PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV, HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR

Vol. 284
1974

SOME RESULTS OF A STUDY OF DC MOTORS WITH VMK AT FIELD SPEED CONTROL RANGES OF 1:6, 1:8

V. A. IVANNIKOV, I. A. MILORADOV

(Presented by the seminar of the power-supply systems sector of the Scientific Research Institute of Nuclear Physics, Electronics, and Automation)

In [1, 2] it is shown theoretically and confirmed experimentally that the main factors (the main field, the active resistance of the sections, and the voltage drop at the brush contact) that influence the width and position of the sparkless zone of electric motors with valve-mechanical commutation (VMK), when the rotational speed is controlled by weakening the field of the main poles, cannot lead to the disappearance of the sparkless-operation zone or to the crossing of its boundary as the armature speed increases.

However, the experimental studies in [2] were carried out on electric motors not intended for speed control by field weakening over wide limits. The minimum speed of these motors when controlled at constant power was 750 and 1200 rpm, which did not make it possible to obtain the indicated control limits, since the upper limit of the rotational speed was restricted by the mechanical strength of the machines under study.

Below are presented the results of experimental studies of electric motors with VMK¹ with the valve portion loaded by the current of the brush set [2] and with a field speed-control range of 1:6, 1:8. The rated data of the electric motors are given in Table 1.

All the electric motors were made uncompensated; excitation was independent, with a light stabilizing winding. The independent-excitation windings were supplied from an AC network through a voltage regulator and a single-phase full-wave rectifier; the motors were of two-collector design with half the number of brush sets. Silicon diodes D215 were used as auxiliary valves. The main brushes were EG74, the auxiliary ones were MGS7N. The armature windings were wave windings. The number of sections in the windings was reduced, and the number of turns in the sections was increased by \(u_{\text{p}}\) times (the number of sections per slot \(u_{\text{p}} = 3\)).

Adjustment of commutation of electric motors

Adjustment of commutation in machines with VMK presents no difficulties if the difference in height between adjacent commutator bars does not

Table 1

...exceeds 2–3 microns, i.e., the permissible difference for ordinary series-produced machines.

The brushes were set on the geometrical neutral with reduced voltage applied to the armature winding and to the interpoles, so that the neutral corresponded to the entire brush assembly. After the brushes had been set on the neutral, according to oscillograms of current and voltage

Fig. 1. Sparkless-operation zones: a—P41, b—P43, c—P61.

on the valves, the quality of contact between the brushes and the collector plates was monitored.

To ensure sparkless operation over the entire range of rotational speeds, the final value of the magnetomotive force (number of turns) of the auxiliary poles was selected so that the sparkless zone would be symmetrical in the middle range of rotational speeds, from 800 to 1500 rpm. The zones of sparkless operation of electric motors P41, P43, and P61 in absolute units for a specified speed range are shown in Fig. 1a, b, c. For each zone of sparkless operation, the range of variation of rotational speed at the upper boundary is indicated when the armature current changes from nominal values to no-load. As can be seen, the tendency for the sparkless zones to shift with increasing rotational speed into the overcompensation region, as indicated in [1], is observed. The exceptions are the lower boundaries of the sparkless zones at rotational speeds above 2200 rpm, when the influence of mechanical factors begins to affect commutation. The decrease in the width of the sparkless zone at the lower limit of the rotational speed of motor P41 (Fig. 1a) is explained by the stronger influence, in this machine, of the main field on the upper boundary of the zone.

Fig. 2. Zones of sparkless operation when the excitation current flows opposite to the armature current in the winding of the auxiliary poles: a — P61, 1, 2 — with pulsating-current supply; b — P63.

