601
750 1806 903
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