The Difference Between the Current Transformer Secondary Rated Current 1A And 5A
We know that the secondary current of the commonly used current transformer is 5A. Under what circumstances should we choose 1A? The secondary load of the transformer is mainly the current line and the ammeter. If the second line is very long and the line resistance is too large, it will affect the display accuracy of the ammeter. Therefore, in the long-distance measurement loop, the secondary current of the current transformer should be 1A type.
GB1208-2016 "Current Transformer" Item 5.2 stipulates that the standard secondary current of the current transformer is 1A and 5A, the preferred value is 5A, and 1A should be selected when the transmission distance is large.
1. Reduced line power consumption
Line power consumption is proportional to the square of the passing current, and a current transformer with a secondary current of 1A reduces power consumption by 25 times compared to 5A. That is, the power consumption of 1A is only 4% of 5A.
Table 1 The power consumption of the current transformer measurement circuit
Measure the power consumption of the line in VA | ||||||
Wire cross-sectional area mm2 | Secondary current A | Transmission distance m | ||||
1 m | 2 m | 5 m | 8 m | 10 m | ||
1.5 | 1 | 0.023 | 0.047 | 0.117 | 0.188 | 0.235 |
1.5 | 5 | 0.59 | 1.17 | 2.93 | 4.68 | 5.86 |
2.5 | 1 | 0.014 | 0.028 | 0.07 | 0.112 | 0.14 |
2.5 | 5 | 0.35 | 0.7 | 1.76 | 2.82 | 3.52 |
4 | 1 | 0.009 | 0.018 | 0.045 | 0.072 | 0.098 |
4 | 5 | 0.22 | 0.44 | 1.1 | 1.76 | 2.2 |
2. Increased transmission distance
Under the same load, the transmission distance of a transformer with a secondary current of 1A is 25 times that of 5A, which can avoid 5/1A intermediate transformers or use large-capacity transformers.
Transmission distance m at different capacities | |||||
Rated Capacity VA | Secondary current A | Wire cross-sectional area mm2 | |||
1 mm2 | 1.5 mm2 | 2.5 mm2 | 4 mm2 | ||
2.5 | 1 | 71 | 106.5 | 178 | 284 |
2.5 | 5 | 2.8 | 4.3 | 7.1 | 11.4 |
5 | 1 | 142 | 213 | 355 | 586 |
5 | 5 | 5.7 | 8.5 | 14.2 | 22.7 |
10 | 1 | 284 | 426 | 710 | 1186 |
10 | 5 | 11.4 | 17 | 28.4 | 45.5 |
3、Small wire cross-sectional area
For large and medium-sized factories, when the installation distance between the instrument and the current transformer is relatively close (for example, 45.5 m), it can be seen from Table 2 that when a 5 A, 10 VA current transformer is selected, the line cross-sectional area is calculated to be 4mm2. When the distance is 71 m, if a 1 A, 2.5 VA current transformer is selected, the line cross-section only needs to be 1mm2. At present, with the popularization and development of computers and numerical control instruments, the selection of current transformers with a rated secondary current of 1 A and below has become more common.
