In the previous issue of E-News, we introduced the basic concepts of electrical power: voltage (V), current (I), and power (W). The article also discussed the subject of direct (DC) power versus alternating (AC) power, and the idea of phase difference between voltage and current signals (power factor).

This article focuses on the primary advantage of alternating (AC) power over direct (DC) power: the ability to change the voltage level either up or down through the use of transformers. This allows electric power to be transmitted at high voltage levels for maximum efficiency, then stepped down to the proper voltage level (and phase) needed for a particular customer or load application. The article also presents the concept of 3-phase power, and explains why this method of distribution is the standard for most industrial power applications.

The signal from an AC electrical source is shaped like a sine wave (see Figure 1), alternating from a positive to a negative value sixty times per second.

Figure 1: AC Signal

Recall that the equation for electrical power is (W = VI), where V is voltage in volts, I is current in amperes, and W is power in watts. In our modern electric utility system, industrial facilities and large groups of residential customers require enormous amounts of power. To transmit that much power, at the low voltage levels used by most electrical loads, would require so much current flow that the conductors (power lines) would have to be prohibitively large and costly.

Transformers are able to raise the voltage of the transmitted power, while simultaneously lowering its current, effectively keeping the power level the same. With such high voltage/low current levels, electrical power can be efficiently transmitted long distances, using smaller conductors and other electrical system components.

Most utility generators create electrical power at voltages of 10kV - 25kV. This power is stepped up to high voltage levels (250kV - 750kV), then injected into the system transmission lines. Distribution substations reduce these transmission voltages to 10kV - 35kV at various point along the system, to safely and economically distribute power to local areas. Transformers mounted at or near the customer's location then step the signal voltage down to the level needed for that particular customer.

The electricity supplied to homes is single phase. Residential customers normally receive a 240 volt, single-phase power signal, which is then split to provide the 120 volts used by most appliances.

Industrial equipment (i.e. pump and fan motors, dryers) generally operates at 208, 240, 480, or 600 volts, three-phase. Most industrial facilities therefore receive power from the utility lines at several thousand volts, and use additional transformers to step the voltage down as needed for particular groups of equipment. Since the power requirements of industrial applications are so much higher, the standard for industries is to use three-phase power. This method of electrical power transmission utilizes three independent signals delivered through three independent conductors, with each signal 120° out of phase with the other two (see Figure 2).

Figure 2: 3-Phase Power Signal

As noted in our earlier article, any calculation for AC power must include the power factor (pf) of the signal, as well as the voltage and current. The power factor for three-phase signals can vary between 0 and 1, as it does for single-phase power. For three-phase electricity, the power equation includes an additional constant, and is written:

W = 1.732 x V x I x pf

Using three conductors (rather than two, as used for single phase power) allows for lower current levels in each conductor, again facilitating the use of smaller conductors. For this and other reasons, three-phase power simply provides a more efficient means of supplying power to large electrical loads like motors.