1400 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



11 



z 

 a. 

 PiO 



Fig. 5 



^ o 



10 



20 



100 



110 120 



30 40 50 60 70 80 90 

 OPERATE FROM BACK- AMPERE -TURNS 



Minimum differential ampere turns as function of magnetic bias. 



BIAS ADJUSTMENTS 



As bias adjustments are made magnetically, rather than with the 

 spring used in some polar designs, the armature flux tends to approach 

 the same value at the operating points as it has in a balanced adjust- 

 ment with the same differential ampere turn value. Such difference as 

 does enter mth increase of bias is due to differences in armature flux 

 distribution between MMF introduced by the coil and that introduced 

 by the magnets. 



The effect of bias is illustrated in Fig. 5, which shows the minimum 

 differential ampere turn value obtainable in a relay as a function of the 

 higher operate value. The minimum differential value of 3.6 NX with a 

 balanced adjustment is about the same as is obtainable with other polar 

 relay designs. For operate values of about 100 ampere turns a "release 

 to operate ratio" of about 90 per cent is obtained. 



Larger differential ampere turn values, both with and without bias, 

 are of course possible. The amount of bias obtainable in combination 

 with larger differentials is somewhat reduced because of the greater 

 magnet strength required. 



EFFECT OF MAGNETIC SOAK ON SENSITIVITY 



The effect of "soak" on sensitivity is illustrated in Fig. 6. It shows 

 the change which takes place when the ampere turns to operate are 



