272 



ALTERNATING CURRENTS 



effected by the use of single-phase current, has attracted a great deal 

 of attention within recent years, and has led to the rapid development 

 of single-phase commutator motors. In the present chapter we shall 

 give an account of the theory and construction of such motors. 



All single-phase commutator motors * may be referred to one or 

 other of three types, viz. (1) the series-wound motor, with or without 

 a compensating winding; (2) the repulsion motor; (3) the com- 

 pensated repulsion motor (Latour-Winter-Eichberg). 



162. Series Motor. Circle Diagram 



The general arrangement of the plain series-wound single-phase 

 motor closely resembles that of an ordinary continuous-current 

 motor ; in fact, such motors may, and in practice actually are, used 

 on both single-phase and continuous-current circuits. The differences 

 between the ordinary continuous- current and the single-phase types 

 are partly constructional, partly differences of design. It is obvious 

 that, in order to prevent excessive eddy-current loss, and to enable 

 the field-magnet to develop its full magnetic flux,| the core of the 

 field-magnet must be laminated throughout. 



In Fig. 164 is given the diagram of connections of a plain series- 

 wound single-phase motor. A is the arma- 

 ture, FW being the field winding. TI and 

 T 2 are the motor terminals. The dotted arrow 

 indicates the direction of the field flux, 

 which, as in a continuous-current motor, is 

 at right angles to the line of the brushes. 



Since in this type of motor the armature 

 is connected in series with the field, it is 

 obvious that the armature and field currents 

 must necessarily be in phase with each other, 

 so that the armature current reaches its 

 maximum value at the instant of maximum 

 field intensity. So long as the magnet is 

 well below saturation i.e. within the range 

 of approximately constant permeability the 

 torque at any instant is proportional to the 

 square of the current. From this it follows 



that the motor will exert equal torques when supplied, in the one 

 case, with continuous current of amount i, and in the other, with 



* The single-phase induction motor is still less suitable for traction than the three- 

 phase induction motor, and its use for this purpose could not be seriously considered. 



t Eddy currents exert a demagnetizing or screening effect on the core, so that 

 with a massive core the central portions would not be appreciably magnetized. 



FW 



FIG. 164. Diagram of Con- 

 nections of Series Motor. 



