Why do we use AC rather than DC?

These days, the electric power we receive from our electricity supplier is AC, or alternating current, power. Why is this so?

In Brief

There is a very good reason for the use of AC. It is only through the use of AC power that cheap mains electricity is possible.

When electricity flows through wires there is always some resistance to its flow. This resistance results in a loss of pressure, or loss of voltage, of the supply. If the voltage drop is too great, we get dull lights, and other electrical equipment does not work properly, or even may not work at all.

So mains power is AC because:

• Direct current power can only be carried a relatively short distance. The distance is limited by the loss of pressure or voltage in the resistance of the conductors.

• Using a higher voltage reduces the effect of voltage drop. With AC power, transformers can be used to change the voltage, to raise and to lower it. So by using transformers and higher voltages, it is possible to transmit electric power over greater distances. Transformers cannot be used to change the voltage of DC power.

• Bulk electricity is transported at a high voltage to your locality, where a transformer converts it down to the 240 volt power we use in our homes.

• Using AC results in better voltage to consumers and much lower losses in the power system. The power system is much more efficient.

• A major advantage of AC electricity supply is that AC generators and motors are much simpler, cheaper, and more reliable than DC generators and motors.

The Story in More Detail

You may wonder why the electricity supply in our homes is AC (alternating current) and not DC (direct current).

Most early electric power supply schemes used DC, yet today, public electricity supply schemes are almost universally AC because of the limitations of DC. Here in Queensland, supply voltage is 240 volts (240 V) AC at 50 hertz (50 Hz) or 50 cycles per second.

The electricity from the batteries in our torch, in our mobile phone and in our cars is DC, a form of electricity which seems much simpler to understand. These use a constant voltage, the voltage does not vary with time – until your batteries go flat! With AC, the voltage is continuously reversing 100 times a second. It makes 50 complete cycles each second. So it said to have a frequency of 50 hertz (Hz). As the voltage reverses so does the current. It flows along  the wires first in one direction and then the other.

A major consideration in electricity supply is to keep the voltage supplied to the consumer (the supply voltage) reasonably constant so that equipment works properly. In Queensland, our equipment is designed to work best and most efficiently on 240 V. If the voltage is too low, filament lamps will be too dull, heating devices such as stoves and toasters will be slow, and motors will not work as well as they should. If the voltage is too high, filament lamps will burn more brightly but their life will be appreciably shortened. Other appliances and equipment will not work as efficiently and their life, too, may be shortened.

The distance DC power can be carried is limited by the loss of voltage (voltage drop) in the wires of the supply system. Voltage drop results in the voltage at the user being too low once it reaches the appliances. This became apparent here in Brisbane in 1886, with Queensland’s first significant electric power installation, the electric lights in Queensland’s Parliament House. Supply to the House was from a DC power station about 400 metres away. A newspaper reported that the voltage at the generators was 115 volts, while the voltage at Parliament House was only 96 volts. This was a loss, or voltage drop, in the supply wiring of 19 volts, or over 16% of the generated voltage. The drop was so severe that special 96 volt lamps were ordered to replace the original 110 volt lamps so as to overcome the problem of excessive voltage drop and the resulting dull lamps.

Voltage drop results from the flow of current in the resistance of the supply wires (the conductors). Conductor resistance increases with the length of the wires and decreases with an increase in their size. Using larger conductors, to lower their resistance and so reduce the resistance voltage drop, would alleviate the problem to some extent, but there is a practical limit to how large one can go. In any case, the conductors to Parliament House were quite large, even by modern standards. (From Ohm’s Law, the DC voltage drop is equal to the resistance of the supply conductors multiplied by the current flowing in them.)

We have therefore seen that excessive voltage drop limits the distance over which a DC power station can supply power. To supply DC power to a larger area, a number of physically dispersed DC power stations would be required. This voltage drop problem dogged the early power supply industry, which in the main used DC power systems. The control of system voltage drop is still one of the most important factors in the design of power systems.

A Solution

The solution to the voltage drop problem was the use of AC, or alternating current, systems.

For a given amount of power, doubling the system voltage requires only half the current and so results in only half the voltage drop. It is even better than this, as the voltage drop as a proportion of the supply voltage is now only a quarter of what it was. Returning to the Parliament house example, if the generator voltage had been raised from 115 V to 230 V, the voltage drop would have fallen from 19 volts to 8.5 volts, and the voltage drop as a percentage of the supply voltage would fall from 16% to only 4%, a more acceptable value.

The key to the success of AC is that the voltage can readily be changed with a transformer. A transformer can be used to change from one voltage to another voltage. With AC supply, power is transmitted from a power station using  high voltages with a small voltage drops relative to the voltage, and then transformed down to a medium voltage in large substations for distribution around a locality. In Queensland, medium voltages used as distribution voltages are commonly 11 000 or 22 000 volts. The power is then transformed in local substations down to 240 volts for supply directly to consumers. These smaller substations are often mounted on a pole, or in the suburbs may be a cubicle substation on the ground beside the footpath.

In many of the remoter parts of rural Australia, the high voltage distribution may be by swer (single-wire earth-return) systems. These have a power line sith a single conductor. The power returns to the source transformer through the ground. Swer systems are only suitable for small amounts of power, but having only the one wire makes them more economical for small loads spread over a large area, such as in our less densely populated rural areas. Swer distribution systems commonly use system voltages of 12 700 and 19 100 volts.

Even higher voltages are used to transmit large amounts of power, longer distances from the power stations to substations where it is transformed down to the distribution voltage. Transmission voltages used in Australia include, 33 000, 66 000, 110 000, 132 000, 220 000, 275 000, 330 000 and 500 000 volts. In general, the larger the amount of power and the greater the distance it is to be conveyed, , the higher the transmission voltage used.

Additional Benefits of Using AC

Generators

Generators, whether AC or DC actually generate AC. A DC generator (dynamo), is fitted with a commutator, or more recently rectifiers, which convert the AC to DC. The commutator with its brushes, is an expensive and high-maintenance piece of equipment, not needed for an AC generator (alternator). Consequently, the alternator is a simpler, cheaper and more robust machine requiring much less maintenance. It has the added advantage that it can be made in much larger sizes that dynamos.

Motors

The simplest electric motor is the AC induction motor. It has only one moving part, the rotor. So it is extremely reliable and relatively cheap. AC induction motors are widely used. It has been reported that in the USA, perhaps 60% of the electricity generated is used by AC induction motors.

In contrast, DC motors like DC generators, use a commutator with brushes to transfer the electricity supply to the rotating part of the motor. As with dynamos, commutators and their brushes are an expensive and high-maintenance item.

DC motors did have an advantage where speed variation was required, such as for electric trains. But even in trains, the AC induction motor has now taken over. High power solid state electronic devices are used to generate a variable frequency AC supply which varies the speed of the AC motor and hence the speed of the train. Brisbane’s recent suburban trains have this system.

J M Simmers

Queensland Energy Museum Inc

25 July 2007