THE OPERATION OF THE DYNAMO AS A MOTOR. 103 



periment. A shunt motor is supplied with current from constant 

 voltage supply mains and runs at a certain speed, as indicated by 

 a tachometer. A slab of iron is now laid across the pole pieces 

 so that a considerable portion of the flux <I> passes through the 

 slab, and the armature flux is consequently reduced. The speed 

 of the motor immediately increases as indicated by the tachometer. 

 This rise in speed is due to the fact that the reduction of <l> re- 

 duces the counter electromotive force, QZ'n, of the motor. A 

 sudden increase of current through the armature results, accord- 

 ing to equation (23*:). This increased current produces more 

 torque than is required to carry the load on the motor, so that 

 the motor speeds up until the increase of its counter electromotive 

 force reduces the current to the value corresponding to the re- 

 quired torque. 



An interesting and important fact in connection with a shunt 

 motor is that such a motor runs at a lower speed when first started 

 than after it has been running for some time. This is due to the 

 fact that the rise of temperature of its field coils increases their 

 resistance, decreases the field current, and decreases 3>. This 

 rise of speed often amounts to as much as 5 per cent, in com- 

 mercial machines. 



Dependence of speed upon brush lead. It was pointed out in 

 Art. 5 1 that the electromotive force between the brushes of a 

 generator running at a given speed with given field excitation, is 

 reduced in value by shifting the brushes from the neutral axis, 

 and that the electromotive force is a maximum when the brush 

 lead is zero. In the case of a shunt motor supplied from con- 

 stant voltage mains, the speed is a minimum when the brushes 

 are in the neutral axis, and the effect of shifting the brushes either 

 forwards or backwards, is to increase the speed. This effect is 

 particularly noticeable at zero load. When the motor is heavily 

 loaded the matter is greatly complicated by the magnetizing or 

 demagnetizing action of the armature current. 



The explanation of this variation of speed of a shunt motor 

 with brush lead is as follows : When the brushes are shifted from 



