SOME ISSUES IN THE COMMUTATION OF TRANSVERSE-FIELD ELECTRIC MACHINE AMPLIFIERS

A. I. Skorospelkin

(Presented by the scientific seminar of the Electromechanical Faculty)

An analysis of the operation of transverse-field electric machine amplifiers shows that they have a number of shortcomings connected chiefly with the commutation process. These shortcomings include:

1. Difficult commutation conditions. They are due to the specific nature of the operation and design of the amplifier.

2. Possible instability of operation associated with changes in the transition resistance of the brush contact.

3. The demagnetizing effect of the commutating currents in the transverse circuit.

4. Closely connected with the indicated shortcomings are the increased weight, dimensions, and cost of the amplifier.

The shortcomings listed limit the possibility of increasing the speed, power, and current density, reducing the overall dimensions, and increasing the stability of operation of the amplifier.

Despite the presence of additional poles along the longitudinal axis, and sometimes the presence of a series winding in the transverse circuit, commutation in amplifiers is often unsatisfactory and serves as the principal limitation in the operation of the amplifier.

Satisfactory commutation is a necessary condition for the normal and prolonged operation of any direct-current machine.

Being a direct-current machine, a transverse-field electric machine amplifier has its own specific design and operating features, which are also reflected in the commutation process.

The peculiarity of commutation in amplifiers is due to the presence of transverse short-circuited and longitudinal brushes, and to the presence of several windings on the stator, each of which exerts its own influence on the process of current change in the section.

In the analytical and experimental study of commutation in transverse-field amplifiers, a possible approach is the separate investigation of commutation in the transverse and longitudinal circuits, and then under the simultaneous operation of the transverse and longitudinal brushes.

The experimental study of commutation in the transverse circuit was carried out under the following conditions:

1. The longitudinal brushes were removed.

1. In the transverse circuit, brushes are installed with a width equal to one commutator division.

2. Power to the armature winding is supplied from an external source. Oscillographic recording of the current and emf curves was carried out under prolonged operating conditions, with the commutator in an established thermal state and with stable current and emf curves.

Under these conditions, the process of current variation under the brushes in the transverse circuit will be determined by the inductance of the section, its active resistance, the transition resistance of the brush contact, and the armature field in the transverse circuit.

Figure 1 shows the current curve for \(I_q = 2a\).

It is clear from the figure that the process of current variation in the section has a distinctly retarded character. The operating conditions of the trailing edge of the brush are much more severe than those of the leading edge. The retardation of commutation is due to the inductance of the section and to the armature field. Although the magnitude of the latter is small, it has a substantial influence. Such a character of the current curve is undesirable, but it occurs because in the transverse circuit there are no additional means that alter the character of the current curve of the commutated section. With such commutation, sparking at the trailing edge of the brush and the demagnetizing action of the commutation current necessarily occur.

Fig. 1. Current curve of the transverse circuit for \(b_{\text{ш}} = b_{\text{к}}\).

The operation of the transverse brushes is eased only somewhat by the fact that the current in the transverse circuit is small. Any increase in current with this character of the curve will undoubtedly be associated with deterioration of brush operation and increased sparking.

The series winding used to improve commutation, when connected into the transverse circuit, does not change the character of the current curve of the commutated section. As the number of turns in the transverse circuit is increased, the required emf is maintained at a lower current. And only by reducing the magnitude of the commutation current is the commutation in the transverse circuit somewhat improved, while the retarded character of the current variation remains unchanged.

But simultaneously with the connection of the series winding, the time constant of the transverse circuit increases and the speed of response decreases, which is undesirable.

Figure 2 shows the EMF curves from the commutated sections. They are located in the trough of the field curve, which corresponds to the zone of the large pitch. The transverse brushes are placed in this zone. In the EMF curves, clearly pronounced peaks are observed, corresponding to the entering (peak a) and leaving (peak b) edges of the brush.

The nature of commutation can also be judged from these curves. It is evident from them that a sharper drop in current occurs at the leaving edge, which also corresponds to the considerable magnitude of the peak.

Undoubtedly, the most effective means of improving commutation is to change the character of the current curve (bringing it closer to a straight line) by external means compensating the action of the armature field and the reactive EMF.

Fig. 2. EMF curves of commutated sections in the transverse circuit.

