THE CONTINUOUS CURRENT MOTOR 165 



where A: is a constant depending upon the reluctance of 

 the magnetic circuit and the number of turns in the series 

 winding. 



If the current is very small the denominator is corre- 

 spondingly small and as a result the speed becomes very 

 high. It is evident that the counter e.m.f. of a motor must 

 always be nearly as large as the impressed voltage and, as the 

 field flux of the series motor is very small at a light load, 

 excessive speed is required to generate the necessary amount 

 of counter e.m.f. 



Calculation of No-load Speed. The no-load speed of a 

 compound or a shunt motor is easily calculated. At no 

 load there is practically no IR a drop so that the counter 

 e.m.f. must be equal to the impressed e.m.f. The no-load 

 speed may therefore be found by calculating what speed 

 the motor would have to run (as a generator) to generate 

 in its armature an e.m.f. just equal to that of the supply 

 line. 



34. Effect of Armature Reaction on Speed. So far 

 nothing has been said regarding the effect of the armature 

 m.m.f. upon the speed of the motor. When discussing 

 armature reaction in Chapter III it was mentioned that 

 armature reaction occurred to the same extent whether the 

 machine is operated as a motor or a generator. But in a 

 generator the armature current flows in the direction of 

 induced e.m.f. and in the motor the armature current flows 

 against this induced e.m.f. (called counter e.m.f.). So that 

 the armature m.m.f. of the motor and generator are in 

 opposite directions. In the generator the main field flux 

 is twisted in the same direction as that in which the machine 

 is rotating; in the motor the armature so reacts on the 

 main field that it is twisted against the direction of rotation. 



Direction of Field Distortion. This is shown in Fig. 102; 

 which shows the direction of the twisted field for generator 

 action in dotted lines and that for motor action in full 

 lines. Hence, if the brushes are to short circuit that coil in 



