Dielectric loss alternating currents

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

The most important dielectric properties are the dielectric constant, e, and the dielectric loss factor, tan 8. These properties are of interest for alternating currents indicates the polarizability in an electric field, and, therefore, it governs the magnitude of the alternating current transmitted through the material when used in a capacitor. For most polymers e is between 2 and 5, but it may reach values up to 10 for filled systems. 

Of more importance is the loss factor, tan 8, denoting the fraction of the transmitted alternating current lost by dissipation in the material. Here large differences occur between polymers, as indicated in Figure 8.10. It appears that polymers with the highest specific resistance also show the lowest dielectric losses. It should be remarked, that the values given are very schematical the losses are strongly dependent on frequency and temperature. 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

The chemical reaction and the electrical characteristics of silent electric discharge have been correlated by studies on an ozonizer. Usually, the space in silent discharge is terminated by dielectrics which act as the stabilizing resistance with a little loss and prevent concentration of the discharge. An alternating current must be used. 

The measurements of the dielectric characteristics of composites were carried out at a frequency of lO Hz using an alternating current bridge P-5058. The specimens prepared had a disc-like shape with a diameter equal to the diameter of the electrode (15-18 mm). The thickness of the specimen was about 1 mm. The temperature dependencies of the dielectric permeability (e) and of the tangent of the dielectric losses (tan 8) were found at the heating rate 3-4 K/min. 

The power loss depends on the phase difference between the alternating current produced by an applied alternating voltage. When the material behaves as a perfect dielectric, the phase difference between the alternating potential and the amplitude of the current is 90° and the power loss is zero. If current and voltage are in phase, then all of the electrical energy is converted into heat and the power output is zero. The ratio of power loss Nv to power output Nb is called the dielectric dissipation factor, tan 6 ... 

The loss is converted to heat (dissipation). For low values of 6, the losses are low so that in the limit 6 = 0, there is no loss (this is the ideal capacitor). Low values of tan 6 may be approximated as tan 6 sin 6 cos 6, where cos 6 is defined as the power factor, 6 represents the angle between the direction of the voltage and current in an alternating current. Then Power = Voltage X Current x Power Factor. The loss factor approximately equals the product of the power factor and the dielectric constant, all values taken at a frequency of 60 Hertz. In nonpolar polymers (Teflon, polyolefins, polystyrene)... 

Chemists often reason in terms of dielectric permittivity (e) or infrared spectra (a) while electrical engineers and physicists consider tangent loss (tg = e"/s ) and alternative current conductivity (free charges (where the complex impedance formalism is well-suited) and bound charges (where the description of the permittivity s = e (co) — ie"(co) is more suitable) can be distinguished. Such a distinction is not straightforward in superionic conductors, however, in which some conducting species are in a quasi-liquid state and in particular, for proton conductors. In the latter, the proton (attached... 

Tan of dielectric loss angle n. In an ideal condenser of geometric capacitance Q, in which the polarization is instantaneous, the charging current Eros Cq is 90° out of phase with the alternating potential. In a condenser in which absorptive polarization occurs, the current also has component E(os"Cq in phase with the potential and determined by Ohm s law. This ohmic or loss current, which measures the absorption, is due to the dissipation of part of the energy of the field as heat. In vector notation, the total current is the sum of the charging current and the loss current. The angle d between the vector for the amplitude of the total current and that... 

The alternating current field also causes additional current flow due to moving ions created by the frictional heating. This leads to poorer resistance to dielectric breakdown under A.C. fields. The effect of the energy absorbtion from the A.C. fields is on both the material and the electrical circuits. In the case of signal carrying circuits, the losses to the insulation can cause destructive loss of signal as well as increased noise due to the effect of heat on the materials. 

As all electrical insulating materials, EP-resins exhibit low electrical conductivity which results in current loss in operation. Thermal losses from current load in an encapsulated conductor or device and the dielectric dissipation loss generated in the EP-resin-molding material result in temperature increases that lower electrical con-ductibility and dielectric strength [885], because in principle, all mechanisms participating in DC current transport and displacement processes also cause dielectric losses in the alternating field. The dielectric behavior of insulating materials is described by the temperature-, frequency-, and stress-dependent (i.e. field intensity dependent) loss factor tan 6 and the dielectric constant s. 

When applying an alternating electric field to a polymer placed between two electrodes, the response is generally attenuated and the output current is out of phase compared with the input voltage. This response stems from the polymer s capacitive component and its conductive or loss component, as represented by a complex dielectric permittivity measured frequencies f, and temperatures T ... 

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