All zones of sparkless operation, including those symmetrical at nominal armature currents, are “especially asymmetrical” at armature currents close to no-load currents, which is due to the influence of the main field and the nonlinearity of the properties of the brush contact. By reducing the magnetomotive force of the auxiliary poles by changing the number of winding turns or the size of the gap under the auxiliary poles, it is not possible to “raise” the upper boundary of the zones and ensure sparkless operation of the motors in modes close to no-load modes. “More-

“symmetrical” arrangement of the zones at low armature currents, as well as a certain weakening of an excessively strong field of the commutating poles at maximum rotational speeds, can be obtained if the independent-excitation winding of the main poles is connected in series opposition with part of the turns of the commutating-pole winding, so that the field-winding current and the armature current in the commutating-pole winding flow in opposite directions [3]. In this connection it is necessary to bear in mind that transferring the machine to the lower limit of rotational speed is associated with an increase in the

Fig. 3. Natural speed characteristics: I—P41 with a light stabilizing winding, II—P61 without a stabilizing winding

field current almost in proportion to the speed-control ratio, as a result of which the weakening of the commutating poles may be very strong. This can be avoided if, for example, as the field current is increased, the number of turns of the commutating-pole winding connected in series with the field winding is simultaneously reduced.

Figures 2a, b show the sparkless-operation zones of the P61 and P63 electric motors, obtained with the field current flowing in the commutating-pole winding opposite to the armature current. The sparkless zone of the P63 motor (Fig. 2b) at a rotational speed of 250–400 rpm was obtained with the usual connection of the field and commutating-pole windings.

Comparing the sparkless zones obtained without shunting and with shunting of the commutating-pole winding by the field current (Figs. 1, 2), preference should be given to the latter. Their position relative to the axis of cur-

of the armature at low currents are “more symmetrical.” In machine P61 (Fig. 2a), sparkless operation is thereby ensured practically when the armature current varies from \(I_n\) to no-load. The greater width of the sparkless zones of this motor in Fig. 2a is due to the fact that, when they were taken, the dimensions and arrangement of the brush elements were changed or, more precisely, the ratio of the commutation periods of the sections at the upper and lower boundaries of the zone. This indicates that in machines with VMK, if necessary, the zone of sparkless operation for given collector dimensions can easily be expanded to a value limited by the width of the commutation zone and by the current density in the brush contact.

Speed (automatic) characteristics

The natural speed characteristics of motors P41 and P61 are shown in Fig. 3. The characteristics of motor P41 were taken with a weak stabilizing winding and with the field winding connected in the usual way. The satisfactory unambiguity of the characteristics is apparently explained by the smaller displacement, in comparison with conventional machines, of the sparkless-operation zones relative to the armature-current axis when the current and rotational speed are varied.

It should be noted that the series-opposing connection of the field windings and of the additional poles, apart from weakening the additional poles, also has stabilizing properties; i.e., the slope of the speed characteristics required for stable operation of the motors in field-weakening modes can be obtained without a stabilizing winding [3, 4].

The speed characteristics of motor P63 with and without a weak stabilizing winding are examined in detail in [4] and are not given here. The speed characteristics of motor P61 (Fig. 3), whose zones are shown in Fig. 2b, were taken without a stabilizing winding, with the field windings and additional poles connected in series opposition.

Potential conditions on the collector

The reduction in the number of collector plates inevitably led to an increase in the voltage between two adjacent working collector plates. In the motors with VMK under investigation, the interlamellar voltages \(U_k\), if the field-distortion coefficient under load is taken as \(k_n = 2\), substantially exceed the maximum permissible value of this voltage recommended for DC machines with speed control by field weakening. For the fourth- and sixth-frame motors with VMK under consideration, \(U_k\) is, respectively, 90 and 70 V.

However, during operation of the experimental machines in various modes, in particular during starting, load application, and double overload, there were no cases of flashover between plates or of ring fire. This can be explained primarily by the presence of a double insulation gap between the working plates and by satisfactory commutation in modes dangerous with respect to flashover.

In our opinion, the most dangerous location, from the standpoint of the occurrence of ring fire in the machines under investigation, is the commutation zone. In this zone the interlamellar voltage is applied only to one

insulating gap, since the other insulating gap is bridged by the brush at the moment of its contact with the idle and working plates. The commutating emfs in the machines are large and, at maximum rotational speeds, may reach 100 V. Therefore, when recording sparkless zones at maximum rotational speeds, the points of the upper boundary of the zone at rated armature current were recorded only when the commutating emf did not exceed the indicated value.

