OPERATION OF SYNCHRONOUS MOTORS. HUNTING 115 



e a series-motor, and it will start as such. When its speed attains 

 synchronism, a centrifugal device doses the circuit with a third col- 

 lector-ring on the right, causing an incandescent lamp to be lighted; 

 the switch-lever is then moved, causing the normal connections to be 

 made; the armature is then connected directly with the circuit; and 

 the commutator circuit is connected with the fine wire excitation-cir- 

 cuit. The inductance in circuit is adjusted in such a manner as to 

 permit the maximum allowable current to pass through the circuit 

 when starting. This maximum current 

 is about two to two and a half times 

 the full load-current. 



The efficiency of this type of motor 

 is 70 per cent for a 2\ H.P. motor, and 

 80 to 85 per cent for motors of 10 to 15 

 .P. They start readily under load, 

 ithout excessive sparking at the brushes, 

 the latter being of carbon and having a 

 fixed position. The power-factor, which 

 is very high when starting, does not fall 

 below 0.95 to i under load. 



For small motors, of i to 5 H.F., the 

 Gramme commutator just mentioned 

 can be replaced by a less complicated 

 commutator having only one segment 

 per pole of the motor. The small Ganz 



motors are of this kind. Their fields are excited by commutated 

 currents obtained from a transformer connected in shunt to the line. 

 The even segments are connected to one terminal of the field-circuit, and 

 the odd segments to the other. To start the motor, which can only be 

 done without load, only one set of brushes need be used on the commuta- 

 tor, the others being put off. The current thus changes direction in the 

 field-coils and in the armature coils, as already mentioned; but there is 

 much sparking at the brushes during the whole time that the motor 

 is starting. When the synchronous speed has been attained, which 

 requires at least a minute, the brushes previously removed are again 

 put on, by hand or else by a centrifugal regulator, or they may be put 

 in circuit by means of a switch ; sparking is then reduced to an allowable 

 amount. Fig. 57 gives the diagram of the arrangement actually used 

 (Blathy system). The field and armature windings are in parallel. 

 The armature-current, coming from the transformer at 100 volts, 



FIG. 56. 



