236 THE DIREfT-rTKKKNT .MOTOR CJI. XI 



axis, but may be a straight Hue, if no part of the iron 

 in the magnetic circuit is magnetised over the bend of the 

 magnetisation curve. Now in this case the induction curve 

 must pass through the point ft, for the motor must have 

 3f=35'5 for 15 amperes; hence the greatest possible value 

 of .}[ for 45 amperes is found by making the induction 

 curve a straight line passing through the point 6. 



Our calculations hitherto have shown us that the motor 

 must have an induction factor of 35*5 at 15 amperes, and 

 that the maximum current at starting must be 45 amperes. 

 We have not, however, determined what must be the value 

 of the induction factor for 45 amperes. All we know is 

 that if the induction factor is constant for all currents and 

 equal to 35'5, we shall cover the given distance in the 

 given time. We shall see later on, that the greater we 

 can make the induction factor for 45 amperes the greater 

 will be the economy of starting. 



It is clear that there are an infinite number of possible 

 induction curves all passing through the point />, but having 

 different values of M for 45 amperes, all less than 106. 

 Any one of these curves would comply with the specification 

 as to time and distance, but none of them would be so good 

 as the line cibc from the point of view of economy. 



We have seen in a previous chapter, that when the 

 maximum current to be carried by a motor is fixed, the 

 weight W increases nearly with M , the maximum induction 

 factor. We shall assume that W=kM : where ft is some 

 constant. Hence of all possible induction curves that 

 might be chosen, abc will give the best results, but will 

 involve the greatest weight. 



In practice the weight of the motor will be limited ; 

 let us suppose that the limit is such that the greatest 



