As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only
As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only
Right Answer is:
33.3 percent of copper
SOLUTION
As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only 33.3 percent of copper.
- For the same conductor length, the same amount of power, same losses and same maximum voltage to earth, 3 wire DC system requires minimum conductor area
- For transmitting the same amount of power at the same voltage, a three-phase transmission line requires less conductor material than a single-phase line; The three-phase transmission system is so cheaper
- For a given amount of power transmitted through a system, the three-phase system requires conductors with a smaller cross-sectional area; This means a saving of copper and thus the original installation costs are less
Important Point:
Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.
System | Same maximum voltage to earth | Same maximum voltage between conductors |
DC system: Two-wire | 1 | 1 |
DC: Two-wire mid-point earthed | 0.25 | 1 |
DC: 3 wire | 0.3125 = 5 / 16 | 1.25 |
Single-phase: 2 wire | 2/cos2ϕ | 2/cos2ϕ |
Single-phase: 2 wire mid-point earthed | 0.5/cos2ϕ | 2/cos2ϕ |
Single-phase: 3 wire | 0.625/cos2ϕ | 2.5/cos2ϕ |
2-phase: 4 wire | 0.5/cos2ϕ | 2/cos2ϕ |
2-phase: 3 wire | 1.457/cos2ϕ | 2.914/cos2ϕ |
3 phase, 3 wire | 0.5/cos2ϕ | 1.5/cos2ϕ |
3 phase, 4 wire | 0.583/cos2ϕ | 1.75/cos2ϕ |