28 DIRECT CURRENT MOTORS. [Exp. 



It is seen that the operation, either as a generator or as a 

 motor, involves ( i ) the generation of an electromotive force and 

 (2) the creation of a torque, both of which depend upon funda- 

 mental laws of electromagnetism. 



2. Generation of Electromotive Force. An electromotive 

 force is generated in a generator or in a motor due to the cutting 

 of lines of force, this electromotive force being proportional to 

 the rate at which the lines of force or flux are cut, as already 

 discussed in i, Exp. i-A. 



In a generator this electromotive force causes (or tends to 

 cause) a current to flow; in a motor, it is a counter electromotive 

 force and opposes the flow of current. 



3. Creation of Torque. A torque is created in a generator 

 or motor due to the forces acting upon a conductor carrying 

 current in a magnetic field. In a motor this torque causes (or 

 tends to cause) a rotation of the armature with respect to the 

 field ; in a generator, it is a counter torque and opposes the rota- 

 tion of the armature. 



The creation of torque depends upon the following funda- 

 mental principle : When a conductor carrying current is located 

 in a "magnetic field, it is acted upon by a force that tends to move 

 the conductor in a direction at right angles to itself and to the 

 magnetic flux, the force being proportional* to the current and 

 to the flux density. 



This force creates a torque, that is a turning moment or 

 couple equalf to the product of the force and the length of the 



* (3a). In C.G.S. units this force is equal to the product of the cur- 

 rent, flux density, length of conductor and sine of the angle between the 

 conductor and direction of flux. This sine is unity when the conductor 

 and flux are at right angles, as in most electrical machinery. When there 

 are a number of conductors, each conductor is subject to this force; the 

 total torque of a motor is therefore proportional to the total number of 

 armature conductors. 



t (3b). Torque may be expressed as pounds at one foot radius, pound- 

 feet, kilogram-meters, etc. Power is proportional to the product of torque 



