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close this bookIntroduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)
View the document(introduction...)
View the documentPreface
View the document1. Importance of Electrical Engineering
close this folder2. Fundamental Quantities of Electrical Engineering
View the document2.1. Current
View the document2.2. Voltage
View the document2.3. Resistance and Conductance
close this folder3. Electric Circuits
View the document3.1. Basic Circuit
View the document3.2. Ohm’s Law
close this folder3.3. Branched and Unbranched Circuits
View the document3.3.1. Branched Circuits
View the document3.3.2. Unbranched Circuits
View the document3.3.3. Meshed Circuits
close this folder4. Electrical Energy
View the document4.1. Energy and Power
View the document4.2. Efficiency
View the document4.3. Conversion of Electrical Energy into Heat
View the document4.4. Conversion of Electrical Energy into Mechanical Energy
close this folder4.5. Conversion of Electrical Energy into Light
View the document4.5.1. Fundamentals of Illumination Engineering
View the document4.5.2. Light Sources
View the document4.5.3. Illuminating Engineering
View the document4.6. Conversion of Electrical Energy into Chemical Energy and Chemical Energy into Electrical Energy
close this folder5. Magnetic Field
View the document5.1. Magnetic Phenomena
View the document5.2. Force Actions in a Magnetic Field
close this folder5.3. Electromagnetic Induction
View the document5.3.1. The General Law of Induction
View the document5.3.2. Utilisation of the Phenomena of Induction
View the document5.3.3. Inductance
close this folder6. Electrical Field
View the document6.1. Electrical Phenomena in Non-conductors
close this folder6.2. Capacity
View the document6.2.1. Capacity and Capacitor
View the document6.2.2. Behaviour of a Capacitor in a Direct Current Circuit
View the document6.2.3. Types of Capacitors
close this folder7. Alternating Current
View the document7.1. Importance and Advantages of Alternating Current
View the document7.2. Characteristics of Alternating Current
View the document7.3. Resistances in an Alternating Current Circuit
View the document7.4. Power of Alternating Current
close this folder8. Three-phase Current
View the document8.1. Generation of Three-phase Current
View the document8.2. The Rotating Field
View the document8.3. Interlinking of the Three-phase Current
View the document8.4. Power of Three-phase Current
close this folder9. Protective Measures in Electrical Installations
View the document9.1. Danger to Man by Electric Shock
close this folder9.2. Measures for the Protection of Man from Electric Shock
View the document9.2.1. Protective Insulation
View the document9.2.2. Extra-low Protective Voltage
View the document9.2.3. Protective Isolation
View the document9.2.4. Protective Wire System
View the document9.2.5. Protective Earthing
View the document9.2.6. Connection to the Neutral
View the document9.2.7. Fault-current Protection
View the document9.3. Checking the Protective Measures

4.6. Conversion of Electrical Energy into Chemical Energy and Chemical Energy into Electrical Energy

For many electrically operated devices it is desired to operate them independently of a central power supply system. For this purpose, small and light-weight electrical energy sources (batteries) are required. Batteries which can be recharged after discharge are called, primary elements. Batteries which can be recharged, several times after discharge are called secondary elements. Below, primary elements are discussed first.

The Italian physicist Galvani (1757 - 1798) was the first to find that a primary electromotive force is produced between two different metals or between metal and carbon in an electrolyte (aqueous solution of an acid, a lye or a salt). The magnitude of the voltage and the polarity are dependent on the metals used. The Italian physicist Volta (1745 - 1827) arranged the individual metals in an electromotive series, named after him, in such a way that magnitude and polarity of the primary electromotive force can be determined. The greater the distance between two elements in the electromotive series, the greater the primary electromotive force that will be brought about.

Of particular importance to engineering is the carbon-zinc element with a primary electromotive force of 1.5 V with carbon forming the positive pole and zinc the negative pole. It is offered in different shapes and sizes and serves for the supply of portable wireless sets, torch lamps, pocket calculators, and many other devices. Fig. 4.24. shows the most frequently used designs; their technical data are given in Table 4.7. The inner structure of such an element (design of the Leclanchlement) is shown in Fig. 4.25.

