Transformers - kVA Ratings vs. Volt and Amps
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 N2 / N1
= (230 V) 3000 / 500
= 1380 V
= 1.38 kV
Single Phase Transformer Full Load Current (ampere)
| Transformer Rating Apparent Power (kVA) | Full Load Current (A) | ||
|---|---|---|---|
| 120 V | 240 V | 480 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 V | 480 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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