2-B] 



EFFICIENCY. 



45 



Constant Potential Supply 



\\ 



STARTING BOX 



electromotive force impressed on the armature terminals, arma- 

 ture current,* field current and speed. 



By using two resistances, B and C, arranged as in Fig. i, the 

 electromotive force impressed 

 on the armature may be varied 

 by short circuiting more or 

 less of B or of C. A single 

 series resistance B may suffice, 

 but the adjustment in many 

 cases can be better made with 

 two. An independent genera- 

 tor can be used as a supply 

 to obtain variable voltages for 

 the armature circuit, or the 

 two voltagesf of a three- 

 wire system. 



8. Results. The losses 

 of the motor include: 



(1) Copper losses of field 

 and armature; 



(2) Iron losses of armature; 



(3) Friction and Windage, or air resistance. 



Losses (2) and (3) are rotation losses and are independent of 

 load. 



* (7c). For the no load run the armature current is small; if a low 

 reading ammeter is used, it should be short-circuited at starting to avoid 

 damage by the initial rush of starting current. 



t(7d). Two-voltage Method. For instruction purposes a complete 

 series of armature voltages and corresponding speeds is desirable. Where 

 two supply voltages (as no and 220 volts on a 3-wire system) are avail- 

 able, accurate results may be obtained by a two-voltage method, by taking 

 8 or 10 readings and averaging first with say 220 and then with no volts 

 impressed on the armature of a 220 volt motor. These points, accurately 

 determined, are sufficient for working up results by the straight line 

 method of Fig. 2, in which they are represented by black dots p and q. 

 By this method the trouble of adjusting armature voltage is avoided. 



FIG. i. Connection for no-load run as 

 a shunt motor for determining losses. 



