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ELECTRICAL MACHINERY 



in Fig. 100. At full load the speed for all three motors is 

 the same and hence, as they grve the same h.p. output at 

 full load, the full load torque must be the same for all three. 

 From these curves may easily be seen one point of superior- 

 ity of the series motor for railway loads, where the torque 

 necessary for starting a heavy train is excessive. It is 

 seen from the curves of Fig. 100 that with 200% full load 

 current (which might be safely put through the motor for 



FIG. 99. Current-torque Curve for Compound Motor. 



the short time necessary for starting) the series motor 

 gives much larger starting torque than either the shunt 

 or compound motor. 



33. Speed-load Curves. The principal factor which 

 determines the selection of one type of motor or another 

 for a certain class of work is the variation of its speed as 

 the load on the motor is varied. The shape of the curve 

 showing this variation of speed is very different for the 

 different types. 



