On the Commutation of a Cross-Field Electric Machine Amplifier with a Smooth Armature
Abstract Generated abstract
This paper reports additional experimental data on the mechanical reliability and commutation behavior of a transverse-field electric machine amplifier with a smooth armature. Using a 0.7 kW, 30 V prototype with a single-turn loop armature winding secured by bandages and epoxy resin, the study compares calculated commutation emfs with oscillographed current and field curves under separate and combined brush operation, including prolonged load testing. The results indicate nearly linear current reversal, an actual commutation period about 25 percent shorter than calculated, absence of brush sparking at high current densities, and no observed mechanical deterioration of the armature winding fastening. The author argues that the smooth-armature design offers favorable commutation and mechanical properties, while noting that further work is needed to improve gain and response speed toward those of conventional transverse-field amplifiers.
Full Text
ON THE COMMUTATION OF A TRANSVERSE-FIELD EMU WITH A SMOOTH ARMATURE
A. I. SKOROSPEsHKIN
Recommended by the seminar of the Departments of Electrical Machines and General Electrical Engineering
In [1] it was noted that we had built and preliminarily investigated a transverse-field EMU with a smooth armature. It was shown that, at significant current densities under the brushes, exceeding \(40\ \text{A}/\text{cm}^2\), sparking under the brushes is absent and commutation remains exceptionally good.
The present article gives some additional data on the mechanical properties and commutation of an amplifier with a smooth armature.
In the first version of the amplifier, the loop armature winding consists of single-turn sections with a diametral pitch.
The winding is secured by means of bandages and epoxy resin. After the main characteristics had been taken—external characteristics and no-load characteristics, which do not differ from the characteristics of an ordinary transverse-field EMU, except for the gain factor and response speed (they decrease)—commutation studies were carried out with separate and combined operation of the brushes.
The amplifier has the following data:
\[ P = 0.7\ \text{kW}, \quad U = 30\ \text{V}, \quad I = 23.4\ \text{A}, \quad n = 2900\ \text{rpm}; \]
with
\[ W_c = 1, \quad r_c = 0.0014\ \Omega. \]
The calculated values of the reactive emf \(e_r\) and the rotational emf \(e_v\), respectively for the transverse and longitudinal circuits of the amplifier, according to [2], are equal to:
\[ e_{rq} = 0.016\ \text{V}, \quad e_{vq} = 0.03\ \text{V}, \]
\[ e_{rd} = 0.04\ \text{V}, \quad e_{vd} = 0.081\ \text{V}. \]
The resulting emfs are obtained, respectively, as:
\[ e_q = 0.046\ \text{V}, \]
\[ e_d = 0.121\ \text{V}. \]
The permeances for the full-pitch winding were calculated by the formulas [3]:
\[ \lambda_{\mathrm{gl}} = 4\lambda_{\mathrm{l}} \approx 0.5. \]
\[ \lambda_{\mathrm{l}} = 0.5 \frac{l_{\mathrm{l}}}{l_{\mathrm{gl}}}. \]
where
\(\lambda_{\mathrm{gl}}\) is the permeance of the winding corresponding to the active part of the smooth armature \(l_{\mathrm{gl}}\);
\(\lambda_{\mathrm{l}}\) is the permeance corresponding to the end part of the armature winding;
\(l_{\mathrm{l}}\) is the length of the end part of the armature winding, cm.
Since satisfactory commutation conditions are obtained in both the transverse and longitudinal circuits of the amplifier, the auxiliary poles from the longitudinal circuit were removed. Accordingly, the field curve of the control winding is obtained (Fig. 1) with sections (a)
Fig. 1. Field curve of the control winding.
and (b) for the large and medium slots. The serration of the curve is due to the presence of teeth on the stator.
For a long time, tests were carried out on the mechanical reliability of fastening the winding with epoxy resin. The tests were conducted for two months, 5 to 8 hours daily, at no load, at rated load, and above rated load. No mechanical changes or disturbances are observed. There are also no changes in the commutation conditions.
The commutation studies were carried out under a prolonged operating regime, with the thermal state of the commutator and armature established, and with a sufficiently stable character of the curves of the sections undergoing commutation.
The current and field curves were oscillographed at various loads. It should be noted that the current curves obtained with separate supply of the transverse and longitudinal circuits and with simultaneous operation of the brushes do not differ from one another. Therefore, the current curves shown in Fig. 2, obtained with simultaneous operation of the brushes (unit compensation), make it possible to judge the commutation conditions in both circuits. Curves 1 correspond to the transverse circuit; curves 2, to the longitudinal circuit for brushes of different polarity.
Fig. 2. Current curves with simultaneous operation of the brushes.
From curves 1 and 2 it is seen that the change in current in the commutated sections approaches a straight-line form. At the same time, determination of the actual commutation period showed that it is on average 25% less than the calculated one. This character of commutation can be explained by the fact that, in the presence of large air gaps in the zones of the large and medium slots and small section resistances, with small values of the reactive emf and the emf of rotation, the commutation conditions in an amplifier with a smooth armature are determined to a considerable extent by the resistance of the brush contact, which varies depending on the current density.
With this character of the current curves and loads, as in [1], sparking under the transverse and longitudinal brushes is absent. This circumstance indicates that the determining factors in commutation are electromagnetic in nature. Nevertheless, the requirements for the mechanical accuracy of manufacturing the commutator must not be reduced. In this connection it is necessary to take into account the reliable operation of the commutator-brush apparatus and of the machine as a whole.
Moreover, under the commutation conditions obtained in the amplifier with a smooth armature, the question of the demagnetizing action ...
reaction of the commutation currents in the transverse circuit. There remains only the demagnetizing action of the eddy-current reaction, which can be determined according to [4].
The curves (Fig. 2) correspond to the oscillogram of the resulting field pattern during the joint operation of the brushes (Fig. 3). The difference from Fig. 1 consists in the displacement and some distortion of section 1, caused by the armature reaction. Here \(a\) is the section of the large slot, \(b\) of the medium one.
Fig. 3. Field curve during joint operation of the brushes.
Thus, the investigations carried out on commutation in an amplifier with a smooth armature reveal a number of its advantages: exceptionally good commutation conditions, the possibility of increasing the current density under the brushes and in the winding, and high mechanical properties. Further work is being carried out to increase the amplification coefficient and to increase the speed of response, with the intention of bringing them closer to the values found in conventional transverse-field EMAs. Then, taking into account the advantages indicated above, it will be possible to pose realistically the question of the practical application of a transverse-field EMA with a smooth armature in various branches of the national economy.
REFERENCES
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A. I. Skorospeshkin, Yu. A. Stepanov. Transverse-field EMA with a smooth armature, Elektromekhanika, No. 10, 1965.
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Ya. S. Gurin, M. N. Kurochkin. Design of Direct-Current Machines, GEI, 1964.
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Arnold, I. L. Lakur. Direct-Current Machines, vol. 1, 1931.
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F. A. Serdyuk, A. I. Skorospelkin. Experimental determination of the reaction of commutation and eddy currents in transverse-field electric-machine amplifiers, Elektromekhanika, No. 3, 1961.