Tutorials > Power module tutorials > Single phase power measurement

We have seen already example how to measure, store and trigger on voltage and current inputs in the "Voltage and current" examples. When measuring those two parameters, it is always interesting to also measure the power consumed or produced by the device being tested.

If we look to the DEWESoft power module, there are lots of parameters which can be calculated. Actually it is amazing how many channels can be produced from of just two input channels. First of all, let's take a look at the common picture from the power measurement. This view is called a vector scope. The name comes from the fact that not only absolute values of the voltage and current are important, it is the also very important to know the phase relations between them. Thus, the vector scope shows the amplitude of voltage (red arrow with black pointer) and current (red arrow with red pointer) as well as the phase in between them.

Why is the phase information so important? This is essential because the machine can use only the part of the current that is in phase with the voltage for producing the work. Therefore we measure the phase angle between the voltage and current as angle phi (in our case -30 deg), and from that also the cos phi (which is just a cosine of that angle, but it is very nice because it is directly the ratio of work against the total consumed current).

Thus we define the power:

S=U · I	this is so called apparent or consumed power; in short, this is a power for which we pay the power distributor
P=U · I · cos phi	active power, which is the power used for doing the active energy or work
Qi=U · I · sin phi	so called reactive power; this is wasted energy, so we need to keep it low

Next, we can calculate the harmonic components. In theory, a line voltage is a perfect 50 (or 60) Hz sine wave. Since nothing in the world is perfect, the line voltage can have distortions, which are nicely shown as harmonics in the frequency spectrum.

The next picture shows the typical line voltage signal in the scope. We can already see in the scope that the voltage is not a pure sine wave. The special display called harmonic FFT nicely shows that the fifth harmonic is quite high - it has 7 volts amplitude on 220 volts of grid voltage. What is the effect of the harmonics? Imagine we have an AC electro motor. The first harmonic (line frequency) is driving the motor. The rest of the harmonics are producing vibrations and noise, but the ironic truth is that there are bad and even worse harmonics.

The 2nd, 5th, 8th... harmonics are really bad ones, since they are breaking the motor. The 3rd, 6th, 9th... harmonics are either driving or braking, while 4th, 7th, 10th... harmonic are driving the motor, but they are still producing higher noise and vibrations. Harmonics on the grid are produced either on the generator side or on the consumer side from for example switching power supplies or nonlinear devices like a transformer on or off.

Actually we can calculate the apparent, active and reactive power for each individual harmonic. We can also calculate the interharmonics. Those are sine waves or other signals appearing in between the harmonic lines, which are not covered by the harmonic calculations.

There are also a few other parameters which that can be calculated. Further, we can calculate THD (total harmonic distortion or a sum of all harmonic values), period values (values for voltage, current and power for each period - this is very helpful for triggering) and the flicker (this is actually a power quality parameter measuring low frequency distortions of the voltage which tells us how much the lights are blinking).