DIRECT-CURRENT MACHINERY 219 



When operated at twice full speed produced by field weaken- 

 ing, the rating may be taken as 10 h. p., 110 volts, 80 amperes 

 but the temperature rise will be less than before, because the 

 armature copper loss is the same, the field copper loss is reduced 

 to about one quarter of its normal value and the iron and friction 

 losses are increased but the ventilation is very much improved. 

 The rated output of the machine for normal temperature rise 

 might be increased but due to the higher speed and consequent 

 reduced time of commutation sparking would occur. 



133. Storage Batteries. A storage battery is an apparatus 

 in which electrical energy can be stored to be used at some later 

 time. 



Batteries are made up of a number of cells connected in series 

 multiple according to the voltage and current required. 



Each cell is composed of two plates or electrodes of suitable 

 materials immersed in an electrolyte. The most commonly used 

 storage battery has a positive plate of lead peroxide Pb0 2 and a 

 negative plate of sponge lead Pb immersed in dilute sulphuric acid 

 H 2 S0 4 . 



When the battery is discharging the electrolyte combines with 

 the active materials of the electrodes and when it is being charged 

 the electrodes are reduced to their original condition and the 

 materials taken from the electrolyte are returned to it. 



The main chemical changes taking place are represented by the 

 following formula: 



charge 



Pb0 2 + Pb + 2 H 2 SO 4 = 2 PbS0 4 + 2 H 2 O. . (224) 



discharge 



Capacity. The unit of capacity of a storage cell is the ampere 

 hour and it is generally based on the eight-hour discharge rate. 

 An 800-ampere-hour battery will give a continuous discharge 

 of 100 amperes for eight hours. If, however, the rate of discharge 

 is increased the ampere-hour capacity of the battery decreases. 

 At a six-hour discharge rate the capacity is only about 95 per cent, 

 at a four discharge rate it is about 80 per cent and at a one-hour 

 rate it is only 50 per cent of its eight-hour rating. Thus the bat- 

 tery mentioned above would give a continuous discharge of 400 

 amperes for only one hour. 



