ALTERNATORS. 247 



ing to the full-load current for which the construction is to 

 be carried out. Draw the triangle a b c so that the point 

 b coincides with 0, the centre of the circle, c a drawn 

 vertically is the short-circuit idle voltage, and c is the total 

 voltage with the same excitation on open circuit; a is the 

 armature ohmic drop and eddy current loss. 



With c as centre, describe a circle with a radius equal to 

 E. Then for any value of the angle of lag in the circuit <f> = 

 B E the terminal voltage of the machine will be given by 

 F and the voltage drop by F E. 



When <t> is so great that the line E coincides with c 

 produced, the maximum drop of voltage will occur. When 

 < is negative, i.e., when there is a leading current in the 

 circuit, the terminal voltage may be greater than on open 

 circuit. Thus, when the current leads by the angle </ in 

 Fig. 117, the terminal voltage is just equal to the voltage 

 on open circuit. When the lead is still greater, as shown at 

 <t>", the voltage is greater than at open circuit by the amount 

 represented by E" F". 



It must be remembered that the triangle a c is drawn in 

 Fig. 117 for one particular value of the current, and that the 

 diagram gives the loss or increase of voltage for this load. In 

 order to determine the voltage corresponding to any other 

 load, its approximate value may be obtained by altering the 

 scale of the triangle a c in the same proportion as the load. 



Relation Between Speed and Voltage of an Alternator. 



When supplied with constant excitation and giving no armature 

 current, the voltage of an alternator varies in direct propor- 

 tion to the speed at which it is driven. This is evident 

 from the formula already given for the voltage of an 

 alternator (see page 194), in which it is seen that the voltage 

 is directly proportional to the speed. A curve comparing 

 terminal voltage and speed of an alternator on no load will 

 consequently be a straight line passing through zero, since 

 the voltage is zero when the speed is zero. A single observa- 

 tion at any speed would consequently be sufficient to enable 

 the curve to be drawn, and the voltage at any other speed 

 can be at once deduced. 



Two other cases, in which the relation between speed and 

 volts is not quite so simple, are considered in the following 

 experiment, viz. : the case where the alternator supplies a 

 constant current, and the case of a short-circuited alternator. 



