Electricity + Control October 2017

Na is the number of turns in the primary coil Nb is the number if turns in the secondary coil

This relationship means that the voltage of the secondary coil can be stepped up or stepped down by using more or less turns in the secondary coil, respectively (relative to the turn number in the primary coil). This ability to easily adjust voltage (and thus the current) in a sys- tem makes transformers an important component in power transmis- sion. This is because by setting the voltage high, the current would consequently be low, thus mitigating energy losses. Owing to the fact that a magnetic flux is attributed to changing the direction of current, transformers only work with ac. There would be no current induced in a secondary coil of a transformer if dc passed through the primary coil, as no magnetic flux would be created. This is one of the main reasons as to why ac is often used instead of dc in large scale power transmission. Ac allows for power to be transmitted at high voltages and hence low currents, the former of which can be easily stepped down to levels suitable for respective applications. Low current levels are desired in transmission systems as they correspond to low power losses across cables, as described by the following equations:

V = IR

(4)

The above equation describes Ohms’s Law. V is the potential difference across a resistor I is current R is resistance

P = IV

(5)

P is power I is current V is potential difference Thus substituting Equation 4 into Equation 5 yields the following:

P = I 2 R

(6)

P is power loss (power consumed by resistance) I is current R is resistance

The strong direct relationship between power loss and current would mean the ability of ac systems to minimise current in transmission makes it the current type selected in most energy infrastructure today. Ac shortcomings Whilst ac is often selected as the current of choice, it is not without its drawbacks. Ac is not as stable as dc, which is why many electron- ics operate on dc. Also, dc requires less cabling than its alternating counterpart in being conducted, thereby incurring less power loss for the same current level. Also, ac incurs power losses across the cable because of a current gradient across the cross-sectional area of the cable (where current is higher towards the outer region of the cable area). However, dc is distributed through a cable such that the current levels across the cable cross-sectional area are uniform.

Electricity + Control

OCTOBER 2017

37