DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVICES 1421 



current at the time h; iis determined by a separate measurement de- 

 scribed above. If its value is not needed, the air core coil can be omitted 

 from the circuit. 



For flux decay measurements made after the A contact has opened, 

 the bridge circuit has to be modified to remove the resistance Rq and KRo 

 from being across the electromagnet and affecting its flux decay. They 

 are transferred to the battery side of the A contact as shown in Fig. 

 3(c), to maintain the same battery drain. This avoids any error due to 

 the internal resistance of the battery, which would othenvise cause the 

 final flux at the end of a rise test to differ from the initial flux at the 

 beginning of a decay test. The added resistor K(Ro + Rm) is connected 

 in series with the meter to make the expression for the flux-turn linkages 

 the same as before. For decay measurements the damping resistor is set 

 at the value 



Re = K{Ro + ie„, + Ra). (11) 



In Figs. 3(b) and 3(d) are shown the linearized equivalent circuits for 

 rise and decay respectively, from which equation (10) above can be 

 derived conveniently. 



MEASUREMENT OF DISPLACEMENT AND VELOCITY 



An optical probe is provided, in which the amount of light falling on a 

 photocell is controlled by the relative position of two flags, one cemented 

 on each of the relatively moving parts to be studied, such as an electro- 

 magnet. The change in output current from the photocell thus is pro- 

 portional to the displacement of the armature with respect to the core, 

 one flag being on each. A block diagram of the system is shown in Fig. 4. 

 Amplifiers effective from dc up to a frequency determined by the resolu- 

 tion required, with substantially no phase shift, deliver a current into 

 the primary of an air core transformer proportional to the instantaneous 

 displacement of the armature. By virtue of the linearity of this trans- 

 former, the flux developed is proportional to the displacement. Thus by 

 operating the electromagnet with the A contact and connecting the 

 secondary of the transformer to the B contact, the instmment gives a 

 dc reading proportional to the armature displacement at the time the B 

 contact closes. The total displacement can be measured statically. The 

 instrument reading at a time after complete operation corresponds to 

 this known displacement, permitting the scale to be converted to a dis- 

 placement scale. 



By changing one amplifier input resistor to a capacitor, the amplifier 



