Measurements on high-speed magnetic servo amplifiers with 2-phase motor load

Measurement of steady-state characteristics and transient-response time of high-speed magnetic servo amplifiers with 2-phase motor load offers two special problems: 1. The output current of a magnetic amplifier with rectangular-hysteresis-loop core material represents a peculiarly truncated sine wave which consists of a fundamental (60- or 400-cycle) component and an array of harmonics having relatively large tnagnitudes. The 2-phase motor. however, responds only to the fundamental component of this output current multiplied by the sine of the actual. phase angle between it and the substantially sinusoidal line-field current of the motor. Thus, in this case, conventional methods for measuring output-input characteristics by means of a separately excited wattmeter are not always applicable. 2. The transient-response time of modern forms of magnetic amplifiers is in the order of 1 to 4 half-cycles of the power-supply frequency. Therefore, for studying the dynamic properties of such high-speed amplifiers it will be advisable to employ a transient analyzer which produces a synchronous square-wave signal voltage and permits, also with higher power-supply frequencies, direct visual observation of a stationary pattern on a dual-beam oscilloscope. This paper is concerned with the solution of these two problems. At first it describes the basic principle and the advantages of the “torque-balance method” which permits direct measurement of magnetic-amplifier output in terms of the “equivalent sinusoidal output voltage” for stalled-motor operating conditions. Then, it illustrates the use of a novel form of square-wave signal-voltage transient analyzer with a “magnetic switch” which consists essentially of a special saturablereactor circuit producing a synchronous unidirectional current with nearly rectangular waveshape. When applying this analyzer, a dual-beam oscilloscope displays the wave forms of input and output voltages of the magnetic amplifier under test simultaneously on a common time base in such a way that these wave forms are exhibited on a long-persistence screen as a stationary pattern. Thus, it is possible to study the effect of various circuit-parameter changes on actual speed of response and to determine optimum design. Finally, this paper gives results of additional measurements on the circuit under test. Measurement of bandwidth on a 400-cycle position servomechanism illustrate the dynamic performance of the circuit when operated in connection with a phase-sensitive rectifier in a closed-loop servo system. Measurements concerning reduction of asymmetry zero-drift errors make it evident that such errors are reduced to about 1/40 of their original values as a result of the application of a large amount of electric negative feedback resistance-coupled into the controlcircuit loop.