The classification of low frequency interference emissions can be identified as:
Radiated magnetic fields
Radiated electric fields
Audio frequency conducted disturbances
Power disturbances: principally harmonics & flicker, but also more specialist types such as commutation notches
The list above covers most LF emissions phenomena that will be found in either commercial or military standard requirements.
Radiated fields
These are measured in the same way as for radio frequency fields, but since the test is always in the near field, it is necessary to be clear whether the electric or the magnetic component is being measured. These tests are almost exclusively performed only for the military standards. They include DEF STAN 59-411 tests DRE01 and DRE02, and MIL STD 461 tests RE101 and RE102.
In the realm of automotive applications, the necessity to receive AM radio signals within a vehicle mandates RF emission tests in the low-frequency spectrum. This requirement holds true for both vehicle-level (GB/T 18387) and component-level (FMC1278)assessments. The most prevalent test employed for this purpose is the Active Rod Antenna test (measuring electric field in dBμV/m). In marine applications, the evaluation of low-frequency magnetic fields is a regulatory requirement, as stipulated in standard EN 60945. It's important to highlight that while magnetic field measurements are employed, the unit of measurement in this particular standard is expressed in dBμV/m. Consequently, a conversion factor of 51.5 dB is applied when transitioning from magnetic field strength to electric field strength to align with the standard's requirements.
This conversion factor of 51.5 dBΩ corresponds to the characteristic impedance of free space, which is 377 Ω. The relationship between these two fields is defined by the equation E/H=Z, where E represents electric field strength (in dBμV), H represents magnetic field strength (in dBμA), and Z represents impedance (in dBΩ). In this dB form, the equation can be expressed as E(dBμV)-H(dBμA)=51.5(dBΩ).
AF conducted
Again, these tests are performed only for military standards. The relevant tests are:
MIL STD 461 CE101, 30Hz-10kHz
DEF STAN 59-411 DCE01/02, 20Hz-150MHz.
In both cases the common mode current is measured on each relevant cable.
Power Quality
Harmonic components of the AC supply input current to an item of equipment arise from non-linearities of the load over a single cycle of the input voltage. Requirements for measuring harmonic emissions are embodied in IEC 61000-3-2, which covers all electrical and electronic equipment with an input current up to 16A per phase. This has a sister standard (IEC 61000-3-12), for higher-powered equipment up to 75A.
The problem of mains harmonics is principally one for the supply authorities, who have to provide a high quality electricity supply. If the total load at a particular mains distribution point has a high harmonic content, the non-zero distribution source impedance will cause distortion of the voltage waveform at this point, and possibly, due to supply network resonances, at other remote points. This in turn may cause problems for other users, and the currents themselves may also create problems (such as overheating of transformers and compensating components) for the supplier.
A plain resistive load across the mains draws current only at the fundamental frequency (50Hz in Europe). Most electronic circuits are not resistive. The universal rectifier-capacitor input draws a high current at the peak of the voltage waveform and zero current at other times; the well-known triac phase control method for power control (lights, motors, heaters, etc.) begins to draw current when the triac switches on, only partway through each half-cycle. These current waveforms can be represented as a Fourier series, and it is the harmonic amplitudes of the series that are subject to regulation. Various methods are used to control supply harmonics, the most common being power factor correction.
IEC 61000-3-2 applies either fixed limits to the harmonic content or variable limits depending on the power drawn by the equipment; choice of limits depends on the class of product. The limits apply up to 2kHz (40th harmonic), although only those up to n = 11 are shown in the graph. Class A is the most universally applicable and has fixed limits for each harmonic.
Certain military standards also have a requirement for harmonic limitation, although this is not found in DEF STAN 59-411 but in other documents such as STANAG 1008, for ship supplies.
A video demonstration of harmonics measurement can be found below. Notice that in order to perform a harmonics test, a clean power source with defined impedance is needed.
Flicker is defined as the “impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time”. Varying loads on a power supply network can result in voltage changes at common points of connection which are of sufficient amplitude to induce flicker in connected luminaires. The affected luminaires may have nothing to do with the load equipment that is causing the variations. Therefore, IEC 61000-3-3 – which applies to the same wide range of apparatus as does IEC 61000-3-2 – regulates the degree to which a given item of equipment can cause perceptible flicker. It does so by limiting the voltage variations that are generated across a reference impedance, and it places limits on three factors:
the relative voltage change;
the short-term flicker value Pst;
the long-term flicker value Plt.
These limits do not apply to emergency switching or interruptions, and the Pst and Plt limits do not apply to manual switching. The voltage change limits do apply to manual switching, however, and this effectively places a limit on allowable switch-on inrush current for any apparatus. Amendment 1: 2001 to the standard makes it slightly clearer that inrush current limitation is intended, and does in fact change the limits in this context, but this aspect of the standard is often not realised by manufacturers.
Equipment that typically will produce flicker includes any device which switches varying loads during its operating cycle; many household appliances fall into this category, and particular offenders are products which have heaters whose temperatures are controlled by burst firing, i.e. power is provided to the heater for a few cycles of the mains supply at a time, and the on/off ratio of the bursts controls the temperature. A good example can be found below, the heater control method is half cycle control, full cycle control can be found in many other products.
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