Advantages and Disadvantages of Alternating Current
Alternating current can be generated at higher voltages than DC with fewer problems of heating and arcing. Some standard values of voltage are 2300; 4600; 6900; 13,800; and 33,100 volts. These values are frequently increased to 100,000; 200,000; and 800,000 volts for transmission over long distances. At the load area, the voltage is decreased to working values of 120, 208, 240, 277, 440, 480, and 550 volts. Figure 6-10 shows a transmission line.
Transmission lines from the generating station to industrial, commercial, and residential customers.
The ease with which the voltage can be raised and lowered makes AC ideal for transmission purposes. Large amounts of power can be transmitted at high voltages and low currents with minimum line loss. Because , the lower the value of transmission current, the less will be the line loss. Because the current is low, smaller transmission wires can be used to reduce the installation and maintenance costs.
Direct current generators, because of their construction, limit their output voltage to 2000 volts or less. The voltage cannot be raised or lowered through the use of transformers. Long-distance transmission requires heavier cables and generally results in greater power loss.
Alternating current generators can be driven at high speeds and constructed in large sizes. Because they generally have rotating fields and stationary armatures, their rotor windings are small and light in weight, thus reducing the centrifugal force. Modern alternators are built with capacities of up to 500,000 kilowatts.
Because of the need for commutators, DC generators are limited in capacity. The maximum power available from any one unit is generally 10,000 kilowatts. If DC were the main source of supply, many more generating stations would be needed. Each station would require a source of power to drive the generators. With the supply of fossil fuels dwindling, this would be a gross waste of energy. AC, on the other hand, can be produced in large central stations and distributed over greater distances with maximum efficiency. It should be noted, however, that with improvements in technology, solar and wind power, which produce DC power, are proving to be viable alternatives to fossil-fuel burning power plants.
An advantage of AC is that it produces a varying magnetic field. This changing field is used in the distribution transformer for raising and lowering the voltage. Transformers will be discussed in Chapter 11 of our Fast Trax® Magenta Course. Lighting units that use transformers produce better and more efficient light. Induction motors utilize the transformer principle for their operation. These motors are less expensive to build, install, and maintain than DC motors. They also require less space than that required by DC motors of the same horsepower. (Electrical generators and motors will be discussed in Chapter 12 of our Fast Trax® Magenta Course.)
AC does have some disadvantages. As you have seen, it is more complicated to understand. Also, AC must be converted to DC in order for most electronic devices to operate. Portable equipment relies on DC power (batteries) to operate. This is not possible with AC-powered equipment. DC motors, by their design, have one distinct advantage over AC motors: They have better speed control. In general, DC motors are used when wide variations of speed and accurate speed adjustments are required. However, with the advent of electronic variable frequency drives (VFDs), the speed control capabilities of AC motors rivals and, in many cases exceeds, that of DC motors.
The above is an extract from our Fast Trax® Magenta Course on Industrial Electricity.