In connection with this, an experimental setup was assembled on the basis of an EMU-12A, in which the additional poles from the longitudinal circuit were transferred to the transverse circuit. At the same time, the current and field curves were also oscillographed.

With a current in the transverse circuit \(I_q = 1.2;\ 1.5;\ 2\ \mathrm{A}\) and in the absence of current in the winding of the additional poles \((I_{\mathrm{a.p.}} = 0)\), the oscillogram of the current curve of the commutated section is shown in Fig. 3.

Fig. 3. Current curve of the transverse circuit at \(I_{\mathrm{a.p.}} = 0\).

The change in current in accordance with the curve of Fig. 3 is even more retarded in character than in Fig. 1. Sparking (very intense) occurs under the entire surface of the brush. Prolonged operation of the ampli-

with such a character of commutation in the transverse circuit is, of course, impossible.

The severe commutation conditions in this case can be explained by the fact that the gap in the transverse circuit, in the presence of additional poles, has decreased, while the commutating field at \(I_{\mathrm{d.p.}} = 0\) is absent.

By connecting the winding of the additional poles it appears possible to regulate the commutation conditions in the transverse circuit, i.e., to change the character of the current and field curves. At a certain value of the current in the transverse circuit, rectilinear commutation is obtained, with all its obvious advantages. With it, sparking under the transverse brushes is almost absent. Under the trailing edge, one or two small sparking points are observed, the appearance of which can be explained by the influence of mechanical factors.

Thus, with the aid of additional poles in the transverse circuit it appears possible to obtain satisfactory commutation conditions. In this case, an increase in current load is possible.

In [1] the results are presented of an investigation of commutation in the longitudinal circuit with the transverse brushes open. It is established that the commutation conditions in the longitudinal circuit are determined mainly by the compensating winding. A range of compensation adjustment is determined at which satisfactory commutation takes place.

In this connection, an investigation was carried out of commutation in the longitudinal circuit with the additional poles removed and, consequently, with an increased gap along the longitudinal axis. The current curve in this case, with the compensating winding disconnected, takes a form analogous to the curve of Fig. 1. Although commutation in the longitudinal circuit also remains retarded, the operating conditions of the longitudinal brushes are considerably facilitated in comparison with those presented in [1].

When the compensating winding is connected and with a certain degree of compensation of the longitudinal armature reaction, the character of commutation in the longitudinal circuit changes noticeably. With undercompensation of the order of 15%, commutation becomes almost rectilinear and the brushes operate without sparking. When the current is increased by a factor of 1.5–2, commutation also remains satisfactory.

A rectilinear change of current in the section corresponds to the field curve (Fig. 4). Commutation peaks are absent from the curve. The serration of the curve is explained by the presence of teeth on the stator.

Figure 5 shows the field curve of the compensating winding obtained with no brushes on the commutator. The depressions of the curve correspond to the increased gaps of the longitudinal circuit.

Thus, in the longitudinal circuit, obtaining satisfactory commutation in the absence of additional poles with the aid of only the compensating winding is entirely possible.

To verify the results obtained in the separate investigation of commutation in the transverse and longitudinal circuits, an investigation was carried out with simultaneous operation of the transverse and longitudinal brushes. The specific feature of amplifier operation consists in the fact that currents flow through the armature winding which are due to the field of the control winding and to the field of the transverse armature reaction. This creates a nonuniform current distribution over the armature winding.

The investigation of the conventional amplifier was carried out under the following conditions:

1. Brushes of width equal to the commutator division were set on the neutral.

Fig. 4. Field curve in the neutral zone during longitudinal brush displacement.

Fig. 5. Field curve of the compensating winding.

1. Normal compensation is adjusted by means of a shunting resistance.

2. In the transverse circuit the current is set to \(I_q = 1.2a\), and in the longitudinal circuit \(I_d = 4a\).

Figure 6 presents the current curves in the transverse and longitudinal circuits of the amplifier. In this case curve I fully corresponds to the curve in Fig. 1 for the transverse circuit, obtained in a separate investigation of commutation. Such correspondence should in fact occur, since in both cases the nature of commutation is determined mainly by the parameters of the commutated loop and by the armature field, which remain the same both in the separate investigation of commutation and in the joint operation of the transverse and longitudinal brushes.

Fig. 6. Current curves during simultaneous operation of the transverse and longitudinal brushes.

