DYNAMIC MEASUREMENTS ON ELErTHOM \c;\KTI(' DEVICES 1415 



Part I — Description of Fluxmeter System 



BASIC MEASURING CIRCUIT 



A schematic of the basic measuring circuit is shown in Fig. 1. A battery 

 switching contact indicated as A, applies and removes voltage to the 

 magnetizing winding of the electromagnetic device under test with a 50 

 per cent duty cycle. The time of one complete cycle is indicated as T, 

 The electromagnetic device is equipped with a search coil of A^ turns, 

 having dc resistance, including wiring, of R^ ohms. A B contact, syn- 

 chronously switched at instants to be described later, connects a dc 

 microammeter to the search coil. A damping contact (7 connects the 

 instrument to a resistance, preset to the same value as the search coil, 

 when the instrument is not connected to the latter, thus providing the 

 same instrument damping at all times. 



The A timing cycle is shown schematically for one interval T. Next 

 below is shown the cycle for the B contact, followed by that for the C 

 contact. The B contact always opens and the C contact closes just before 

 the A contact closes. The point of closure of the B contact, indicated as 

 ti may be adjusted to occur at any time during the cycle T. The cycle 

 time T is chosen sufficiently long so that the flux in the electromagnet 

 has sufficient time to reach a substantially steady state during both 

 the closed and the open intervals. 



The flux rise and decay have the general characteristics shown as the 

 <p curve, while below it is the induced voltage in the search coil, exactly 

 given by Lenz's Law 



« = ^t a) 



The first property of this last curve to observe is that the positive 

 and negative areas are exactly equal. Othenvise a dc instrument con- 

 nected permanently to the search coil would show a dc current flow. The 

 second property is that the area of the curve up to some time ti is 

 proportional to the instantaneous flux at the time ti. Finally, this un- 

 shaded area is numerically equal but opposite in sign to the shaded re- 

 mainder of the area, taking into account the signs of the two components 

 of the remainder. The operation of the test set depends upon measuring 

 this shaded area. 



Let the flux linking the search coil of N turns at any time be ^. As- 

 sume that the microammeter has a resistance Rm and inductance L„. 

 The current flowing in the meter mesh while the B contact is closed is 



N^ + Ljj-+(R^ + Rc)i = 0. (2) 



at at 



