Dynamic Measurements on 

 Electromagnetic Devices 



By M. A. LOGAN 



(Manuscript received July 6, 1953) 



A sampling switch with adjustable make and fixed break times can be 

 used to obtain dynamic measurements of reciprocating phenomena. A test 

 set has been developed using this principle to measure the flux-time, current- 

 time, displacement-time, and velocity-time response of electromagnets and 

 similar devices. The tested device is steadily cycled. A dc instrument is 

 switched in by the synchronous control at any preselected instant in the 

 cycle, and out when a steady reference condition has been reached. The 

 steady reading of the instrument is proportional to the value, at the closure 

 instant, of the variable being measured. The instrument switching is con- 

 trolled by an electronic timing system. This system operates mercury cmitact 

 relays, which do the actual switching. For the displcu^ement-time and velocity- 

 time measurements, an optical transducer with associated dc amplifiers is 

 added. The design of these devices is described. The results of an investiga- 

 tion of dynamic flux rise and decay in solid core electromagnets are reported. 

 Modified first approximation equations are developed to give a better repre- 

 sentation of eddy current effects. 



Introduction 



The application of common control methods to telephone switching 

 systems has led to the widespread use of high-speed relays. The actua- 

 tion time of these relays is affected by many parameters such as the 

 power supplied, how far the armature has to move, the mechanical work 

 the armature has to perform during its motion, the winding design, the 

 magnetic structure, and eddy currents introduced in the magnetic mem- 

 bers caused by the application of current to the winding. The eddy cur- 

 rents act to oppose a magnetic flux change and hence retard a building 

 or decay of flux. This causes the actuation time to l)e increased compared 

 to a relay without such effects. An analytical determination of the de- 

 velopment and effects of eddy currents can l)e made for simple sym- 



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