176 THE DIRECT-CURRENT MOTOR CH. VIII 



Hence for equal force factors the weight will increase 

 slightly with induction factor. 



In Fig. 42 the force factors of a complete set of four- 

 pole dynamos of the same type are plotted on a base of 

 weight. The numbers opposite each machine indicate 

 the rated power in kilowatts. The force factors are 

 plotted vertically in kilodynes. All these machines are 

 for 125 volts. 



Example 48. To find the lightest dynamo of this 

 type that would give a power of 40 k.w. with 125 volts 

 induced tension. The current is 320 amperes. The weight 

 of a machine to cany this current must be 1,250 pounds, 

 and a dynamo of this weight has to run at 960 r.p.m. and 

 must have an induction factor of 7*81 in order to give 

 the required induced tension at the maximum permissible 

 speed. This is the lightest dynamo of this type that will 

 give the specified power at the given induced tension. 

 The force factor of this machine will be 2'5 kilodynes. 

 We might have taken an induction factor greater than 

 7'81. Suppose we took If =10. The current would be 

 unaltered, but the speed would be reduced to 750 r.p.m. 

 and the weight would be increased to 6,250 pounds. 



We have already seen that a motor must have a larger 

 force factor at the moment of starting than when giving 

 the maximum power for which it is designed, and that 

 when the speed at the maximum power is fixed, the 

 increased force factor at the start must be obtained by 

 increasing the current if M is constant. Hence a motor will 

 be heavier in proportion as the force factor at the start 

 is greater. 



Example 49. The motor in Example 18 has a force 

 factor of 2 '5 kilodynes when developing its maximum 



