Phase measurement is not witchcraft, but one should think about the optimal choice of device as well as a reliable measurement result. Help promises a pocket-sized phase meter.
The variety of portable phase measuring devices is not as high as, for example, in the case of universal multimeters. The latter often have no phase measurement mode. The resulting lower popularity of this particular measurement technique is not without consequences: Users of a multimeter are often unfamiliar with possible sources of error and are unaware of the practical benefits of measuring a phase.
When it comes to the analysis of an electrical variable, such as voltage or current, the determination of less characteristic values often spares the handling of larger data sets or course descriptions. A typical periodic signal may be substantially determined by the parameters amplitude, frequency, and phase relative to a comparison signal. To complete the signal description still helps the waveform, which inspires a spectral representation.
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The phase angle – not without confusion!
If the fundamental values of amplitude and frequency are to be determined, no complex signal processing is necessary and the user can use a simple multimeter for this purpose. If several signals originating from a familiar source are to be measured, the phase determination gives much information about faulty system behavior, parameter settings or line assignments. A common source is the basis for frequency equality of the signals and thus for meaningful phase measurement.
Unlike the determination of a voltage lurking in the phase measurement hiding potential confusion. While frequency or RMS measurements are taken with great care when using two color-matched laboratory cables, the tendency for two-color line selection in a DC voltage measurement is considerably higher. The desire for color diversity in a phase measurement, however, pays off.
A small example should clarify inconspicuous sources of error. The typical 120 degrees between the potentials, lines or transformer windings of a three-phase system are supposedly determined quickly. Compared to the correct choice of the four connections, a phase meter falsifies an angle of -60 degrees in the event of reverse polarity of an input pair. If you swap both connection pairs, the measuring device will display a treacherous -120 degrees. Even worse: If reverse polarity and reversed connection pairs combined, then the meter shows an apparent 60 degrees.
Incorrect readings and electrical hazard
If the phase meter has a handy switching option between half and full circle display (-180 ° .. 180 ° / 0 ° .. 360 °), the clutter is completed with additional false 240 degrees or 300 degrees. Inadvertent misallocation of the four lines would also create some other false readings, all of which may be mitigated by knowledge and awareness on the part of the user or, where possible, prevented by special four-pin connection adapters. Consistently isolated entrances and colored markings also provide valuable allocation help.
In the propulsion, the energy supply as well as the traditional railway signaling technology, the individual phase signals, although peacefully rigidly coupled concerning the phase angles to each other, but often not in galvanic terms. Measuring equipment with the so-called common ground, with the black COM socket leading to the standard internal reference point, would not only deliver incorrect values but would also electrically endanger the system and the user. Measuring and testing technology with isolated inputs becomes inevitable.
The variety of different measuring methods
If a phase value is estimated in the vicinity of the boundary between the negative and positive semicircles or near the full circle angle, measured value fluctuations can occur either in the form of irritating flickering of the sign (-180 ° / 180 °) or in apparently very coarse value jumps (0 ° / 360 °). Express. Since you can switch between full and semicircular display, a stable phase value display is no problem (Figure 1). These are two interpretations of the same angle. When it comes to the analysis of phase shifts due to signal delays, one should be careful in the evaluation. Reason: If phase shifts of more than 180 ° or 360 ° occur with periodic signals, these can be erroneously interpreted as negative or ambiguous.
A variant to determine the phase is to evaluate the distance of polarity changes (“zero crossings”) of the signals concerning the signal period. To emphasize is the enormous signal form dependency of this principle. While providing a useful measure complex networked module, measurement for low-noise digital signals, measure ripple voltage, it is subject to other techniques in most analog signals, particularly at low zero crossings, as well as noise or higher-frequency contributions. As a result of several polarity changes within a period, the underlying frequency determination can fail.
Handy live multimeter with a graphics display
The phase can be determined by comparison with a temporally and formally exact fundamental vibration reference. The Best Hobbyist Oscilloscope Reviews of 2018 offers the VSR (Virtual Selective Reference) technology. Also, the multimeter inherently offers good interference immunity, waveform independence and time resolution within a wide frequency range.
In addition to permanent frequency control, the waveform can be graphically checked. The isolated inputs circumvent the disadvantage of the necessary galvanic coupling of system parts in simple devices and the so-called three-voltage method – a phase determination method on a geometric basis. Rotary multiphase drives as in motors form their torque mainly based on the phase angles of the fundamental frequency component of the drive currents.
The phase measurement concerning the temporal position of the fundamental wave provides particularly practical results. Apart from the SICO 2074, only a few phase meters use this advantage. Another typical limitation of the zero crossing method encountered in many devices is the maximum measurement frequency as a result of the internal reference clock needed for angular resolution. At least 36 MHz would be necessary to resolve 10 kHz signals at 0.1 °, as dominated by the SICO 2074.
Attention, the sign is essential!
If signal B appears delayed concerning signal A, then signal B (phase to be measured) has a negative phase angle concerning signal A (reference signal). A delayed signal appears in an oscillogram shifted to the right (towards higher time values), a leading signal corresponding to the left. The phase angle of more than 360 ° against mutually shifted signals is displayed incorrectly due to the periodicity, which also applies to phase angles outside the display range. Thus, a signal delayed by 25 ° can be viewed as a signal leading by 335 ° or by a lead of 370 °, leading only by 10 °. A 180 ° / 360 ° switchover option supports the interpretation.