IMPROVED SPARKING LEVEL METER

Z. F. Idrisov, G. A. Sipailov, V. S. Stukach,
A. Ya. Tsirulik

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Sparking of brushes is the principal and determining factor in unsatisfactory commutation of direct-current machines. According to GOST 183–66, the intensity of sparking at the commutator is a characteristic determined by the intensity of the light emission of spark-arc discharges. The intensity of the light emission is assessed visually, which is the reason for the nonobjectivity of this method of measurement. For this reason, an objective assessment of the intensity of sparking and an analysis of sparking in electrical machines are of very substantial importance both for establishing the causes of unsatisfactory commutation and for its adjustment. Attempts are being made to develop objective methods for measuring sparking with the aid of instruments that record and measure the parameters of certain objective signals generated by the sparking process. All known instruments use one principle—the averaging of the parameters of signals from discharges on all lamellae and the obtaining of a certain integral index in arbitrary units, which is considered proportional to a definite sparking level expressed in points on the GOST scale. In our view, such a principle does not correspond to the physics of the phenomenon [1].

In a previously published work [2], a new method was set forth and an instrument developed on its basis for determining the intensity of sparking was described. A three-channel version of the spark meter was tested on the test bench of a factory laboratory. The tests made it possible to conclude that the instrument correctly reflects the quantitative and qualitative aspects of the phenomena associated with spark formation, but that it is not free of shortcomings and requires further refinement. A block diagram of the improved instrument is shown in Fig. 1. The voltage pulses of spark-arc discharges are picked up by a high-impedance capacitive-type sensor 1, which is a copper plate with an area of $(0.6 \div 1)\ \text{cm}^2$, mounted by means of a special bracket on the brush holder at a distance of $(2 \div 6)\ \text{mm}$ from the commutator surface. The sensor is simple in design and convenient in operation, which has its undoubted advantages in monitoring and adjusting the commutation of mass-produced machines under the conditions of the test station of the manufacturing plant. The use of such a sensor also increases the reliability of the instrument as a whole. To match the sensor with amplifier 3, a high-frequency emitter follower 2 is used. From the amplifier the pulses are fed to oscilloscope 4. The image of the voltage pulses on the oscilloscope screen makes it possible to judge the nature of the commutation processes and, approximately, the causes of spark-

Fig. 1. Complete functional diagram of the instrument

formation. The amplified signals are also fed to two identical channels, one of which additionally contains inverter \(4'\), to ensure pulses of positive polarity at the input of Schmitt trigger \(T.Sh.\ 5\). The presence of two channels makes it possible simultaneously to determine the sparking level for over- and under-commutation, which is necessary in the process of adjusting the commutation of machines. Each channel consists of Schmitt trigger 5, which performs the function of a pulse shaper and is triggered by positive pulses; electronic switch 6, which creates the necessary potential step for reliable operation of the sawtooth-voltage generator 7; and the sawtooth-voltage generator itself, which converts pulse duration into amplitude. From the sawtooth-voltage generator the sawtooth voltage is fed both to the multichannel amplitude discriminator and to amplitude voltmeter 8. The circuit of the amplitude voltmeter is designed so that the pointer instrument installed in the AV responds only to the amplitude of the pulses, but does not respond to the pulse repetition frequency or to their duty factor. The amplitude discriminator consists of ten identical channels, each of which includes amplitude comparator 9, monostable multivibrator 10, logic circuit 11, and indicator lamp 12. In the AC circuit, the amplitude of the sawtooth is compared with the level of the reference voltage \(U_{\text{op}.i}\). The reference-voltage levels are set during adjustment of the instrument from a rectangular-pulse generator. The monostable multivibrator is used as a pulse stretcher. Owing to the logic circuit, each channel of the amplitude discriminator operates only for its own duration.

Thus, in the instrument only one channel will operate, namely the one that is set to a duration equal to the duration of the input pulse.

In view of the fact that the voltmeter, although it is an amplitude voltmeter, retains (even if only to an insignificant degree) integral properties, it will introduce some error into its readings. To avoid this error, the instrument provides indicator lamps, which ensure the necessary speed of response and simplicity of readout.

The described instrument uses the same principle of estimating sparking by maximum durations, which corresponds to the physics of the phenomenon; however, unlike the previously developed instrument, the new one uses a duration scale without reference to scores, which eliminates subjectivity in the assessment of the sparking level.

The instrument described, possessing objectivity in the assessment of sparking and correctly reflecting the quantitative and qualitative aspects of the phenomenon, has been tested under laboratory conditions and has demonstrated reliable operation.

LITERATURE

1. A. Ya. Tsirulik, V. S. Stukach, Z. F. Idrisov. Investigation of the relationship between the parameters of spark discharges on a commutator and the intensity of sparking in points. Izvestiya TPI, vol. 284, Tomsk, TTU Publishing House, 1974.

2. Z. F. Idrisov, G. P. Trofimov, A. Ya. Tsirulik, V. S. Stukach. A device for measuring the intensity of sparking on the commutator in direct-current machines. Izvestiya TPI, vol. 284, Tomsk, TTU Publishing House, 1974.

3. Calculation and Design of Pulsed Transistor Devices, ed. by M. D. Shterk. Moscow, “Soviet Radio,” 1964.

4. M. I. Gryaznov, M. L. Gurevich, Z. V. Magrachev. Measurement of Pulse Voltages. Moscow, “Soviet Radio,” 1969.

5. L. M. Goldenberg. Fundamentals of Pulse Engineering. Moscow, “Svyaz,” 1964.