Table 4.7. Technical Data of Some Carbon-zinc Cells

Specification

Designation


R6

R14

6F22

R20

3R12

diameter in mm

14

26


34


height in mm

50.5

50


61.5


length in mm



26.5


62

width in mm



17.5


22

height in mm



48.5


67

voltage in V

1.5

1.5

9.0

1.5

4.5

medium capacity in Ah

1.0

2.6

0.38

5.8

1.6


Fig. 4.24. Frequently used primary elements


Fig. 4.25. Design of the carbon-zinc element

1 - Metal cap
2 - Sealing compound
3 - Carbon
4 - Zinc cup
5 - Electrolyte (thickened, ammonium chloride solution)
6 - Linen bag with pyrolusity and graphite filling

When current is drawn from the element, the zinc sheath is disintegrated. The chemical energy liberated in this way is the equivalent of the produced electrical energy. It is disadvantageous that a disintegration of the zinc also takes place during storage although no current is drawn. The service life of such elements therefore is only about 6 months. Another disadvantage is the risk of leakage of the thickened ammonium chloride solution when the disintegration of the zinc sheath begins. As ammonium chloride solution is chemically aggressive, devices and equipment in which such elements are incorporated can be damaged. For sensitive device, the expensive tightly enclosed elements are made. In this case, a carefully sealed steel-sheet coat is the enclosure of the element proper; in this way, leakage is effectively prevented.

An improved design is the alkali-manganese cell which can deliver a considerably higher quantity of energy with the same dimensions as the above element. Because of the higher price, which is due to manufacture, it was not yet in a position to supersede the above described Leclanchlement. For practical use it is advisable to use in a device always batteries of the same manufacturer and of the same type designation and to replace the batteries in the device when they are in the same state of discharge. At the end of discharge, the voltage drops to about 0.9 V per cell. Recharging is not possible. Consumed batteries must be removed immediately from the device.

In contrast to primary elements, secondary elements show the advantage that they can be reacharged after discharge. It is disadvantage that the quantity of energy that can be stored is smaller in an element of the same size as the primary element. In practice, two designs of the secondary element have gained particular importance, namely, the lead accumulator and the nickel-iron or nickel-cadmium accumulator.

In a lead accumulator, there are lead plates as electrodes and sulphuric acid as electrolyte. In accordance with the various applications, the lead plates - consisting of a frame and pressed-in lead powder - are made in different shapes. Because of this plate construction, lead accumulators are sensitive to vibrations. Lead powder dissolved out of the plates is deposited under the plates as lead sludge in the course of time. When the accumulator is overloaded, a formation of large amounts of lead sludge will occur. When the lead sludge can touch the lower edge of the lead plates, self-discharge will occur and the accumulator become useless.

In the charging process, lead oxide is formed at the positive plate and lead at the negative plate. Due to discharge, the two plates are converted into lead sulphate. This shows that the sulphuric acid takes directly part in the process of conversion of chemical energy into electrical energy. Since during the process of charging the concentration of the sulpheric acid increases, the state of charge can be determined by measuring the acid density by means of a hydrometer. There are specifically made hydrometers where the state of charge can be read directly.

The voltage of each cell shows during charging and discharging a typical behaviour (Fig. 4.26.). When charging, the voltage will at first rapidly rise from 2.0 V to about 2.15 V, and another rapid increase in voltage will only take place a short time before the end of the charging process, namely, to 2.7 V per cell. If, after this voltage rise, the process of charging is continued, a further chemical change cannot take place in the plates. The supplied electrical energy will cause a decomposition of the electrolyte and, consequently, an intense evolution of gas.


Fig. 4.26. - Course of charging and discharging voltage at the lead accumulator

1 - Charging
2 - Discharging

The gas produced is highly explosive (oxyhydrogen). Therefore, in accumulatorrooms any use of open fire or smoking is strictly forbidden. Overcharging should be avoided in any case. The rise of the cell voltage to 2.7 V is used in automatic charging equipment for switching off the charging process. In discharging, the mean cell voltage first drops to about 1.95 V and then it again rapidly drops near the end of discharging. A discharging voltage should not fall short of 1.8 V per cell. Depending on the magnitude of the discharging current, this voltage will be reached after different periods of discharging.