It is only necessary to bear in mind that, during the joint operation of the transverse and longitudinal brushes, in the circuit of the section commutated by the transverse brushes there is a component \((i_{ad})\) of the current of the longitudinal circuit. The influence of this component will be discussed somewhat below.

Curve II (Fig. 6) approaches a linear dependence. Current curves of the same form were obtained for normal compensation when studying commutation only in the longitudinal circuit (with the transverse brushes open). This is in fact what should occur. The commutation conditions in the longitudinal circuit of an ordinary amplifier are determined by the parameters of the commutated loop, the armature field, the field of the compensating winding, and, to some extent, the field of the interpoles. All these factors are present both in the separate and in the joint operation of the transverse and longitudinal brushes. Here it is also necessary to bear in mind that in the circuit of the section commutated by the longitudinal brushes (during simultaneous operation of the transverse and longitudinal brushes) there is a component \((i_{aq})\) of the current of the transverse circuit.

The correspondence of the curves in Fig. 6 to the curves obtained in a separate investigation may serve as confirmation of the conclusions drawn earlier regarding commutation conditions in the transverse and longitudinal circuits of the amplifier.

Obviously, when investigating the commutation process in electric-machine amplifiers with a transverse field, it appears possible to carry out a separate analysis of commutation in the transverse and longitudinal circuits, and then to proceed to more complicated conditions—operation under load.

An investigation was carried out of commutation during operation under load in an amplifier with displaced auxiliary poles, i.e., when the auxiliary poles were removed from the longitudinal circuit and installed in the transverse one. In this case, in the transverse circuit of the amplifier, rectilinear commutation was adjusted by means of the auxiliary poles, and in the longitudinal circuit—by means of the compensating winding. In this arrangement the transverse and longitudinal brushes operate almost without sparking, which confirms the results obtained in the separate investigation of commutation.

The influence of \(i_{ad}\) and \(i_{aq}\) in the corresponding circuits can be checked on the basis of the propositions of the classical theory of commutation.

For the commutated loop of the transverse circuit (Fig. 7), for \(b_{\text{п}} = b_{\text{к}}\), the equation can be written as:

Fig. 7. Diagram of the commutated loop of the transverse circuit.

\[ r_c(i_q + i_{ad}) + i_2 r_2 - i_1 r_1 = e_{zq}. \tag{1} \]

For the commutated loop of the longitudinal circuit (Fig. 8), for \(b_{\text{п}} = b_{\text{к}}\), the equation has the form

\[ r_c(i_d + i_{aq}) + i_2 r_2 - i_1 r_1 = e_{zd}. \tag{2} \]

Here

\[ i_1 = i_{aq} - i_q,\quad i_1 = i_{ad} - i_d,\quad r_1 = r_{\text{п}}\frac{T_{\text{к}}}{t},\quad e_{zq} = e_r + e_q; \]

\[ i_2 = i_{aq} + i_q,\quad i_2 = i_{ad} + i_d,\quad r_2 = r_{\text{п}}\frac{T_{\text{к}}}{T_{\text{к}} - t},\quad e_{zd} = e_r + e_{\text{к}}. \]

In the figures and in the equations the following notation is adopted:

\(i_q, i_d\) — currents of the commutated sections in the transverse and longitudinal circuits;

\(i_{aq}, i_{ad}\) — currents of the parallel branches corresponding to the transverse and longitudinal circuits;

\(r_1, r_2\) — brush-contact resistances corresponding to the entering and leaving edges of the brushes;

\(e_r\) — reactive emf;

\(e_q\) — emf from the armature field in the transverse circuit;

\(r_{\mathrm{ш}q}, r_{\mathrm{ш}d}\) — resistances corresponding to the full width of the brush in the longitudinal and transverse circuits;

\(e_{zq}, e_{zd}\) — resultant emfs in the transverse and longitudinal circuits;

\(i_1, i_2\) — currents corresponding to the entering and leaving parts of the brush;

\(T_k\) — commutation period;

\(t\) — time from the beginning of commutation;

\(r_c\) — section resistance;

\(b_{\mathrm{ш}}, b_k\) — brush width and commutator-segment width.

