SYNCHRONOUS MACHINERY 



279 



compounding curve for unity power factor. If the phase char- 

 acteristics are very steep a slight change in field excitation pro- 

 duces a large change in armature current, or a large component 

 of wattless current is required to correct for a slight variation in 

 field excitation. This is the case in a motor with small synchro- 

 nous reactance or small armature reaction and the motor is un- 

 stable. If the synchronous reactance is large only a slight change 

 in armature current is produced by a change in field excitation 

 and the phase characteristics are flat and the motor is stable. 



168. Synchronous Compensators. Since by varying the field 

 excitation of a synchronous motor the power factor can be made 

 either leading or lagging, such machines can be used to improve the 

 power factor of transmission lines or distributing circuits by draw- 

 ing wattless leading currents to compensate for the wattless lagging 

 currents required by the load. The fields must be over excited 

 and the synchronous reactance should not be very large. This 

 is one of the most important characteristics of the synchronous 

 motor and is being applied to an ever-increasing extent. The 

 synchronous compensator usually operates without load drawing 

 the required wattless leading current and a small power current 

 to supply its own losses. In some cases, however, it may be 

 advantageous to supply some load from it. 



169. Starting. In order to improve the starting torque of 

 synchronous motors and also to prevent hunting short-circuited 

 grids are placed in the pole faces, 



Fig. 257, or between the poles, 

 and in addition the poles are 

 sometimes made solid. The field 

 winding at start may either be 

 open or else short circuited. In 

 the following discussion it will 

 be assumed that the field circuit 



is open. Torque is produced in __^_ 



two distinct, ways. FlG . 2 57. Copper grid in a pole face. 



(1) The revolving field sweeps 



across the grids and the solid pole faces and generates e.m.f.'s and 

 currents in them. These currents react on the field and produce 

 torque which makes the rotor follow the field. The rotor can 

 never be brought up to synchronous speed by this torque because 

 the e.m.f.'s and currents are only induced below synchronous speed. 



