An alternator is an A.C. Generator. Why don’t we use D.C Power?

Question

give answerin term of ac current and dc current

Answer 1

I'm not sure exactly what you're asking and what you're looking for, but DC is impractical for power transmission.  It's not so much a question of "AC Current" and "DC Current" but voltage drop due to transmission losses.  Tom Edison swore by using DC, but voltage drop over long transmission lines is the killer.  There would be higher voltage near the generating station and all the lights would be too bright and burn out quickly.  Farther away the voltage would start to drop and all the lights would be dim.  The same thing happens with AC but the voltage can be increased again very easily with transformers.  Transformers also enable long transmission lines to run around a million volts to minimize the loss in the wires from heating.  Also with AC, the ground acts as one of the conductors thus cutting the amount of copper needed by half.  So today, over a century later, we're still using the Tesla system pioneered by Nikola Tesla and George Westinghouse.

Answer 2

Autos used to have DC generators, with relay circuits to control the charge rate. With relays, the charge rate was adjustable to only several values.

The generators used brushes, and they wore out. The relays wore out. And the generators were not too efficient,

With alternators, there are no brushes, no relays, a lot less to go bad.  And the effeciency is better. And the batteries have a better controlled charge rate.

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Answer 3

Billrussel42, there are brushes, they just have a different function. Both alternators and generators actually produce AC.

We use AC and rectify it, because it is simply more efficient. Both halves (the positive AND negative going excursions of the sine wave) are used in an alternator, and rectified into DC.

In the older generators either one or the other half of the sine wave was discarded mechanically, via the commutator and brushes.

The alternator uses solid state diodes for rectification.

It is the advances made in rectifiers that has allowed using AC in automotive alternators practical.

Answer 4

The DC generator system is designed and optimized to deliver the high currents at low voltages required for battery charging and operating DC loads.  No battery chargers or power supplies are required.

DC generators do not require a transfer switch. Transfer switches lower system reliability.

In prime power applications the DC generator lowers the overall cost of the system.

Certain AC generators and switch mode power supplies are incompatible.  These AC generators have voltage regulators that cannot regulate voltage due to the current pulsing load of the switch mode power supplies.  Polar’s DC generators when connected to a battery do not suffer this incompatibility.

DC generators are more fuel-efficient. Site operators want the longest run time with the least amount of fuel on site.

Polar’s DC generators are simpler in design, have considerably less maintenance and are more reliable than AC generators.

Answer 5

Alternators replaced the old DC generators because they are lighter, cheaper, more efficient, easier to control, more reliable, and work well over a wide speed range. The only real difference between AC and DC is that the AC must be rectified, i.e. turned to DC. This is inexpensively accomplished with diodes and by winding the alternators for 3 phase.

Compared to modern DC generator designs, the main advantage of the alternator is low cost and easy control over a wide speed range. However, they typically require 2A to excite the armature, and still have slip rings with brushes to convey this to the armature - not as bad as the old commutator/brush system, but still wear parts.

Part of the reason they still use alternators is tradition. Everyone is set up to use them and they work well. As efficiency becomes more important and permanent magnets and power electronics get cheaper, I expect high efficiency DC designs to eventually replace the alternator again. Controlling the voltage over the necessary speed range and cost are the biggest obstacles.