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1125 



Load and Pull Characteristics 



Load and ])ull curves ai'e incasuicd under essentially static conditions. 

 Spring load and ai'mature motion are both ol)served at the center line 

 of the card. They are measured and simultaneously recorded in chart 

 form in a modification of a (ensile testing machine. " A ty])ical load and 

 pull chart is shown in ¥'ig. 14. The abscissa shows armature motion, as 

 the armature moves the card and the spring load, through a distance 

 of 0.030 inch to the operated position, and back again. The ordinate 

 shows s])ring load on the armature on both operate and release, and 

 also the magnetic pull which is (kn^eloped in the armature; for varions 

 numbers of ampere turns in the winding. 



The load and pull chart provides a comprehensive picture of over-all 

 relay performance. For example, starting from the released position, 

 the force or back tension, holding the card against the core is about 140 

 grams. Following the upper curve, the spring load increases slowly as 

 the armature mo\'es toward the core, until the first contacts make at a 

 load of about 200 grams. The load increases rapidly as the remaining 

 contacts are closed until the last contacts are closed at about 650 grams. 

 Further travel of the armature to the operated position increases the 

 spring load to a final value of about 700 grams. As the armature is 

 allowed to return to the original position, the lower curve is traced. 

 The area between the two curves is a measure of mechanical hysteresis, 

 or friction, in the relay. This energy loss is a very small fraction of the 

 spring load at all values of armature travel. 



The pull curves show ampere turns necessary to assure operation of 



LEAKAGE 



RELUCTANCE 



SHUNT 



Fig. 13 — Core assembly for replaceinent type relays showing leakage reluc- 

 tance shunt. 



