Power measurement and analysis
Rick Nelson, Chief Editor -- Test & Measurement World, 8/1/2005
Whether you are dealing with a three-phase high-power industrial-distribution system or an Energy Star-compliant computer in standby mode, you're likely to have to make power measurements. Instruments that can make such measurements range from current sensors and wattmeters to precision power analyzers as well as digital oscilloscopes equipped with power-analysis capabilities.
In the Webcast, "Power Measurement and Analysis," produced by Test & Measurement World, sponsored by Yokogawa's Test and Measurement Division, and originally broadcast May 24, Bill Gatheridge, Yokogawa's product manager for power-measuring instruments, describes how to apply those instruments to make single-phase and three-phase measurements in the presence of inductive and capacitive loads powered by sinusoidal or pulse-width-modulated sources.
Gatheridge begins with the basics, covering Ohm's law as it relates to AC power measurements. He reviews the Blondel transformation, which states that to measure power, you need one wattmeter fewer than the number of wires in your system: one wattmeter for a two-wire single-phase system, two wattmeters for a three-wire single-phase system or three-wire three-phase system, or three wattmeters for a four-wire three-phase system.
Specific applications covered include analysis of low-power systems, switching power supplies, and inverters and pulse-width-modulated drives. Gatheridge also covers power factor, which comes into play with inductive loads (such as AC motors), where current lags voltage, as well as with capacitive loads (such as fluorescent lighting), where current leads voltage. He describes the difference between displacement power factor (the cosine of the phase shift between pure voltage and current sine waves) and true power factor (power in watts divided by total volt-amperes). He concludes by describing a three-wattmeter method for measuring power factor in three-phase systems.
An archive of the Webcast is available at www.tmworld.com/webcasts.