After transforming and solving equations (1), (2), the following expressions are obtained for the currents \(i_q\) and \(i_d\):

Fig. 8. Circuit diagram of the commutated contour of the longitudinal circuit.

\[ i_q = \frac{e_{zq}(T_k-t)t}{r_c(T_k-t)t + r_{\mathrm{ш}q}T_k^2} - i_{aq}\, \frac{2r_{\mathrm{ш}q}T_k t - r_{\mathrm{ш}q}T_k^2} {r_c(T_k-t)t + r_{\mathrm{ш}q}T_k^2} \tag{3} \]

\[ i_{aq} = -\frac{r_c(T_k-t)t}{r_c(T_k-t)t + r_{\mathrm{ш}q}T_k^2}; \]

\[ i_d = \frac{e_{zd}(T_k-t)t}{r_c(T_k-t)t + r_{\mathrm{ш}d}T_k^2} - i_{ad}\, \frac{2r_{\mathrm{ш}d}T_k t - r_{\mathrm{ш}d}T_k^2} {r_c(T_k-t)t + r_{\mathrm{ш}d}T_k^2} \tag{4} \]

\[ i_{ad} = -\frac{r_c(T_k-t)t}{r_c(T_k-t)t + r_{\mathrm{ш}d}T_k^2}. \]

The expressions (3) and (4) are solved for the following data:

\[ \begin{aligned} r_c &= 0.1\ \Omega,\\ L_c &= 1.48\cdot 10^{-4}\ \text{H},\\ T_k &= 4.61\cdot 10^{-4}\ \text{s}, \end{aligned} \qquad \begin{aligned} i_{aq} &= 0.6\ \text{A},\\ i_{ad} &= 2\ \text{A},\\ e_{zq} &= 0.58\ \text{V},\\ r_{\text{sh}q} &= 0.84\ \Omega,\\ r_{\text{sh}d} &= 0.61\ \Omega. \end{aligned} \]

The solution of equation (4) is carried out under the condition

\[ e_{zd} = e_r - e_k = 0. \]

Figure 9 presents the curves of the currents \(i_q\) and \(i_d\), obtained with allowance for \(i_{ad}\) and \(i_{aq}\) in the corresponding circuits. Of these, curves (1) are obtained with allowance for \(i_{ad}\) and \(i_{aq}\), and curves (2) without allowance for \(i_{ad}\) and \(i_{aq}\).

Fig. 9. Calculated current curves allowing for simultaneous operation of the transverse and longitudinal brushes.

Comparison of the curves shows that the components \(i_{ad}\) in the commutated contour of the transverse-circuit section and \(i_{aq}\) in the contour of the longitudinal circuit exert an accelerating effect on the change of current in the section.

In the figure, region I corresponds to the zone of the transverse brushes, and region II to the zone of the longitudinal brushes.

The change of current in the transverse circuit is of a retarded character. Moreover, this retardation is somewhat reduced owing to the action of \(i_{ad}\).

In the longitudinal circuit, an almost rectilinear change of current is set up. In this case curve (1) corresponds to a more rapid change of current owing to the action of \(i_{aq}\).

The same can be seen in a qualitative comparison of the current curves obtained in a separate investigation of commutation (Fig. 1) and in the joint operation of the transverse and longitudinal brushes (Fig. 6).

Thus, when carrying out a separate investigation of commutation in the transverse and longitudinal circuits under simultaneous operation of the transverse and longitudinal brushes, one should take into account the accelerating influence of the currents \(i_{ad}\) and \(i_{aq}\) in the corresponding commutated circuits. It should only be borne in mind that the accelerating action of the currents \(i_{ad}\) and \(i_{aq}\) is comparatively small.

On the whole, from the results of the work the following conclusions may be drawn:

1. In electric-machine amplifiers with a transverse field, a separate investigation of commutation in the transverse and longitudinal circuits is possible, with transition to the more complicated case of simultaneous operation of the transverse and longitudinal brushes.

2. In electric-machine amplifiers with a transverse field, satisfactory commutation can be obtained in the transverse circuit by means of additional poles, and in the longitudinal circuit by means of a compensating winding.

3. The components \(i_{ad}\) and \(i_{aq}\) in the corresponding circuits of the commutated sections exert a comparatively small accelerating action.

REFERENCES

A. I. Skorospelkin. Some results of an experimental investigation of the commutation process in an EMA with a transverse field. Elektromekhanika, No. 11, 1959.

Received by the editors
in June 1962.