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Energize How to measure energy consumption 
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How to measure energy consumption 

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Information from Fluke  –

If you haven’t measured energy before, take a minute to read Fluke’s explanation here.

Power, kW, indicates the rate at which energy is expended in one second. Watt-hours (Wh) describes the total energy expended over other time periods, such as a month, as recorded for ac energy use by our electrical utilities. Wh measures actual work, such as heating or cooling buildings, moving objects or liquids, etc.

Demand, kVA, measures the total requirement that a customer places on the utility to deliver voltage and current, without regard to the efficiency of that delivery or whether it does actual work.

Figure 1: Fluke power screen. This is a real time view of all the common power calculations. The kW, kVA, kvar and PF are displayed for each phase and total. The symbols to the left of the kvar readings indicates whether the load is inductive or capacitive.

Now let’s start measuring. Use a regular digital multimeter with an accessory current probe to measure the voltage and then the current, and then multiply the two values to get demand – kVA.

This works for a simple single-phase circuit where the load remains stable for the period of the two measurements. For a real-life load, we need to account for a few other elements.

Power factor, imaginary power, and harmonics

Power factor

If the circuit is operating at 100 % efficiency (which rarely happens), then demand is also a measure of power. Power is usually less than demand. The difference, kW/kVA, is called power factor (PF). Utilities often collect a penalty charge if PF falls below 0,95. Some utilities are setting the bar even higher. Remember: Low power factor is bad; high power factor is good.

Figure 2: Fluke 435-II energy screen when you scroll down. This allows you to track accumulated kWh, kVAh, kvarh, kWh forward and kWh reverse for each phase and total.

The typical industrial or commercial facility uses three phase energy distribution, and then uses that energy in several ways – to provide heating, operate three phase motors and motor drives, or handle single phase loads such as computers and lighting. Three phases make it harder to measure power or energy usage, particularly if you plan to use improved efficiency to reduce energy use.

Imaginary power

Volt-Amps Reactive (var) is a strange type of current flow that produces no work but is present on your electrical distribution system. It’s part of the difference between power and demand and thus contributes to lowering power factor. It’s usually caused by motor inductance and is greater when those motors are not loaded to their full capacity. A constant speed motor driving a large air movement fan is an example where mechanical dampers have been used to regulate air flow, making a fan less efficient. This also reduces the load on the drive motor and increases imaginary power in the electrical supply system.

Figure 3: PowerLog “Energy.” This 30-day load study graph allows us to view the kWh and kVARh for each phase and total. An averaging period can also be selected to help match the billing period used by the local utility (typically 15 minutes).

Many facilities opt to change their motor supply from direct line drive to an adjustable speed motor drive, so that they can optimize the motor’s operation and speed to its load. That optimization uses energy more efficiently in the fan and motor and increases power factor.

Harmonic currents

Current reflected back into the supply system is produced by the input rectifier loads of adjustable speed motor drives, computers, and similar electronic devices. Harmonics also reduce power factor.

How to measure power

To measure real power, we need a meter that can simultaneously measure voltage, current, and all the stuff mentioned above that lies in-between, over a one second period. A digital multi-meter can’t do that. The solution lies in a power quality tool.

Figure 4: PowerLog “Power” view. In this view of the 30-day load graph we can view the kW and kVAR for each phase and total. From here we can identify our maximum values along with time and duration.

Depending on the make and model you select, you can 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 of the recording models also provide the means to record measurements over time to report the energy readings used by the utility: kWh, kVAh and kvarh.

The good news here is that these tools will account for all the issues mentioned above and accurately report energy use when it happens, as a function of instantaneous voltage and current measurements over time.

Timing

Here’s the last complication. You can only measure energy as the work that your electrical system delivers to your loads, and that takes time. You can estimate what energy usage will be by observing power use for a short period of time. Using that information, you can project longer term energy usage with some simple math.

Example: A 100 W lamp burning for one hour consumes 100 Wh of energy. That same lamp would use 100 x 24 x 365 = 864 kWh over a year.

It gets a little more complicated with motors, variable speed motor drives, and computers, but if you measure the power usage for one hour and then apply some assumptions to the results, you can estimate the energy usage for a month or a year, pro-vided the rate of energy usage stays the same. The other option is to do a 30-day load study with a power logger.

If you really want to save money

So yes, you could use your multimeter to measure voltage and current, make your calculations, and go from there. But the whole point of energy reduction is that for the first time, electrical measurement accuracy makes a monetary difference. If your energy calculations are inaccurate, because they don’t account for power interferences in your system, then you really don’t know how much you’re consuming, or what impact your reduction efforts have. It’s worth using a power quality tool like Fluke’s 435-II to get real energy values, and to then track those over time.

A video is available here: https://youtu.be/UWpO7V38bng

Contact Comtest, Tel 010 595-1821, sales@comtest.co.za

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