Power Capacitors Information
Power capacitors are passive electronic components that provide a static source of reactive power in electrical distribution systems. They consist of two conducting plates separated by an insulating material called the dielectric. Multilayer dielectrics provide excellent temperature stability and frequency characteristics. Single layer dielectrics are also available. Capacitance, a measure of energy storage ability, is typically expressed as C = K A/D, where A is the area of the electrodes, D is their separation, and K is a function of the dielectric between the electrodes. Power capacitors are used in the aerospace and automotive industries, power factor correction and lighting circuits, power supplies, and motor starters. Some devices can handle one-phase voltages while others are designed for three-phase voltages.
Selecting Power Capacitors
Selecting power capacitors requires an analysis of dielectric materials. Aluminum electrolytic capacitors are polar devices that feature a high volumetric density but cannot withstand reverse voltages. Ceramic capacitors are made of resistive ceramic materials and provide bonded metal contacts. Examples include ceramic Z5U, a Class III ceramic dielectric, and ceramic X7R, a temperature-stable material that is suitable for bypassing and coupling applications. Glass, mica, oil, air, and paraffin paper are other commonly used dielectric materials. Polycarbonate, polypropylene, and polystyrene are suitable for applications such as fillers and timers. Tantalum, a metallic element, is used to make a variety of alloys. Niobium, a hard ceramic material characterized by high conductivity, has similar chemical properties to tantalum.
Performance specifications for power capacitors include capacitance range and capacitance tolerance, a percentage of total capacitance. Other considerations include working DC voltage (WVDC), working AC voltage (WVAC), rated current, power rating, dissipation factor, insulation resistance, and temperature coefficient. WVDC and WVAC are, respectively, the maximum DC and AC voltages that can be applied continuously at any temperature between a lower category temperature and the rated temperature. Rated current is the maximum current and power rating is the maximum power that can be applied continuously across these same temperature intervals. The dissipation factor (DF) is the ratio between the resistive and reactive parts of a capacitor’s impedance when a sinusoidal voltage at a specified frequency is applied. Insulation resistance is the ratio between an applied DC voltage and the resulting leakage current. The temperature coefficient is the change in capacitance measured over a range of temperatures.
Power capacitors can use axial, radial, flying, tab, screw, gull wing, or J-leads. Some devices bolt into place while others require or include mounting brackets. Pole-mounted capacitors are also available. Surface mount technology (SMT) adds power capacitors to a printed circuit board (PCB) by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Through hole technology (THT) mounts components on a PCB by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
Power capacitors are packaged in tape reels, trays or rails, shipping tubes or stick magazines, and in bulk packs. Tape reel assemblies include a carrier tape with embossed cavities for storing individual components. A cover tape seals the carrier tape in place and the composite tape is wound on a reel that can be loaded into industry-standard, pick-and-place board assembly equipment. Power capacitors with leads on four sides are often packed in trays or rails that are made of carbon-powder or fiber materials and molded into rectangular outlines that contain matrices of uniformly spaced pockets. Shipping tubes or stick magazines are containers made of rigid polyvinylchloride (PVC) and extruded in industry-standard sizes. Bulk packs are used to distribute components as individual parts.