SETUP FOR EXPERIMENTAL STUDY OF SLIDING BRUSH–COLLECTOR CONTACT

A. I. Skorospeshkin, L. Ya. Zinner, E. K. Damm, V. P. Karataev

(Recommended by the scientific seminar of the departments of electrical machines and general electrical engineering)

A carbon brush is a very active element of the commutated circuit of a collector machine; therefore, the study of the role of the brush and of the nature of its influence on the commutation process is an important task.

The behavior of a sliding brush contact is influenced by a combination of electrical, mechanical, and thermal factors; neglect of any one of them may lead to incorrect conclusions. Therefore an artificial apparatus which, on the one hand, correctly reproduces the phenomena in the brush contact of a real machine and, on the other hand, makes it possible, within the necessary limits, to vary and control the operating conditions, has undoubted advantages in investigations; the proposed setup, in our opinion, fully satisfies these requirements.

The basic electrical circuit of the setup is shown in Fig. 1. The circuit includes a device for forming current pulses, a collector, and also various arrangements to make it possible to oscillograph the processes.

Fig. 1. Basic electrical circuit of the setup

The main element of the setup is the collector, whose rotational speed can be varied over wide limits. The order in which the collector plates are connected is clear from the figure.

During the time when plate 1 passes under the selected brush (in this case under the lower one), the circuit sends through the contact “lamella 1—brush” a current pulse whose amplitude, duration, and also spatial position relative to the brush can be varied smoothly and over wide limits.

The circuit operates as follows. By means of a simple device consisting of a disk with a hole, a light pulse is applied at the required moment to photodiode D\(_1\), which is connected into the base circuit of transistor T\(_1\). As a result, the resistance of the photodiode drops sharply, which changes the dc operating mode of triode T\(_1\). A positive rectangular voltage pulse appears at the collector of triode T\(_1\). This pulse is then amplified by triodes T\(_2\) and T\(_3\), limited by diode D\(_2\), and fed to an RC generator assembled on triodes T\(_5\) and T\(_6\). The RC generator produces a triangular voltage pulse. Through emitter follower T\(_7\), this pulse is applied to a power amplifier assembled on power transistor T\(_8\). The signal obtained at the emitter of triode T\(_8\) triggers the final power amplifier, assembled on power triodes T\(_9\) and T\(_{10}\), connected in parallel. (In the actual circuit, to increase reliability, 6 triodes operate in parallel.)

As a result, a triangular current pulse with an amplitude of up to 40 A flows through the sliding contact. The power source for the circuit is a storage battery.

The shape, duration, and amplitude of the current pulse are adjusted, respectively, by potentiometers R\(_{13}\), R\(_{14}\), R\(_{17}\). To make it possible to oscillograph the current, a shunt is connected into the brush circuit; the potential of the collector plate is applied to the input of the oscilloscope by means of a slip ring.

In Fig. 2, as an example, oscillograms of the current and transient voltage drop are shown, recorded on a C1-16 dual-beam oscilloscope by the method described. The current pulse is shifted to the leading edge of the brush contact, as occurs under sharply accelerated commutation.

Fig. 2

An additional synchronization circuit, assembled on photodiode D\(_3\) and transistor T\(_4\), makes it possible to stretch in time and examine any portion of the curves of the lamellar current and the transient voltage drop.

Figure 3 shows in detail the initial stage of the process, the general form of which is shown in Fig. 2. The voltage scale here is reduced by a factor of 10; one grid division corresponds, in the horizontal direction, to 50 μsec.

Fig. 3

When the brush overlaps lamellae 2, 3, 4, or K-2, K-1, and K, rectangular pulses appear on the oscillogram of the voltage drop; their heights are determined by the supply-source voltage, and the trailing edge of the first and the leading edge of the second pulse coincide in time, respectively, with the beginning and end of contact of lamella 1 with the brush. In processing the experimental results, this circumstance makes it possible to take accurate account of the changing contact area. In this respect the present setup compares favorably with that described in [1]. In addition, the current density here is forcibly varied during the period of contact of the lamella with the brush.

The collector has artificial heating. The temperature of the lamella can be monitored continuously by means of a thermocouple circuit, which is not shown in Fig. 1. By shifting the current pulse to the leading or trailing edge, accelerated or delayed commutation can be simulated.

In summary, it may be said that the setup described makes it possible to study the volt-ampere characteristics of sliding contact under various conditions, and to investigate the conditions of spark formation and arc formation.

Together with an instrument for measuring small displacements [2], it makes it possible to study the influence of the mechanical properties of the brush-collector assembly on the commutation process.

REFERENCES

1. Z. B. Vartanov. Pulsed volt-ampere characteristics of brush contact. Vestnik elektropromyshlennosti, 1957, No. 2.

2. L. Ya. Zinner, A. I. Skorospeshkin. Bridge instrument for studying the dynamics of collectors of electric machines. Izvestiya TPI, Vol. 160, 1966.