TRANSACTIONS OF SECTION A. 



545 



speed considerably greater than wlien it -was connected in the circuit, slowly 

 decreasing- in speed until it stopped. 



An examination showed that its fields had been inadvertently connected as a 

 shunt on the armature instead of in series with it. On joining it up as a series 

 motor it ran without exhibiting any imusual action. The cause of the unexpected 

 reversal under the above-described circumstances will be understood from the 

 following explanation : — 



As the motor was series-wound, but improperly joined up as a shunt, it will 

 be understood that a strong field was produced, the current through the field- 

 magnet coils being large, the armature being traversed by onlj' a small portion of 

 the total current. 



It will also be understood that on breaking the external circuit the momentum 

 of the armature converts a shunt motor into a generator, the motion of the arma- 

 ture producing a strong current, circulating in the armature in the reverse direction 

 to the previously received current, but in the fields in the same direction. In fact, 

 a shunt motor under such conditions acts like a series generator with its terminals 

 short-circuited. The intensity of this action in any motor depends to a great 

 extent upon the form of the generating ' characteristic ' of the machine, Properly 

 constructed shunt motors, having fields wound to a comparatively high resistance, 

 do not exhibit this action to any marked degree unless a suitable external circuit 

 is provided for them, while shunt motors which are incapable of acting as shunt 

 generators do not exhibit it at all. In the case in question, as the fields were of low 

 resistance, the generation of current was considerable, and instantly stopped the motor. 



A reference to the figures will show why, when it had stopped, the armature 

 started again in the reverse direction. 



Fig. 1 shows the connections and the course 

 of the current when the motor was supplied 

 with current from an external source, and when 

 it was running in its proper ' forward ' direction. 



Fig. 2 shows the direction of current under 

 the two conditions which successively occur 

 when the external circuit is broken. In the 

 first place the momentum of the armature 

 causes a current to be generated which, it will 

 be seen, is in the same direction in the field as 

 before, but in the opposite direction in the 

 armature. This at once stops the armature. 

 Then the momentary induced (' extra ') current 

 follows in the same direction at the moment 

 when motion has ceased, and causes the arma- 

 ture to start again in the reverse direction. 

 This extra current is produced in both the armature and the field-magnet coila, 

 but mainly in the latter, as in them the coefficient 

 of self-induction is very considerable. 



■ It may be added that the motor in question 

 has for several months been working very suc- 

 cessfuDy in New Zealand, where it is used to 

 supply power to the machinery at the Phoenix 

 Gold Mine. The current is transmitted to it 

 fi'om generators driven by water power at a con- 

 siderable distance from the mine, the conductor 

 consisting of three miles of copper wire, "IGo 

 inch in diameter, supported on telegraph poles. 

 The effective return is over 65 per cent. 



Eeferring to the proposed utilisation of the braking power possessed by motors 

 the author pointed out that sudden strains should be avoided, and that the power 

 should rather be wasted in a brake-block, where it did no harm, than in heatin"- 

 the motor itself. The exceptions were those cases where accidents could be pre'- 

 vented, or the generated current could be actually used. 



1886. N N 



Fig, 



