Direct and Alternating Current

There are two types of electrical current. Direct current (DC) has a constant flow rate from a constant voltage source, like a battery in which one terminal (or pole) is always positive and the other always negative. The flow is always in the same direction, or polarity. Any current in which each wire is always of the same polarity, with one wire always positive and one always negative, is a direct current. Direct current is produced in batteries and photovoltaic equipment.

With alternating current (AC), the voltage difference between the two points reverses in a regular manner. This means that the electrical current changes direction back and forth at a fixed frequency (rate). The change from positive to negative to positive again is called one cycle, and the speed with which the cycle occurs is the frequency of the current. Commercial power from utility companies in the United States and Canada is AC, typically supplied at 60 cycles per second, or 60 hertz (Hz). Many other countries supply commercial power at 50 Hz. By the way, Hertz was not the same fellow who started the rental car franchises, but Gustav Hertz, a German atomic physicist born in 1887.

Equipment made for one frequency is not compatible with any other frequency. Motors won't perform as desired at the wrong frequency, and may overheat, burn out, or have a shortened life. In AC circuits, resistance (measured in ohms) is called impedance.

The advantage of AC over DC is the ease and efficiency with which the level of voltage can be changed by transformers. Generators (Fig. 27-2) put out currents at many thousands of volts. Transformers at the generating plant further increase the voltage before the electricity is passed to the main transmission lines, to keep amperage at a minimum. When the amperage is kept low, large amounts of energy can be transmitted through small wires with minimum transmission losses.

The electricity passes through substations on its way to local transmission lines. Once the electrical energy has reached the local area, it is reduced in voltage at another transformer for distribution to buildings. The local lines have higher transmission losses per mile than the main lines, but are much shorter.

The voltage that reaches the building is still too high for consumer use, so each building or group of buildings has a small transformer to reduce the voltage still further before it enters the building. Electrical service for small buildings is provided at 230 or 240V. You have probably seen large, cylindrical transformers on utility poles that reduce the voltage for small buildings. The voltage is again reduced to around 120V for household use. Some older homes have only 120V service. Near large cities, the supply may be 120/208V. Large buildings and building complexes often buy electricity at the local line voltage and reduce it themselves with indoor transformers before use.

Figure 27-2 How electricity is supplied to a building.

Transformers for large buildings are usually mounted on poles or pads outside the building, or inside a room or vault. The transformer for a large building steps down 4160 V service to 480V for distribution within the building. A second transformer in an electrical closet steps 480V down to 120V for receptacle outlets.

The electricity within a home may not be exactly 120 and 240V. Typically, a city dweller might have 126V at an outlet, while a suburbanite may receive only 118V. Outlets at the far end of a branch circuit have lower voltages than those near the service entrance panel, but the wiring in a home shouldn't vary by more than 4V. The minimum safe supply is 108V in order to avoid damage to electrical equipment.

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