DIRECT-CURRENT MACHINERY 



209 



employed. Fig. 178 shows such a system. Two generators, one 

 giving 90 volts and the other 160 volts, are connected in series 

 and three wires are run from them to the machine shop. The 

 motor field is connected across 250 volts from a to b and a rheostat 

 is placed in series with it. For low speeds the armature is con- 

 nected across 90 volts from a to c and an increase in speed of 70 

 per cent can be obtained by the field rheostat. For medium 



16C 



Volts 



To Motors 



Volts 



FIG. 178. Three-wire system. 



speeds the armature is connected across 160 volts from c to b and 

 the speed can again be increased 70 per cent. Finally for high 

 speeds the armature is connected across 250 volts from a to b and 

 the speed can be increased again by the field rheostat. In this 

 way a range of 4 : 1 can easily be obtained. 



With interpoles a speed range of 4 : 1 can be obtained by field 

 weakening without using a three-wire system. 



(2) The characteristics of a series motor are (a) variable speed 

 and (b) torque proportional to the square of the current below 

 saturation. 



When a load comes on a series motor it responds by decreasing 

 its speed and supplying the increased torque with a small increase 

 of current, thus preventing a sudden shock on the supply system. 

 A shunt motor under the same conditions would hold its speed 

 nearly constant and would supply the required torque with a 

 large increase of current and would thus make a heavy demand on 

 the system. 



Series motors must not be used for belt drives or in any case 

 where the load may be removed suddenly since they run at ex- 

 cessive speed at light load. 



Series motors are used in railway work and for cranes, hoists, 

 etc., where very large starting torque is necessary. 



