Basics of Power Monitoring

Voltage and Current Values

An accurate measurement of voltage supplied by the utility and the current produced by the connected load is necessary in identifying power usage and power quality problems.

DC

Voltage is either direct current (DC) or alternating current (AC). DC voltage produces current flow in one direction. DC voltage can be obtained directly from sources such as batteries and photocells, which produce a pure DC. DC voltage can also be produced by applying AC voltage to a rectifier.

Measuring DC Voltage

The value of DC voltage varies. Low level DC voltages, such as 5 – 30 VDC, are commonly used in electronic circuits. Higher levels of DC voltage, such as 500 VDC, can be used in many industrial applications to control the speed of DC motors. A voltmeter is used to measure DC voltage.

AC Voltage, Current and Frequency

Current flow in AC voltage reverses direction at regular intervals. AC voltage and current are represented by a sine wave. Sine waves are symmetrical, 360° waveforms which represent the voltage, current, and frequency produced by an AC generator. If the rotation of an AC generator were tracked through a complete revolution of 360°, it could be seen that during the first 90° of rotation voltage increases until it reaches a maximum positive value. As the generator rotated from 90° to 180°, voltage would decrease to zero.

Voltage increases in the opposite direction between 180° and 270°, reaching a maximum negative value at 270°. Voltage decreases to zero between 270° and 360°. This is one complete cycle or one complete alternation. Frequency is a measurement of the number of alternations or cylces that occur in a measured amount of time.

If the armature of an AC generator were rotated 3600 times per minute (RPM) we would get 60 cycles of voltage per second, or 60 hertz.

AC voltage can either be single- or three-phase. While singlephase power is needed for many applications, such as lighting, utility companies generate and transmit three-phase power. Three-phase power is used extensively in industrial applications to supply power to three-phase motors. In a three-phase system the generator produces three voltages. Each voltage phase rises and falls at the same frequency (60 Hz in the U.S., 50 Hz in many other countries); however, the phases are offset from each other by 120°.