A small-size instrument is described for measuring the specific resistance of carbon-graphite electrodes of different shapes. The device includes a microcomputer allowing repeated measurements and statistical processing of the obtained results according to standard requirements. The specific resistance is measured by the four-probe method.A most important parameter of carbon-graphite products (CGP) is their specific resistance (SR) that must be measured during intermediate technological operations and at the completion of the production cycle. Two widely used methods of measuring SR are defined in [1]. The first method is used to measure the SR of elongated products with a constant cross section. According to this method, a constant current is passed along the product and the voltage is measured across the central portion of its side surface along the axis of pressing. The current and potential probes are placed at considerable distances from each other so that the measuring setup becomes quite cumbersome. The second method can be used for measuring the SR of products of any dimensions and consists in placing a special four-probe pickup on the side surface of the measured product. A constant current is passed through the end probes and the voltage across the selected part of the product is measured with the aid of the inner probes.Two problems arise in the design of SR measuring instruments. First, since the relatively low specific resistance of graphite (I0 ~f~.m, or less) and considerable transverse dimensions of electrodes demand very sensitive measuring circuits or large measuring currents (up to 100 A), traditional measuring equipment is large and heavy. Secondly, many carbon-graphite products have an irregular structure. Because of this, the measurement of SR with a four-probe pickup according to the standard [1] requires repeated measurements followed by averaging of the obtained results.Let us consider the design principles of a small-sized instrument for measuring SR with the aid of a four-probe pickup. The instrument is small and light, automatically processes the results of repeated measurements, and measures SR with high accuracy.The instrument block diagram is shown in Fig. 1. The individual functional modules execute the following operations. The input amplifier ICL 7652 preliminarily amplifies the voltage across the potential probes PP. The analog-digital converter ADC converts the operational amplifier output signal into a digital code and enters it by request into the microcomputer for further processing. The MAX 110 integrating ADC suppresses the interference of the 50 Hz power line. The stabilized constant current generator produces the current flowing through the current probes CB during measurement, the storage capacitor allowing the generation of a current of 5 A when the instrument operates from low-capacity nickel-cadmium batteries. The single-chip 1830VE751 microcomputer controls the instrument through the control knobs and the ADC, switches on the stabilized current generator, displays t...
The article provides a comparative assessment of various ways of powering a synchronous motor with permanent magnets (PMSM) in the mode of a thyratron motor (TM). Numerically, on analytical and computer models, mechanical ω = f(M), the dependence of speed on electromagnetic torque, and energy ηэ = f(M), the dependence of elec-tromagnetic efficiency are calculated from the moment, the characteristics of the motor with sinusoidal power supply and discrete switching with different values of the relative inductance of the winding. Moment pulsations are investigated. The three most common methods of discrete degree commutation are considered – 120, 180 and 150. It is shown that discrete 120 degree commutation provides the highest value of ηэ in comparison with other methods, and the mechanical character-istics are approximately the same in appearance with different power supply methods. The study of moment pulsations has shown that discrete switching without additional measures cannot compete in this indicator with the case of sinusoidal power supply, which theoretically has zero moment pulsations. However, the use of 150 degree commutation makes it possible to significantly reduce them, bringing them closer to 2…3 %. Curves are given that allow an approximate estimate of the expected moment pulsations for all discrete switching methods. Practical recommendations on the choice of the method of powering the PMSM are given.
The article considers the possibility of implementing phase vector control (FVU) of a synchronous motor with permanent magnets in the thyratron motor mode. The characteristics of the FVU with three methods of discrete commutation of the three-phase winding are considered: six-stroke 120- and 180-degree and twelve-stroke 150-degree. A comparative assessment of the efficiency of the PVF is carried out in terms of expanding the range of implemented mechanical coordinates and energy indicators, both with different methods of discrete switching and with respect to sinusoidal power supply. It is shown that 120-degree switching is the most advantageous in terms of energy indicators. However, with it, the values of the maximum achievable speeds and moments weakly depend on the angle of commutation θ, therefore, operation is recommended at a constant θ = 0, corresponding to the efficiency value close to the maximum in a wide range of speeds. With 180-degree commutation, the FVU allows you to adjust both the maximum speed and the maximum torque. However, with small values of the relative inductance of the winding, which refers to the ratio of inductive resistance to active, this method of regulation is ineffective due to low efficiency and is inferior in this indicator to both 120-degree commutation and the case of sinusoidal power supply. As the relative inductance increases, the difference between the case of sinusoidal power supply and 180-degree switching decreases. The use of 150-degree commutation allows, on the one hand, to increase efficiency while maintaining the ability to adjust coordinates, on the other hand, reduces the pulsation of the motor torque, expanding the possible scope of the drive.
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