The capacitor, similar to the rechargeable batteries, has the ability to store the energy in the form of an electrical charge producing a potential difference across its plates. The basic capacitor consists of two or more metal plates separated by various insulating material such as mica, waxed paper, plastic film or some form of a liquid gel as used in electrolytic capacitors or air could be the other option. The insulating material layer between a capacitor plates is known as dielectric.
Inductor, capacitor and resistors i.e. Trimurti’s of electrical and electronics engineering are not given equal focus in research and academics. As a result, most of the application finds use of resistor or inductor as its base. Capacitor is most neglected of all three. Most of the working engineers are not aware of desirable values of time constants or capacitance or internal resistance of capacitor in specific application. Role of capacitor in AC and DC circuits is not clear to them. Concept of pulse current and pulse power is always confused. In academics, one can only find a chapter on electrostatics covering capacitor partly at first year level. In-fact, electrostatics is not explored by our scientist and engineers as much as electromagnetic. Capacitor primarily uses dielectric materials and it is neither covered in material science nor is covered in polymer engineering. There are very few universities/ institutes across the globe which are offering special course on capacitor technology. If mathematics is important in engineering then so is the case with material science. Material science subject needs to be introduced in every year at UG level. Instead of keeping material science as generalised subject, it needs to keep as material science for specific component/ technology.
In DC system, capacitor is an energy storage device. Same is the case with battery and fuel cells. On energy and power scale they are different from each other. There are many applications where one can see or read that battery and fuel cells are giving stored electricity for some system for its operation. However, capacitor is not giving energy for doing some useful work which can be seen. Battery and fuel cell energy can be directly used as electrical energy and can be converted in other form like mechanical, sound, heat etc. In case of capacitor, it does not happen that way. Then the question arises, how do one say that it is energy storage device and what is then it’s role. Basically, capacitor storage energy for electronic circuits as a functional requirement of same. Amount of energy it stores is very small and it can be used only by electronic circuits. In AC system, energy storage and release is at very high rate and usually at high voltage. In such applications, phase shifting and frequency dependency plays a key role. Capacitors used in AC and DC systems have different structure, materials and manufacturing method.
Materials & Manufacturing
The small grain size of nanomaterials has significant effect due to drastic change in the physical properties of the materials used for components. The dielectric property for nanomaterials is very attractive for its applications in capacitor, sensors and memory devices. The dielectric property of nanomaterials exhibits unusual property which will give rise to develop new materials for capacitors. The frequency behaviour of dielectric materials gives the valuable information about the conduction phenomena of nanostructures materials. The dielectric properties in nanomaterials have some odd aspects especially for metal nanoparticles in which the cappants are insulating and the core is metallic. However, when they form complex system of nanoparticles, then their insulating and conducing properties gets reversed due to reduced particle size of the order of 1 nm, due to quantum size effect. Dielectric strength of a material is the ability to the material to act as an insulator. Dielectric strength is a measure of the electrical strength of a material as an insulator. The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. Table 1 shows conventional dielectric materials with their dielectric voltage and dielectric constant. There is need to introduce nanomaterials as insulating materials in academics covering all basic properties of new insulating materials. Insulating material properties need to be co-related to technical specifications of capacitors. Fundamental research in materials for capacitor can lead to improvement in parameters (increase in operating voltage, reduction in ESR etc.) of capacitor and will increase its application range.
Capacitors are not manufactured (in true sense) in India but they are just assembled from imported components. Manufacturing includes preparing raw material and processing it to get complete product. Making impurity free materials is a persistent problem which Indian industries are facing. Most of the main parts of capacitors are imported in India. Process of assembling capacitors has not changed over decades. New and advanced methods need to be adopted. Use of IoT in industry 4.0 environment is likely to increase quality of capacitor and it will reduce the price. Use of IoT in capacitor manufacturing and applications needs sensing lot of data from capacitor. Capacitance value, internal resistance, capacitor voltage, capacitor charging and discharging currents, number of charge/ discharge cycles, rusting, dimensional changes, temperature needs to be sensed for better manufacturing adopting predictive maintenance of this device. Over time, the performance of unmaintained capacitors can deteriorate, reducing power system’s power factor, leading to power factor loss. In DC applications, if capacitor is not maintained properly, it can lead to malfunctioning of the system and can result in huge financial loss. Adoption of flexible manufacturing concept can increase profit and range of the capacitor manufacturers. Many aspects related to materials and manufacturing are missing in engineering syllabus. New approach of research in this area need to be adopted as fast as possible.
Capacitors & Applications
Figure 1 shows the classification of capacitor. All capacitors have their own capacitance, internal resistance values and suitability for particular application. Fundamental of each of these capacitors need to be covered at UG level in depth.
