Why current &voltage levels cannot be changed in d.c as in a.c?

Question

Why current &voltage levels cannot be changed in d.c as in a.c?

Answer 1

Actually they can to some extent with DC-DC converters, but they use expensive semiconductors, and are very limited in power and voltage. And not too efficient.

For AC we can use transformers, and they are very efficient, and will operate at very high power levels and very high voltage and current levels.  But transformers don't work with DC.

Answer 2

A current in a conductive loop will create a magnetic field.  This is true for DC and AC.  With DC as the source of power, current in a loop creates a steady state magnetic field.  With AC as the source you get a time varying magnetic field.  A time-varying (but not a steady state) magnetic field can induce a current in a conductive loop.  This very useful property is the basis for transformers.  You can use transformers to change the voltage and current with very low power loss if the system is AC.  With DC, this doesn't work.  "DC-DC converters" work by having a time-varying current in the middle somewhere; in effect they change the DC to AC.

Answer 3

For AC, the current/voltage varies sinusoidally so by wrapping the conductors around a metal loop, you can have a different loop with different number of turns to "change" the voltage/current in an efficient manner.  Thus, the heavy metal transformers which couple the flux from the first "primary" loop through to the secondary winding.  

In DC systems, the current/voltage does not vary, it is relatively constant (so to speak) once the circuit is energized, so the varying flux does not happen and a typical metal core transformer won't work.  Many DC-DC transformers will actually use a semiconductor circuit to "chop" or "create" an AC current which can then be transformed.  The difficulty is the power capacity of the circuits and the expense when compared to "relatively" cheap metal core loop transformers.

One of the reasons AC systems are so popular is their ability to be transformed (network, neighborhood, home) and transmitted from generation facilities relatively inexpensively.

So yes, DC can be transformed, but generally not with as much power or as inexpensively as AC systems.  Many systems start with AC generation, AC transmission, AC step-down and step-up transformers and finally conversion to DC for use (your power supply in your PC converts 120VAC to 5VDC, 12VDC and so on).

However, much work in high-voltage DC transmission systems are of great interest because heating losses in the metal power lines can be related to the current flowing (P = I^2 R).  Thus with a higher voltage, the entire wire is used (lower resistance) and lower losses can result making for more efficient generation/transmission network.  AC transmission lines essentially use only the outer portions of the wires.

Answer 4

due to semiconductors.