Quality Factor & Dissipation Factor
With a perfect dielectric capacitor, current will lead the
applied voltage by 90º. When dealing with less than perfect dielectric
materials, current in a capacitor will lead the applied voltage by less than
90°. This can be visualized in the current-voltage phase relation of a
dielectric.
Current-Voltage Phase Relation of a dielectric
Here theta is the dielectric phase angle and 90° minus theta is delta, the loss
angle. The loss angle is usually expressed as the tangent of delta, or just tan
delta. The permittivity or relative dielectric constant of materials invariably
decreases with increasing frequency. This is due to the inability of the
polarizing charges to move with an increasing rate of alterations of the
electric field. This can be seen when tan delta is plotted with frequency for a
particular material.
Tan Delta vs. Frequency
The Polar Polymer curve is typical of polymers and liquids. The flat Non-polar polymer curve represents materials such as glasses, ceramics and inorganic
crystals.
The polar polymer curve indicates ionic migration conduction at low frequency.
The peak is associated with a molecular dipolar rotation and occurs when the
rotational mobility of molecule rotation can just keep up with the alterations
of the electric field, sort of a molecular resonance condition in the dielectric
material. The dissipation factor peak occurs below room temperature for viscous
liquids and lower still for more mobile liquids. Some mobile liquids may
crystallize before a dissipation peak occurs. The peak for polar polymers is
likely to occur, at power frequencies, at a temperature close to a softening
point. Some times dielectric dissipation factor and mechanical modulus can be
correlated at the same temperature for comparable frequencies. In any case the
peak should occur well away from the frequency that is being employed and should
remain stable through the usable lifetime of the dielectric material.
As indicated on the dissipation factor-frequency curve, the dissipation factor
peak will move to higher frequencies with higher ion mobility or lower
viscosity. Water, the universal solvent, is particularly effective in increasing
ionic concentrations and the mobility of ions in dielectric materials. Water
associates with impurity ions or the ionizable constituents within or on the
surface or interface of dielectric. This provides a local environment for
greater mobility and higher dissipation factor for the material than what would
be expected at a particular frequency. This effect is aside from the electrolyte
conduction that impure water may provide.
The ECAD System 2005 acquires data and calculates the
dissipation factor at 9 frequencies between 100Hz and 40kHz. This information is
useful for either troubleshooting or condition monitoring all of the ECAD
testable components of an electrical circuit whether it be the interconnecting
cable or an end device. The data is especially powerful if a baseline
dissipation factor has been established for the circuit. The trend of
dissipation factor is an indicator of how well a dielectric material is aging.
Without this data the effects of aging have to be accepted rather than managed
with some type of engineered controls.
