Energi och Effekt

En elmätare eller energimätare används för att mäta mängden energi som distribuerats.

I den enklaste formen är effekten lika med spänningen gånger strömmen (V*A). Energi är effekt över tiden.

I en AC-krets är situationen dock lite mer komplex eftersom spänning och ström alternerar och kan vara ur fas beroende på last. Det är därför det krävs en mätare som hela tiden mäter spänning och ström och beräknar effekten av detta hundra eller tusen gånger per sekund.

För en korrekt redovisning av av effekt och energi krävs att spänning och ström mäts samtidigt.

Den enklaste formen av kommunicerande energimätare har en pulsutgång. Varje puls motsvarar en viss mängd energi som passerat mätaren. Till exempel, en mätare märkt med “500 k/kWh” innebär en mätare som räknar 500 pulser per 1 kWh. Varej puls är akWh/500 = 2Wh. Vanliga typer av pulser är “S0” eller “KYZ”. Dessa typer av pulser är direkt avläsbara från ezeio, upp till 4 st pulsmätare per ezeio.

Vissa mätare har en digital utgång. ezeio stödjer mätare med Modbus direkt, och via Modbus är det möjligt att läsa ut betydligt mer information än via puls. En Modbus-mätare ger information om energi, effekt för varje fas, ström, spänning, effektfaktor mm. Till varje ezeio kan anslutas upp till 20 st Modbus-mätare.

För mindre installationer kan används direktmätande mätare, vanligen upp till 63 eller 80 A. För mätning med högre ström krävs extern strömtransformator.

The current through an AC wire can be monitored using a Current Transformer (CT). There are several types of CT’s to choose from.

Solid Core vs Split Core

Split core transformers can be opened. That way they can be clamped around the monitored wire without interfering with the circuit – sometimes even without switching the power off. While convenient, split core CT’s usually costs a little more than solid core CT’s, and sometimes suffer from lower accuracy.

Output types

The simplest form of CT outputs a current that is relative to the current flowing through the eye of the CT. An unconnected CT output may carry a dangerous voltage, so safety may be an issue with this type of CT. The most common type is rated 5A at full scale, so for example a 5/400A CT would output 5A when 400A flows through the eye, and 2.5A when 200A flows through it.

Some CT’s have a calibrated resistor built in that makes the output from the CT a voltage rather than a current. These CT’s are safe to work with even when they are not connected to the meter. A common rating is 1/3V (0.333V) at full scale.

Some CT’s have a DC output. This type of CT cannot be used with an energy meter, but may be convenient if only a current reading is required, for example to monitor run time. Note that monitoring only current will not be satisfactory to calculate energy/power in an AC circuit. Many of the “home energy monitors” only monitor current, and thus will not be very accurate.

Rogowski coil (a.k.a Rope CT)

Rogowski Coils are very convenient for monitoring large diameter, or oddly shaped conductors, and since they are flexible they offer many mounting options. Rogowski coils are suitable for very high loads (1000A ++), but are usually less accurate than regular CT’s at lower currents. They require a separately powered integrator circuit to work with most meters.

Selecting the right CT

These questions must be answered in order to select an appropriate CT:

• What is the diameter of the cables? How large must the eye of the CT be?
• Are there any space constraints where the CT’s are to be mounted?
• What is the max current that will flow through the circuit? (Usually the fuse rating gets you in the ballpark).
• What kind of CT does the meter support (5A/0.333V/other?)
• What accuracy is required for the application?

To measure the amount of energy transported through a gas line, the type of gas needs to be known, and a flow meter is needed.

Gas flow meters typically have a pulse output. Each pulse will correspond to a certain volume of gas. The ezeio can directly monitor the pulse output from a gas meter.

Selecting a gas meter require specific knowledge about the installation requirements, local regulations and the properties of the measured medium. eze System does not carry or specify these meters. Please contact your local utility or contractor to work out the specifics.

Water is commonly used to transport thermal energy to or from a facility, sometimes called District Heating (or cooling). Pool heaters are also examples of heat transport systems where the water is heated up by the sun, moved through hoses and tubes with a pump, and leaves off that energy to the pool water.

To calculate the energy transfer in such a system, the following must be known:

• The type of fluid (usually water)
• The temperature of the fluid entering the consumer.
• The temperature of the fluid returning from the consumer.
• The rate of flow.

Two temperature probes and a flow meter will provide the data needed to calculate the energy transfer. There are special integrators designed to do the calculation and conversion to kWh or BTU and keeping track of the energy transfer. The ezeio is also capable of acting as an integrator, reading the flow and temperatures directly from the sensors.