Computer-Aided Magnetic Circuit Design for a Bell Ringer

In 1952 the Bell System introduced the standard 500-type telephone set which uses a "universal" two-gong ringer (C-type) to meet a wide variety of service conditions. 1 A universal ringer must: (i) have two voltage sensitivity modes that ensure adequate operation under worstcase conditions and provide protection against cross ringing on party lines, (ii) be electrically polarized to protect against bell tapping due to dialing transients, (Hi) have high impedance («8 kohms) at ringing voltages so that multiple ringers can be bridged across the line, (iv) have high impedance («120 kohms) at voice frequencies to prevent speech signal attenuation, and (v) have two coil windings for use with multiparty ringing circuits. 2 Recently a project was undertaken to design a miniature single-gong ringer that would have the low cost, reliability, sensitivity, and sound 179 output of the C-type ringer but would be significantly smaller. The small single-gong P-type ringer commonly used in TRIMLINE® telephones and other new telephone sets did not optimally meet all of these objectives and previous attempts to meet them were unsuccessful. To achieve these objectives, effort was channeled toward a rocking armature type motor3,4 that drives a single clapper and fits under the gong. A mathematical model was essential for the design process in order to perform parametric analyses and design optimization. The mathematical model had to realistically account for flux saturation of materials and flux leakage paths in the design, both of which have significant influence on the performance of a compact ringer motor.