CHAPTER VIII 

 ALTERNATING-CURRENT COMMUTATOR MOTORS 



227. Alternating-current Series Motor. The alternating-cur- 

 rent series motor is very similar to the direct-current series motor 

 and can be operated on direct-current with increased efficiency 

 and output. 



If a direct-current series motor is connected to an alternating- 

 current supply circuit it will rotate since the currents in the field 

 and armature reverse together and therefore the torque is always 

 in one direction, but it will be very inefficient and will spark very 

 badly. 



With alternating current flowing in the field winding an alter- 

 nating magnetic flux is set up through the magnetic circuit and 

 causes very large losses due to hysteresis and eddy currents. To 

 reduce these to a minimum the whole magnetic circuit _of an 

 alternating-current series motor must be laminated. The field 

 circuit must be very heavily insulated to prevent short circuits 

 between turns which would burn out the motor on account of 

 the large induced currents. 



The relation between the e.m.fs. and current in the direct- 

 current series motor is given by the equation 



E= &+I(r a + 77), ..... (332) 



where E = impressed e.m.f., 



& = counter e.m.f. generated by rotation, 



/ = current, 



r a = resistance of the armature, 



TV = resistance of the field. 



In the alternating-current series motor the alternating flux sets 

 up large e.m.fs. of inductance in both the field and armature 

 windings, which consume components of the impressed e.m.f. in 

 quadrature ahead of the current. If L f is the inductance of the 

 field and L a the inductance of the armature, their reactances are 



358 



