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ELECTRICAL ENGINEERING 



(2) The second way in which torque is produced can be under- 

 stood by reference to Fig. 258. 



The current in phase 1 grows to a maximum and produces a 

 north pole on the armature at N and a south pole on the rotor 

 at S. As the north armature pole moves around to the left the 

 south pole on the field becomes weaker but it cannot immediately 

 disappear since eddy currents are induced in the solid field poles 

 which retard the decrease of flux. The armature pole exerts a pull 

 on the residual field pole and produces torque. The revolving 

 north pole is succeeded by a south pole which induces a north pole 

 on the rotor and exerts a torque on it. The torque produced in this 

 way combines with the torque produced by the currents induced 

 in the pole faces and grids and brings the motor up nearly to 



FIG. 258. Starting torque of a polyphase synchronous motor. 



synchronous speed. When the armature pole is moving very 

 slowly across the field pole the two lock together in the position 

 of minimum reluctance and the motor runs at synchronous speed. 

 The field circuit can then be closed and the impressed e.m.f . raised 

 to its full value. When starting in this way the motor draws a 

 very large lagging current since the impressed voltage is consumed 

 by the synchronous impedance of the armature, and the power 

 factor is very low. The impressed voltage at start must be 

 reduced to about one third of its full value in order to reduce the 

 starting current. It is the flux of armature reaction which pro- 

 duces the starting torque and, therefore, a motor with high arma- 

 ture reaction will give better starting torque than one of low 

 armature reaction. 



When the motor is running below synchronous speed large 

 voltages sometimes reaching 5000 volts or more are induced in 

 the open-field winding by the revolving armature flux. Such 

 voltages are dangerous and may puncture the insulation of the 

 field or endanger the lives of operators. Both the magnitude and 

 frequency of these induced voltages become zero when the motor 

 reaches synchronous speed. 



