DEVICE FOR OBJECTIVE EVALUATION OF COMMUTATION IN DIRECT CURRENT MACHINES

A. I. Skorospeshkin, E. K. Damm

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

Among the existing methods for the objective evaluation of commutation, the photoelectric method and the method based on analysis of the voltage between the trailing edge of the brush and the commutator deserve the greatest attention.

It is clear that the photoelectric method can be applied only to a sparking machine. If the machine does not spark, however, its commutation can no longer be judged, although it may be on the verge of sparking. In addition, when using the photoelectric method it is impossible to draw a conclusion regarding the nature of commutation in the given machine. This, in particular, explains why, in the study of commutation, increasing preference is given to direct measurement of the voltages under the trailing edge of the brush.

An instrument for objective evaluation of commutation was proposed earlier, separately measuring and summing the voltage pulses under the trailing edge of the brush corresponding to undercommutated and overcommutated sections [1]. The instrument is convenient for determining the nature of commutation and makes it possible to obtain initial data for recalculating the parameters of additional poles for optimal commutation.

A shortcoming of the instrument is that with its aid it is impossible to determine the commutation stability of the machine. The machine may be adjusted for optimal commutation, which the instrument will indicate, and at the same time operate with sparking. It is difficult to judge commutation stability or the intensity of sparking from the integral value of the measured voltage, since this quantity increases greatly with an increase in the power of the machine. Therefore, a clearer criterion is needed for evaluating the commutation stability of machines.

It is known that one of the main causes of brush sparking is the presence in the section of an unreversed current toward the end of the actual commutation period. A rapid reversal of current at the end of the commutation period leads to a sharp change in the flux linkage of the commutated section, and under the trailing edge of the brush a voltage pulse then arises which, upon reaching a certain magnitude, causes an electric discharge. In this connection it appears possible to judge the commutation stability of direct current machines by the amplitude value of the voltage under the trailing edge of the brush.

The readings of an instrument that records the amplitude value of the measured voltage will not depend on the number of sparking commutator bars, which is not a shortcoming of the method, since, first, in machines with a satisfactory mechanical condition of the commutator-and-brush assembly a sufficient number of bars begin to spark simultaneously, and second, although the intensity of visible sparking does depend on the number of sparking commutator bars, the quality of commutation does not depend on this. In other words, the machine is equally poor whether several bars spark or all of them do.

Using the amplitude value of the voltage under the trailing edge of the brush as a criterion for the onset of sparking, one can evaluate the commutation stability of the machine. In this case, by commutation stability, in our opinion, one should understand the ratio of the actual peak value of the voltage under the trailing edge of the brush to a certain value, constant for brushes of the given grade and the given polarity, corresponding to the onset of sparking.

Combining this method of evaluating commutation with a method based on measuring integral voltage values makes it possible to investigate the state of commutation of machines more comprehensively.

We have developed an instrument that makes it possible to measure both the integral and the amplitude values of voltage pulses under the trailing edge of the brush. Tests of it have shown that, in addition to the possibility of determining the character of commutation, it makes it possible to construct zones of sparkless operation and to determine the commutation stability of the machine.

Figure 1 shows the zones of sparkless operation of the P-51M machine, one of which was obtained by the visual method and the other with the aid of the instrument. The mean lines of the zones practically coincide; the difference in the width of the zones is explained by the excessively small value of the selected critical voltage. With the aid of the instrument the zone was taken in the usual way; only the excitation current of the additional poles was not recorded—

Fig. 1. Zones of sparkless operation of the P-51M machine. 1 — zone obtained by the visual method; 2 — zone obtained with the aid of the instrument.

of the poles corresponding to the appearance of sparking, and the excitation current corresponding to an increase in the amplitude value of the voltage to the critical magnitude.

On the whole, an instrument that measures the integral and amplitude values of the voltage under the trailing edge of the brush can also be used to determine the influence of the machine operating conditions, the brush grade, and the mechanical condition of the commutator–brush assembly on the character and quality of commutation.

REFERENCES

1. M. F. Karasev, V. P. Paramzin, I. V. Senkevich. Spark indicator. Transactions of OMIIT, no. 54, 1965.