A Practical Primer On Motor Drives (Part 7): Power Calculations In Three-Phase Systems
by Ken Johnson, Teledyne LeCroy, Chestnut Ridge, N.Y., How2Power Today, Aug 15 2016
Focus:
In this part, the discussion moves on to power calculations in three-phase systems as
encountered in motor drive applications. After reviewing the basics of calculating power values
in systems with resistive and non-resistive loads, techniques are explained for measuring power
given various three-phase system configurations including four-wire and three-wire wye-
connected systems, as well as three-wire delta-connected systems. Both line-to-neutral and
line-to-line measurements techniques are discussed as they apply to these configurations as is
the conversion of line-to-line voltage measurements to their line-to-neutral equivalents for
the purpose of calculating power per phase. Both the two-wattmeter and three-wattmeter methods
of measuring power are explained. As in previous parts, the Motor Drive Analyzer is used to
demonstrate the various measurement techniques on an example motor drive and how the MDA’s
built-in functions automate the power calculations described.
What you’ll learn:
- How to calculate power in three-phase ac systems
- How to measure three-phase ac power in four-wire and three-wire wye-connected systems, as well as three-wire delta-connected systems
- How to understand the two wattmeter and three wattmeter methods of power measurement
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