DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVICES 1429 



duce a B cycle for flux decay where h is greater than T/2, a break contact 

 on the B relay is used in series with the C make contact. Note that in all 

 conditions the meter circuit closure is performed by a B contact and the 

 meter circuit open is performed by the C contact, in fixed phase relation- 

 ship to the A contact. The addition of the switched meter damping 

 resistor is accomplished by other contacts on the same relays. 



A minimum contact circuit^ to perform all these switchings is shown 

 in Fig. 9 but was found to be unsatisfactory because of failure to meet 

 requirement (g) above. Only a single transfer switch is necessary to change 

 from flux rise to decay, by transferring the wire marked "x" from the 

 search coil to the damping resistance. 



The relays are shown in the released position with the break contacts 

 closed. For the flux rise circuit, contact B can have a momentary open 

 interval when it operates. However, it must not bridge or the damping 

 resistor will momentarily be across the instrument at the initial connec- 

 tion, causing an error. But after contact C operates and then B releases 

 at {T/2 + ^i), an open interval at B will momentarily disconnect the 

 instrument. This would cut out part of the current which must be inte- 

 grated, causing an error. Thus there are conflicting requirements for flux 

 rise measurements, and transfer contacts of a single relay cannot meet 

 both simultaneously. For flux decay, the B contact cannot bridge because 

 when it releases to connect the instrument, a bridging would momen- 

 tarily connect the damping resistor across the instrument at the instant 

 of greatest interest, causing an error. The requirements for the C contact 

 are of the same type but less stringent, as the flux is never changing 

 rapidly whenever it is switched. 



The foregoing discussion was directed toward establishing the basis 

 for the requirements (f) and (g) above. The former is because of the 

 need for phase reversals in contact operation to perform the necessary 



sc 



♦ \*- 



Rd: 



RISE ^ DEC^\ . . 



Fig. 9 — Unsatisfactory minimum contact circuit because of transit time 



requirements. 



