Three-phase current measurement :
Selecting the right test voltage source

A precise single-phase or three-phase supply with optimal phase symmetry in terms of amplitude and phase angle is extremely important for certain tests. If one—or even both—of these requirements are not met, this can lead to significant measurement errors and, in the worst case, even to an incorrect GO.

With our 6 selection criteria, everyone can find the right test voltage source:

selection criteria

1fixed or variable TEST VOLTAGE?

2 Low or high voltage stability?

3 Fixed or variable frequency?

4 Low or high phase symmetry?

5  Precise phase shift of 120° between the 3 phases?

6 Low distortion/harmonics?

There are different solutions for all these requirements—which vary in terms of technical complexity and, accordingly, price—see also our overview at the end of this article.

What asymmetries exist in the three-phase current network?

1. Unbalanced phase voltages
In the mains supply, the voltage differences between the 3 phases may amount to several percent.

2. Asymmetrical phase angle between the phases
The deviation the phase angle of the three phases relative to each other should not be underestimated. Ideally, it should be 120° electrically. However, even a phase difference of a few tenths of a degree from the ideal value can have a significant effect on the current consumption or cos φ of the test object.
This applies in particular to electrical machines.

3. Distortions/harmonics
These factors also have an impact on the test depending on their severity, but are not considered in this article.

What effect do asymmetries have on the test result?

Unbalanced phase amplitudes and/or phase shifts can lead to significant differences in current consumption in the individual or linked phases.
Of course, phase imbalances always occur in the real grid – and then the effects described always occur as well ...

For testing purposes, however, a high degree of symmetry in terms of amplitude and phase angle is often necessary in order to reliably detect certain fault effects. Examples of high symmetry include no-load current consumption and locked rotor testing of asynchronous machines.

Ohmic, inductive, or capacitive test objects

Almost all test objects can be reduced to one of three basic variants or hybrid forms of these three variants in terms of power supply and the directly associated current consumption.
Typically, this affects the phase shift between voltage current.

Purely ohmic test objects
Typical examples here are electric heat generators consisting of resistors.
Since they are purely ohmic in nature, voltage and phase imbalances can be corrected relatively easily using mathematical methods. The current voltage not voltage any phase shift relative to voltage , meaning that power measurements are possible despite the imbalance.

Inductive, capacitive test objects
Typical examples here are electric motors, compensations, etc.
The relationships and effects here are clearly more complex and cannot be easily compensated mathematically due to many influences. Theoretically, compensation is always possible—but in practice, many parameters of components such as inductances, stray inductances, capacitances, and couplings between components are unknown.

At this point, at the latest, a good symmetrical power supply becomes necessary.

Power supplies from SCHLEICH an optimal TEST VOLTAGE