ELECTROMAGNETIC DRIVING SYSTEM 103 



A. Reed Armature Type. — A reed armature driving system consists of 

 an electromagnet operating directly upon an armature of steel (Fig. 6.15). 

 The steel armature is spaced at a small distance from a pole piece wound 

 with insulated wire carrying the alternating current and supplied with 

 steady flux from the poles of a permanent magnet. 



The flux, in maxwells, due to the permanent magnet is given by 



M 

 ,. = ^ 6.7 



where M = magnetomotive force of the magnet, in gilberts, and 



Ri — reluctance of the permanent field magnetic circuit, in oersteds. 

 The flux, in maxwells, due to the sinusoidal current z'max sin co/ in the 

 coils is given by 



47rA^Vax sin oj/ 



— o.o 



R 



where A^ = number of turns in the coil, 



Vax = maximum current in the coil, in abamperes, 

 R2 = reluctance of the alternating magnetic circuit, in oersteds, 

 CO = lirf, 



f — frequency, and 

 / = time. 

 The force, in dynes, on the armature is 



^ ((/>! + 02)' ^ M^ MNi^^^ sin CO/ 



TrA'^/^niax TrA^^/^tnax COS 2co/ 



RM R^^A 



where A = effective area of the pole in square centimeters. 



The first and third terms of the right-hand side of equation 6.9 represent 

 a steady force, the second term represents a force of the same frequency 

 as the alternating current and the last term represents a force of twice the 

 frequency of the alternating current. Referring to equation 6.9 it will be 

 seen that the driving force is proportional to the steady flux 01. Also, 

 01 must be large compared to ^i, in order to reduce second harmonic dis- 

 tortion. For these reasons the polarizing flux should be made as large 

 as possible. 



The motional impedance of this system will now be considered. If all 



