Power Analyzer: A Power meter for high precise demands

The neutral conductor is often not present. For example, many electric drive applications like electric machines do not provide the star point for external access. By using an artificial neutral point, this missing reference center is generated externally. Used as a reference point, the power of each phase and the total power can be determined as in the 3P4W arrangement with three power measurement channels. The total power is the sum of the measured individual powers. The figure below shows the corresponding connection diagram.

If the neutral point is not available, the total power can be determined using the Aron circuit without an artificial neutral point. With this method of power measurement, two power measurement channels are sufficient. If the sum of the instantaneous values of the phase currents is zero, one of the phases can be considered as a common return. The figure shows the corresponding measurement setup.

Two phase currents are recorded by the two current sensors. The two voltage inputs of the measuring cards measure phase-to-phase voltages. As a result, the services displayed do not correspond to the individual services of the respective strands. However, the total power can be determined by adding these two powers. This only applies under the condition that the sum of all instantaneous phase currents is zero. When operating electrical machines on frequency converters, leakage currents can occur due to capacitive coupling to ground, which falsify the measurement result. In this case, the sum of the phase currents is not equal to zero. Therefore, the Aron circuit is not recommended for a measurement on a converter.

With the 3V3A configuration, 3 power measurement channels are used. However, since there is no neutral conductor or neutral point available here either, a phase is used as a reference. The figure shows the corresponding connection diagram.

The first power measurement channel LMK1 measures the current in phase L1 and the voltage between phases L1 and L3. LMK2 measures the current IL2 and the voltage between L2 and L3. Phase 3 represents the reference point for the first and the second power measurement channel. The third measuring channel LMK3 measures the phase current IL3 and the line-to-line voltage UL1-L2.

In the configuration shown above, the total power is obtained from the sum of the two powers LMK1 and LMK2. It is important to note that none of the individual values (LMK1 to LMK3) corresponds to the correct individual power of the respective phases! Even if two power measurement channels are sufficient for calculating the total power, it is still advisable to use the 3V3A method. By sensing all three phase currents and voltages, statements can be made about symmetries and any leakage currents that may occur.

When measuring power, a distinction is made between electrical and mechanical power.

The values

• Active Power (Unit W)
• Reactive Power (Unit var)
• Apparent Power (Unit VA)

are considered to be electrical power values.
An input for torque and speed is required for the calculation of mechanical power. The actual sensing of these values takes place within a torque measuring shaft.

Current and voltage are measured directly. After the analog quantification, the signals are digitized. These values are then multiplied and averaged in real time. This is how the real power is calculated. The digital availability of the measured values also enables to derive other variables. Furthermore these data also offer the option of filtering.

The basic measuring method does not depend on AC or DC. With AC, the entire signal period must be taken into account. With a sine, for example, the power also looks like a sine. However, it is essential to average the entire period, otherwise there will be beats (flickering measured values). That makes an exact reading impossible. With DC signals the power can be read off at any time.

Reactive power: Reactive power does not affect the energy balance of the system. But it still has to be made available. It is a pendulum power that does not affect the energy balance due to the recurring energy flow. It is not entirely necessary to keep the reactive power low, but this is desirable. The greater the reactive power, the stronger the electrical components have to be designed. Resulting in more volume, more weight and more costs.

Real power: Unlike reactive power, real power actually has an effect on the energy balance. It is the electrical power that "actually" matters. It is an effective gain or loss in energy.

Apparent power: Apparent power describes the sum of active and reactive power. The apparent power does not allow any information about whether there is actually a lot or little "real" power. For example, a high apparent power with high reactive power can have a lower real power than a low apparent power with low reactive power.

In the segment of power meters, a distinction must be made according to the application use. For end user applications, "simple" power meters, also called watt meters, are more interesting. These have a limited range of functions and are also less precise than highly professional devices in terms of their accuracy. The areas of application for watt meters are particularly in the home for tracking down "energy guzzlers".

Power analyzers differ from watt meters in that they are much more accurate. They work with a larger range of functions, but are also a lot more complicated. They are used in professional environments for R&D activities and EOL testing.