|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|6. Electrical Field|
For the different fields of application, a great variety of designs of capacitors is available. In heavy current engineering, primarily paper capacitors in a metal cup are used (Fig. 6.11.). Two metal foils and two paper strips are placed one upon the other in the way shown in the illustration and then properly rolled up. The two metal foils are attached to connections and the roll is mounted in a metal cup. Such a paper capacitor is also know as roll-type capacitor. The MP-capacitor (metal-paper capacitor) is designed in a similar manner; in this case, the foil is replace by a coat of metal which is produced by vapour deposition. These capacitors are smaller than paper capacitors of the same capacity.
Fig. 6.11. Encased capacitor
Fig. 6.11.a Design of the roll
1 - Connections of the foils
2 - Metal foil
3 - Paper strips
Fig. 6.11.b External view of the capacitor
1 - Metal enclosure
2 - Connections
Another advantage of MP-capacitors is the fact that, after a breakdown or puncture of the dielectric, the extremely thin metal coat in the close vicinity of the puncture evaporates and, thus, removing the short-circuit - that is why MP-capacitors are called self-healing capacitors.
For the practical use, the value of the capacity printed on the device and the rated voltage up to which the capacitor may be used have to be observed.
An arrangement consisting of two plates with a dielectric between them is called capacitor. The capacity of a capacitor is a measure of the charge which the capacitor is capable of storing at a certain voltage, and it is also dependent on the design. The total capacity in series connection and in parallel connection of capacitors is expressed by the equations 6.5. and 6.6.
When a capacitor is connected to a direct voltage, a current will only flow during charging and discharging. There are not sudden voltage changes in a capacitor. A charged capacitor can retain is charge for a longer period of time (danger!) and it should never be discharged via a short circuit. The capacitor may be used as an energy store.
In heavy current engineering, the paper capacitor arranged in a metal cup is sued. For use, pay particular attention to the value of the capacity and the rated voltage.
Questions and problems:
1. Describe the basic design of a capacitor and, in particular, the design of a paper capacitor!
2. Which property of the capacitor is described by the capacity?
3. Explain the course taken by current and voltage during charging and discharging of the capacitor!
4. Why should capacitors not be discharged via a short circuit?
5. What should be strictly observed when working at installations incorporating capacitors?