The product of discharging time times discharging current is termed as capacity of the accumulator in ampere-hours. A certain discharging time is always used. The capacity can be used for classifying accumulators and it is stated in Ah (ampere-hours).

The internal resistance of lead accumulators is low. A high current can, therefore, be draw for a short time (e.g. starter battery in a motor-car); it should be noted, however, that any short circuit must be avoided in any case because of the extremely high short-circuit current.

The lead accumulator is used as starter battery in motor-cars, for emergency current supply in plants which have to be serviceable even when the mains voltage fails (e.g. telephone exchanges, emergence lighting) and, in special designs, it is used for power supply to portable electronic devices.

In maintaining accumulators care should be taken to see to it that the gas escape valves are clean so that the gas evolved in charging and discharging can escape. The poles must be kept clean and protected by means of a special grease. The plates must always be covered by electrolyte; for topping up only distilled water (no sulphuric acid) has to be used. An excessive discharge must be avoided.

The plates of the nickel-iron accumulator consist of nickel hydroxide and iron hydroxide, aqueous solution of potassium hydroxide or caustic potash is used as electrolyte. In contrast to lead accumulators, the density of the electrolyte does not change in charging and discharging. A measurement of the state of charge by means of a hydrometer is not possible therefore.

The voltage behaviour during charging and discharging o£ the nickel-iron accumulator is shown in Fig. 4.27. Since no distinct voltage rise takes place at the end of the charging process, the state of charge cannot be determined, on the basis of the charging voltage. During discharge, the cell voltage first rapidly drops from 1.4 V to 1.25 V and only at the end of the possible discharge again rapidly drops to 1.1 V. The state of charge can therefore be conveniently derived from the discharging curve.

The mean cell voltage of the nickel-iron accumulator is for about 0.5 V lower than that of the lead accumulator while the internal resistance is higher. The nickel-iron accumulator requires less maintenance, it has a longer service life and a lower weight. For some applications, the higher internal resistance and the lower cell voltage are disturbing factors.


Fig. 4.27. Course of charging and discharging voltage at the nickel-iron accumulator

1 - Charging
2 - Discharging

The nickel-cadmium accumulator has gained great importance to electronic devices. It can be made gas-tight and thus be installed in pieces of equipment in any desired mounting position. The service life is stated to be about 5 years or about 5000 charging cycles. The charging specifications must be strictly observed. In contrast to nickel-cadmium accumulators, carbon-zinc accumulators have the three-fold to ten-fold energy content so that the accumulator of the former type must be charged at least three times during the same operating time and when of the same size as the latter. Nevertheless, the use of the gas-tight nickel-cadmium accumulator is of advantage to frequently employed electronic devices even after a short time of operation.

For the supply of power to mains-independent devices and equipment, primary elements and secondary elements can be employed. Primary elements are useless after discharge while secondary elements can be recharged after discharge. In primary elements, the carbon-zinc element is primarily used. It has a cell voltage of 1.5 V. Manufacture is in a great variety of shapes and sizes; the battery voltage ranges from 1.5 V to 9 V depending on the number of cells connected in series, in exceptional cases the battery voltage may be even higher.

Lead accumulators are mainly used for large current consumers. Careful maintenance will considerably increase the service life. Due to the low internal resistance, high currents can be drawn for short periods. When a long service life and limited maintenance are required, the nickel-iron accumulator is employed. In portable electronic devices, the gas-tight nickel-cadmium accumulator is used which is on offer with capacities from about 10 mAh (button cell) to 1 Ah and it does require practically no maintenance.

Questions and problems

1. What is the difference between primary elements and secondary elements?
2. What are the mean cell voltages of the described voltage sources?
3. How can the state of charge be measured at the lead accumulator?
4. Quote examples of application and designs of electrochemical sources of voltage!