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AC and DC electricity

This article requires just a minimal understanding of electricity, so before you go on, you may like to find out more about the basics of electricity here.

We make most of the electricity that we use in two ways:

  • as a chemical reaction in a battery
  • by moving wires through a magnetic field in an alternator.

DC electricity

Electricity from a battery is a result of the chemical reactions that occur inside it.  When we use electricity from a battery electrons flow from one terminal, called the negative terminal, through the load and back to the other terminal, called the positive terminal, forming a circuit.  The load is whatever it is that we are using the electricity for: a lamp, music player, or motor.  In the small batteries that we use for many compact appliances the terminals are on the ends of the battery.  The electricity moves in one direction only, so this flow is called direct current (DC).  A simple direct current electric circuit using a battery looks like this:

AC Electricity

Electricity from an alternator is produced by a magnet rotating near coils of wire.  The magnet produces a magnetic field between its two ends, which are called poles.  The poles are named the north pole and the south pole.  As each alternate pole sweeps past a coil of wire electricity is induced into the coil in opposite directions, and the electricity flows back and forth from the coil through the circuit.  The electricity moves alternately in each direction, so this flow is called alternating current (AC).  Alternators inherently generate AC electricity.  A simple AC electric circuit using an alternator looks like this:

 


Many alternators use a coil of wire as an electromagnet to generate the magnetic field; if so, the coil of wire is called the field coil.  

AC Waveform

AC electric current swings back and forth in a smooth curve that is called the waveform.  The best waveform is a sine wave.  The rate at which AC electricity changes direction depends on how quickly the magnet rotates; this rate is called the frequency.  The frequency is defined by how many times the electricity completes a cycle of flow in both directions in each second.  A single cycle in a sine-wave waveform looks like this:

 

The unit of measurement of frequency is the Hertz.  In Australia, the mains electricity alternates at 50 Hertz.

The advantages of AC electricity

The voltage of AC electricity can easily be changed using a device called a transformer.  Changing to a high voltage allows electricity to be moved over long distances efficiently, as our mains electricity is when it moves from the power plant to our homes. Changing the electricity to a high voltage reduces the amperage by the same proportion, so smaller and lighter conductors can be used to carry it with less loss to resistance.  
The high voltage needed to move electrical energy over long distances is difficult and dangerous to use with home appliances.  High-voltage AC electricity is reduced using a transformer to an easy-to-use low voltage for home appliances.  Changing voltage is much more difficult to do with DC.

For an independent power plant such as Eniquest’s independent energy solutions the ease with which voltage can be changed is not very important because the electrical energy does not need to be moved vary far.
Many of our uses for electricity involve electric motors.  AC electric motors can be simpler than DC motors, making them cheaper and more reliable.

The advantages of DC electricity

AC electrical energy is difficult to store efficiently, particularly at a small scale.  This is a problem when electricity must be made at one time but will be used at different time.  The best way to store electricity at a small scale is with a rechargeable battery, such as the storage battery bank that Eniquest’s independent energy solutions use.  Rechargeable batteries require DC electricity, which can be made from an alternator’s AC electricity by the process of rectification.  You can find out more about the advantages of DC electricity here.