A Method for Oscillographing the Voltage of Spark and Arc Discharges at the Trailing Edge of Brushes in Commutator Electric Machines That Does Not Require the Use of a Measuring Brush

V. S. Stukach, A. Ya. Shirulik

(Submitted by the scientific seminar of the Department of Electrical Machines)

Researchers of commutation in direct-current machines and electrical-machine builders are well acquainted with the method developed by M. F. Karasev and Suvorov for oscillographing the voltage at the trailing edge of the brushes of commutator machines. With the aid of an oscillograph, this method makes it possible to observe the distribution of sparking pulses over all commutator bars, to assess the condition of the commutator and the commutation qualities of the machine. The oscillographing circuit is shown in Fig. 1a, where

Fig. 1. Circuit for measuring the voltage between the working brush and the commutator bar moving out from under it: a) using a measuring brush; b) using a capacitive sensor

1 — working brush; 2 — additional measuring brush, installed in the plane of the trailing edge of the working brush to form electrical contact with the trailing commutator bar.

The method is widely used by commutation researchers, since it is very effective from an informational point of view, but it has not become widely used in the practice of factory testing stations. This is explained by the difficulties involved in manufacturing and installing the additional measuring brush. This brush, first, must be very narrow in the direction tangent to the circumference of the commutator, narrower than the interlamellar slot, so that it does not itself perform commutation. Such a brush is difficult to manufacture. Second, it must be installed in the plane of the trailing edge of the working brush and must provide reliable contact with all commutator bars as the latter rotates.

Third, it must be insulated from the working brush and the brush holder. Some researchers glue the measuring brush into a slot made in the body of the working brush; however, in this case the brush does not provide reliable contact with the commutator segments. This is explained by the vibration of the working brush during rotation of the commutator and by the rigid connection of the measuring brush with the working brush. The authors of the present article usually use a separate brush holder of special design for installing the measuring brush, which makes it possible to obtain reliable contact with all segments during rotation of the commutator. In this case the measuring brush is glued together from two brushes insulated from one another, one of which (the measuring one) has a thickness of 0.5–0.8 mm, while the other (the supporting one) has a thickness of 3–4 mm. The supporting brush is insulated from the brush holder. Such a method of installation can be used in research practice, but under factory conditions, when checking the commutation of production machines, it is of little use, since installing a measuring brush holder with a brush is laborious and is not possible on every machine.

The authors have developed a contactless method for forming an electrical connection with the commutator segments when oscillographing the voltage between the working brush and the trailing segments. For this purpose a capacitive sensor 2 is used (Fig. 1b), installed at the edge of the working brush 1 above the trailing commutator segment. The capacitive sensor may be made in the form of a plate of conductive material, fastened to a rod and brought up to the commutator with its plane at a distance of 0.5–1.0 mm from the commutator surface. In the simplest case, the sensor may be made in the form of a brass strip with its end bent at a right angle. This end is brought up to the commutator. Such a sensor is fastened to a clamp insulated from it, and the clamp is placed on the standard brush holder in such a way that the capacitive sensor is positioned above the commutator segment emerging from under the brush. To set the sensor at the required distance from the commutator, a spacer 0.5–1.0 mm thick may be used; it should be placed under the capacitive sensor, after which the clamp together with the sensor is lowered to the commutator until it stops. The spacer must then be removed.

The fastening clamp embraces the working brush holder of the machine. It can be easily made for a brush holder of practically any design and is easily installed on a production machine.

Fig. 2. Comparison of spark oscillograms over the entire commutator circumference obtained using a measuring brush (top) and using a capacitive sensor (bottom)

Fig. 3. Comparison of oscillograms of a separate spark pulse on one commutator segment obtained using a measuring brush (top) and using a capacitive sensor (bottom)

This makes it possible to use the method of analyzing machine commutation and the condition of the commutator surface by oscillographing the voltage at the trailing edge of the brushes under the conditions of the manufacturer’s test station.

Distortion of the shape and parameters of the signal with the new oscillographing method practically does not occur, as is evident from the oscillograms shown in Figs. 2 and 3. No strict limitations are imposed on the area of the capacitive sensor or on the accuracy of its installation above the commutator in terms of distance. The surface area of the sensor may vary from one to several tens of square millimeters. The smaller the area, the greater the amplification factor required when oscillographing the signal. The distance from the sensor surface to the commutator surface may vary from 0.2 to 5 mm; in this case the shape and parameters of the signal remain practically unchanged.

Conclusions

A simple method has been developed for oscillographing the voltage at the trailing edge of the brushes of commutator electric machines, making it possible to use more widely the method of evaluating machine commutation on the basis of oscillograms of brush sparking in the practice of test stations at electrical-machine manufacturing plants.