CH. V SERIES-WOUND MOTORS 97 



speed curve for a motor with constant induction factor, 

 since the speed is infinite when the current is nothing. 



When the current is reversed, the sign of the torque 

 remains unchanged, hence the direction of rotation must 

 change at the origin. The reversed speed curve may be 

 plotted by taking the ordinates of the induction curve 

 and the heat-drop as before. The dynamo is now acting 

 as a generator, and sending a current into the line, but 

 unlike the dynamo with constant induction factor, the 

 change from motor to generator has been accompanied by 

 a change in the direction of rotation. Motion is also 

 reversed at the point l> as with a dynamo of constant in- 

 duction factor. The form of the speed curve is not 

 symmetrical about the axis of speed. An inspection of 

 the curve for reversed current shows that the speed first 

 decreases and then increases. 



In comparing the speed curve of a series-wound motor 

 with that of a motor of constant induction factor, we see 

 at once that the former gives much greater variations of 

 speed for small loads. The line ab in the diagram would 

 be the speed curve for a motor having a constant induc- 

 tion factor equal to that of the series-wound motor when 

 running at a speed given by the intersection of its speed 

 curve with the line ab. For loads greater than that 

 corresponding to this speed, the behaviour of the two 

 motors is not very different, but for smaller loads, the 

 speed of the series-wound motor exceeds that of the other, 

 the ratio of the speeds increasing to infinity when the 

 load is nothing. 



The energy curves for a series-wound dynamo will be 

 similar to those for a dynamo with constant induction 

 factor. The general form of the heat, line, and mechanical 



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