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Measuring energy is a power quality proposition

With the rising costs of energy and the push to reduce consumption, more and more facilities are paying increasing attention to analyzing all facets of their monthly energy usage. Thorough and proper measurement of energy can usually uncover ways to reduce both usage and costs.

The typical industrial or commercial facility uses three-phase energy distribution, and then uses that energy supplied in a number of ways - for example, to provide heating, operate three-phase motors and motor drives, or handle single-phase loads such as those from computers and lighting. Under these circumstances, getting a real picture of overall energy usage involves more than taking a few basic voltage and current measurements.

When looking at energy usage and costs, it's important to first understand the two primary components that make up the energy supplied by the utility: power and demand. Power - the rate at which energy is expended - is measured in Watts, with Watthours describing the total energy expended over time. Watthours measure actual work, such as heating or cooling buildings and moving objects or liquids.

Demand or apparent power, which is expressed in kVA, measures the total requirement that a customer places on the utility to deliver voltage and current, without taking into account the efficiency of that delivery or whether or not it does actually work.

The Variables

For a simple single-phase circuit, where the load remains stable over a period of measurement, the process is simple: take the voltage and then the current readings, and multiply the two values. However, when measuring "real-life" loads, other factors should be taken into account.

For example:

  • The power factor (PF) is the ratio between power (kW) and apparent power (kVA). Power is typically less than apparent power. A low PF is bad, and a high PF is good. Utilities will usually apply a penalty charge if a facility's power factor falls below a certain limit, typically 0.95.
  • Reactive power is a type of current flow that produces no work but is present in an electrical distribution system. Known as Volt-Amps Reactive (VAR), it will be present whenever the power factor is less than 1.0. It's usually caused by motor inductances, and is greater when those motors are not loaded to their full capacity.
  • Harmonic currents are usually produced by the input rectifier loads of adjustable-speed motor drives, computers and similar electronic devices. There are actually two different power factor measurements. Total power factor includes the effects of these currents; displacement power factor includes only the 60Hz fundamental current.

An additional consideration when measuring energy is that it's best measured over time as the electrical system delivers power to the loads. It is possible to estimate energy usage by observing power use for a short period of time and then using that information to project longer term usage. This will work for something that consumes a consistent amount of energy, such as a 100-Watt light bulb. However, it gets a little more complicated with motors, variable-speed motor drives, and computers because consumption patterns vary over time.

How to Measure Power

A multimeter can be used to measure voltage and current and the readings can be multiplied together to get VA. However, other key measurements needed for energy reduction, such as Watts, require a power quality (PQ) tool that can measure real energy values and track them over time.

A PQ meter can simultaneously measure many values. Depending on the make and model, a PQ meter can be used to test single-phase, split-phase, three-phase (3-wire or 4-wire) measurement configurations, and measure or record V, A, W, VA, VAR, PF and harmonics. Some also have built-in functions that record measurements over time and to report energy readings such as kWh and kVAh.

To start monitoring, simply connect the voltage and current probes of the PQ tool to the supply and check the power (kW), demand (kVA), and the resulting power factor as well as VARs and harmonics. In terms of energy consumption, kW and kVA are generally the values to compare over time, in order to determine what changes need to be made within a facility to reduce consumption.

With the right PQ meter, facilities can account for all mitigating factors in energy measurements, provide an accurate report of energy usage as it happens, and take the steps needed to improve operations and reduce energy costs. It's all a matter of getting the numbers right in the first place.