DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVICES 1423 



2 4 6 8 10 12 14 16 



^d/^ METER 



Fig. 5 — Decay time constant of DC instrument. 



20 



eddy currents. The induced voltage in the search coil then consists of two 

 equal and opposite impulses occurring respectively at and T/2. To 

 measure the constant flux, the switching epoch can be anywhere between 

 these two values. The switching omits the impulse at zero time. For limit- 

 ing cases ^1 can approach either or T/2. The instrument reading of 

 course should be independent of the choice, as the flux remains at the 

 constant maximum throughout this interval. The instrument receiv^es 

 only the decay impulse, and thus there will be an average dc component. 

 For the case of U approaching zero, the instrument is always connected 

 to the search coil, which usually is of negligible resistance and the short 

 circuit meter decay time constant applies. Just before a succeeding im- 

 pulse, the meter will have decayed to a relative value of: 



r^"'^ ^ 1 - 





(12) 



where T is the cycle time as before and Ti is the short circuit decay time 

 constant. The pointer next abruptly rises to its original maximum value, 

 followed again by another equal decay in a cyclic manner. 



For the case of h approaching T/2, the instrument is connected to the 

 the search coil only half the time, being open circuited for the remainder 

 of the time, if no damping resistor is provided. It thus decays at two 

 different rates following the impulse. The first half is the same as before 

 but the second half is under the condition of open circuit. For small 

 errors the relative decay will be: 



-TI2T^^-TI2T, _ J 



-21A+A]' '''' 



where T2 is the open circuit decay time constant. 