Capacitor is energy storage device which can store energy in the form of electrical charges and release it as and when required by the circuit. It is extensively used in electronic circuits to perform variety of tasks, such as smoothing, filtering, bypassing etc as shown in Fig 2. One type of capacitor is never suitable for all applications. Ceramic capacitors are generally better than other types and therefore are used in many applications. Some of the typical capacitor applications in electrical and electronic systems are:
DC blocking capacitor: In this application capacitor blocks the passage of DC current (after completely charged) and yet allows the AC to pass at certain portion of a circuit.
Filter Capacitor: Capacitors are the most important component of filters. There are several types of filters that are used in electronic circuits. Since the reactance of the capacitor is inversely proportional to the frequency, therefore it can be used to increase or decrease the impedance of the circuit at certain frequencies and therefore does the function of signal filtering.
Capacitor as power source: Capacitors used as a charging unit and energy stored is used for ignition and triggering.
Pass capacitor: The reactance of capacitor is frequency dependant and hence in certain application it is used in parallel with other components to bypass it at a specific frequency.
Coupling capacitor: In electronics circuit it is common to use capacitor to pass signal from one stage to other.
Decoupling capacitor: In high speed electronic logic, switching causes sinking of current. It results in disturbance in the logic values due of change in voltage level. Decoupling capacitor is connected close to the output of IC and provide the needed extra current and thus minimises the disturbances to the logic signal.
Snubber capacitor: In relays and power electronic switching devices high inductance loads could induce voltage in the contacts and may damage the device. Snubber capacitor is used to limit the high voltage transient in such circuits.
There are other applications such as signal processing and tuned circuits. Table 2 shows capacitor types and their applications. Academics need to cover capacitor parameters such as dielectric constant, capacitance, time constant, ESR, EPR and then which values are suitable in various applications. Scaling these applications in AC and DC systems against these parameters can give good insight to engineers and researchers.
SDM & Stray Capacitance
Dielectric constant values of various materials need to be included in academics. Today, low and high value dielectric constant material capacitors use is not known properly. In some applications capacitor with very high dielectric constant is desirable. Evidence is provided that a class of materials with dielectric constants greater than 100K, herein called Super Dielectric Materials (SDM), can be generated readily from common, inexpensive materials. Specifically, it is demonstrated that high surface area alumina powders, loaded to the incipient wetness point with a solution of boric acid dissolved in water offers remarkable increase over the best dielectric constants previously measured. It is postulated that any porous, electrically insulating material (e.g. high surface area powders of silica or titania), filled with a liquid containing a high concentration of ionic species will potentially be an SDM. Lot of research work is being done in the area of SMD and it needs to be converted in to the product. Capacitors created with the first generated SDM dielectrics (alumina with boric acid solution), herein called New Paradigm Super (NPS) capacitors display typical electrostatic capacitive behaviour, such as increasing capacitance with decreasing thickness, and can be cycled. However, operating voltage is limited to about 0.8 V. Potentially NPS capacitor stacks can surpass supercapacitors in volumetric energy density.
Concept of wanted capacitance and unwanted capacitance is completely missing from academics. Stray capacitance is unavoidable capacitance existing between live parts and equipment case or surroundings, or between live parts of different polarities. Capacitance always exists between two conducting paths or components, simply by virtue of their being separated by some kind of electrical insulating materials. For example, capacitance exists between two wires or between wire and surroundings (including earth). Stray capacitance is neither designed nor desirable. Its frequency dependence makes it more complex to understand. Stray capacitance affects electrical machines such as transformer, semiconductor devices like BJT/ CMOS/OPAMP, Sensors like thin film piezo-electric and embedded system. There are many systems which experience bad effect of stray capacitance and hence it should be properly and collectively included in UG syllabus.
Capacitor is one of the most extensively used components in all electrical and electronic circuits. Current engineering syllabus covers only fundamental aspect of it. Many important issues of it are covered in distributed manner in various subjects. Material properties, type of capacitor it forms and use of it, needs to be co-related to give better understanding of this device. Some aspects such as advances in materials and manufacturing need of material parameters co-relation to applications, development of SDM and importance of stray capacitance is presented briefly in this article. There are many techno commercial aspects which can be included in engineering syllabus. Indian capacitor industries need to put money in research and development. Due to computer sector lucrative jobs, there is tremendous shortage of skilled manpower at production and research work level. In this scenario, industries should collaborate with public and private sector research labs/ institutes. Industry sponsored materials and equipment with research lab sponsored manpower model should be adopted. Use of locally available material can be key for addressing commercial issues.
Pursuing M. Tech in
Electrical Power System
Deemed University, Pune
Prof R M Holmukhe
College of Engineering,
Dr P B Karandikar
Army Institute of