184 ELECTRICAL ENGINEERING 



overload limits of output set by armature heating can be ob- 

 tained. 



Interpoles are not so necessary on generators as on motors but 

 they are very useful in the case of a generator which supplies a 

 rapidly fluctuating load since the interpole m.m.f. follows exactly 

 the fluctuations of the armature m.m.f. and so prevents sparking. 

 Fig. 148 shows approximately the flux distribution in an interpole 

 generator or motor. 



112. Sparking. When a current i amperes flows in a coil of 

 inductance L henrys, energy is stored in the magnetic field sur- 

 rounding the coil of value 



i 2 

 co = L TT watt-seconds. 



a 



At the beginning of commutation the current in coil c is I c and 



1 2 



the energy stored in its field is L -~ watt-seconds. While the cur- 

 rent is falling to zero this amount of energy must either be given 

 back to the electric circuit as useful work or wasted as heat in the 

 resistance of the short-circuited coil. At the end of commutation 



1 2 



the same amount of energy L - must be stored in the field by a 



& 



current I c flowing in the opposite direction in order that the cur- 

 rent from coil 6 may flow freely through coil c, and that there may 

 be no tendency to spark. 



If when the brush BI (Fig. 144) breaks contact with bar 1 the 



i 2 

 current has only decreased to a value i, the energy L = stored in 



the field of the coil will appear as a spark and the current I c from 

 the coil 6, since it cannot immediately flow through the coil c 

 against the inertia of its magnetic field, will try to follow the brush 

 and will produce an arc which will increase the sparking. Behind 

 this arc is the energy stored in the magnetic field of all the coils 

 on one side of the armature, since any decrease in the current in 

 coil 6 is accompanied by a decrease in all the coils in series with it. 

 This energy is not, however, available instantaneously due to the 

 inertia of the magnetic field, and commutation is complete before 

 the current has time to decrease appreciably. At the instant of 

 breaking contact with bar 1 the current density in the brush tip 

 and in the edge of the bar is very high and a very high temper- 

 ature will be produced locally which may volatilize a small amount 



