Wattmeter


The watt meter measures and displays the power dissipation by a circuit element.

Energy (power over time) is measured on a watt-hour meter, such as the one that connects each house to the power company’s lines. If we remove time from the equation, we end up with a power meter, as a power meter measures the instantaneous dissipation of power, much in the same way a speedometer measures a car’s instantaneous speed. The measuring of power dissipated in a purely resistive circuit with a DC voltage source is trivial, as power equals voltage against current, P = VI. The math becomes a little trickier with AC-power sources, as the waveform dictates the averaging formula used to find the power dissipated. Circuits containing reactive parts, such as capacitors and inductors, on the other hand, are much more complex to measure.

Pure reactive components dissipate zero power, which makes sense in a DC circuit, as the capacitor passes no DC current and the inductor displaces no voltage. Yet, in an AC circuit, the reactive components “seem” to dissipate power, as current passes through the capacitor and the inductor sees a voltage drop. This counterfeit power is called “reactive power;” it is measured not in Watts, but in VARs (Volt-Amps-Reactive). In contrast, actual power is labeled “true power” or “active power” or “real power;” it is measured in Watts. To this two powers, a third must be added, “apparent power.” Much in the same way as impedance (Z) is the combination of reactance (X) and resistance (R), apparent power is the combination of pure power with VAR. Apparent power is measured in VAs (Volt-Amps) and it’s mathematical formula symbol is “S.” Transformers are rated in VAs. For example, a 100VA transformer might hold a secondary with a 10Vac winding that can sustain 10A of current output, which if attached to a 10-ohm resistor would realize 100 watts of true power (P) and 100 VA of apparent power (S), but zero VAR of reactive power (Q). And if hooked up to a 265µF capacitor, the transformer would realize 0 watts of true power and 100 VA of apparent power and 100 VAR of reactive power. If the 10-ohm resistor and 265µF capacitor were placed in series and hooked up to the transformer’s secondary, the transformer would realize 50 watts of true power and 70.7 VA of apparent power and 50 VAR of reactive power. The power triangle shown below displays the relationships between the three powers. (Note how Pythagoras theorem nicely applies to the values from the previous example and how apparent power will always be equal to or greater than pure power.)

Connecting the power meter to a circuit can be done in two ways in the Circuit Setup tab: by selecting an individual part or by selecting two nodes within the circuit.
The Meter Options tab control the display of the meter.

The meter displays its measurements in two ways: text and a moving bar graph. The bar’s length expands and contracts with the power being measured, which makes it an analog readout. When the measured current exceeds the power meter’s range, the bar changes color from its normal blue to red. Pressing the “Auto” button overrides the fixed range and auto adjusts the range to twice the highest power reading it sees.

The Measure Power As section specifies how the instrument measures Wattage.
Real/Active - In contrast to apparent or reactive power, actual power refers to the power dissipation by resistive components. It is labeled "true power" or "active power" or "real power", and is measured in Watts. Its mathematical formula symbol is "W".

Reactive - Pure reactive components dissipate zero power, which makes sense in a DC circuit, as a capacitor passes no DC current and an inductor displaces no voltage. Yet, in an AC circuit, the reactive components "seem" to dissipate power, as current passes through the capacitor and the inductor sees a voltage drop. This counterfeit power is called "reactive power" and is measured not in Watts, but in VARs (Volt-Amps-Reactive). Its mathematical formula symbol is "Q".

Apparent - Much in the same way as impedance (Z) is the combination of reactance (X) and resistance (R) apparent power is the combination of pure power (P) with reactive power (VAR). Apparent power is measured in VAs (Volt-Amps) and its mathematical formula symbol is "S". Transformers are rated in VAs.

Power Factor - Power Factor (PF) or Total Power Factor (TPF) equals the ration of real power (W), which performs the actual work of creating heat, light, motion, etc., over apparent power (VA) that is the combination of real power (W) and reactive power (VAR).