THE DYNAMO. 49 



the supply mains. A series motor has a coarse wire field wind- 

 ing which is connected in series with the armature to the supply 

 mains. A compound motor has a field winding in two parts ; one 

 part is of fine wire connected in parallel with the armature, and 

 the other part is of coarse wire connected in series with the 

 armature. The connections of motors to the mains differ from 

 the connections of generators on account of the starting resistance 

 which must be used with a motor. Motor connections are dis- 

 cussed in detail in Chapter IV. 



Shunt motors are almost always used for constant speed driving 

 when the current is taken from constant voltage mains. The 

 series motor is used for variable speed driving when the current 

 is supplied at approximately constant voltage as exemplified by 

 the use of motors for hoisting and for propelling street cars. 

 Compound motors have characteristics which adapt them to 

 special kinds of service as explained later. 



32. Eddy currents. Lamination. When a piece of iron is 

 magnetized or demagnetized, the changes of the magnetic flux 

 through the central portions of the iron induce electromotive 

 forces in the surrounding portions of the iron, and these electro- 

 motive forces produce what are called Foucault currents or eddy 

 currents in the iron. Eddy currents are also produced in a mass 

 of metal, iron or copper, which is near a moving magnet, or 

 which moves in the neighborhood of a stationary magnet. 



Those parts of a dynamo which are subject to rapid and fre- 

 quent changes of magnetization are built up of thin sheets of iron 

 so as to leave the iron continuous in the direction of magnetization, 

 but discontinuous in the direction in which the eddy currents tend 

 to flow. Such a mass of iron is said to be laminated. The iron 

 cores of dynamo armatures are always laminated, inasmuch as 

 any given part of the iron core of an armature is subject to 

 magnetic reversals as it moves past the field magnet poles. 



Eddy currents are produced in the solid iron of a dynamo field 

 magnet when the flux through it varies in value. Eddy currents 

 are also produced in the surface layers of the pole pieces when 

 4 