Supply with pulsating current

In Fig. 2a, lines 1 and 2 show the sparkless zones recorded when the armature of motor P61 was supplied from a single-phase two-half-period uncontrolled rectifier through a smoothing inductance. The same figure shows the variation in the ripple factor of the armature current \(k_j\) as a function of the armature current. The coefficient \(k_j\) was defined as the ratio of the amplitude of the fundamental harmonic to the dc component of the armature current. As follows from Fig. 2a, the width of the sparkless zones decreases when the motors are supplied with pulsating current.

Thus, at a rotational speed of 900 rpm and rated armature current (\(k_j=0.12\)), the relative value of the positive compounding current decreases from 17.7 to 13.1%; at a rotational speed of 2100 rpm, from 16.2 to 10.1%. At the same time, although the width of the sparkless zones decreases, the character of the zones remains the same. Therefore, sparkless operation of the motors at rated load can be expected over the entire speed range, even with large pulsations of the armature current. This is also indicated by the results of a commutation check of motor P32 with a VMK (\(P_n=1.5\ \text{kW}\), \(U_n=320\ \text{V}\), \(I_n=5.7\ \text{A}\), \(n_n=1300\ \text{rpm}\)) when supplied from a controlled rectifier in a three-phase half-wave circuit. Operation of the motor in the regulating-characteristic mode up to a rotational speed of 3200 rpm was accompanied by dark commutation. The magnitudes of the current and voltage pulsations in this mode were, respectively, \(I_{\sim\max}=42.6\text{--}39.2\%\) of \(I_n\), \(U_{\sim\max}=55.3\text{--}50.5\%\) of \(U_n\), where \(I_{\sim\max}\) and \(U_{\sim\max}\) are the alternating components of the armature current and voltage.

It should be noted that the machines studied had unlaminated auxiliary poles and yoke; therefore, the degree of lag of the commutating flux was considerable.

Operation in long-duration modes

Operation with the experimental machines showed that, under normal conditions of the brush-commutator assembly with respect to vibration, the VMK circuit ensures reliable operation in long-duration modes, at least in low-power machines, to which the motors under study belong.

It is sufficient, for example, to say that the sparkless zones and speed characteristics of motor P41 were recorded after its operation in intermittent duty for a duration of 800 h, of which 135 h at \(i_y=5.5\ \text{A}\), \(n=2400\ \text{rpm}\); 500 h at \(I_y=5.7\ \text{A}\), \(n=3600\ \text{rpm}\); 155 h at \(I_y=5\ \text{A}\), \(n=600\ \text{rpm}\), as well as 12 h at a rotational speed of 650 rpm at no load. Sparking of intensity \(1 \frac{1}{2}\) points was observed only in the no-load mode.

In connection with the nonuniformity of the commutator (the presence of idle and working copper plates), nonuniform wear of the ...

plates after prolonged operation of the motor; however, measurements of the height differences between commutator segments with a profilometer, with the armature rotating, did not reveal uneven or wavy wear of the segments around the circumference of the commutator, although the wear on the commutator tracks was 0.2–0.3 mm. Between measurements the motor operated for about one thousand hours. On the remaining motors, tests of such duration were not carried out, but each of them operated in the laboratory for at least 500 h. In this case the mechanical contact of the brushes with the commutator also remained satisfactory (monitoring was carried out from oscillograms of valve currents and voltages).

Conclusions

1. Electric motors with valve-mechanical commutation make it possible to obtain the specified control range and have satisfactory commutation throughout the entire range of rotational speeds.

2. Motors with VMK can operate satisfactorily, in terms of commutation, when supplied with pulsating current.

REFERENCES

1. V. V. Ivashin, V. A. Ivannikov, I. A. Miloradov. Commutation of direct-current motors of shunt or compound excitation with VMK in the rotational-speed control mode. Izvestiya TPI, vol. 212, 1971.

2. I. A. Volkomirskii, V. A. Ivannikov, V. V. Ivashin, I. A. Miloradov. Experimental studies of the commutation of direct-current motors with VMK in the mode of rotational-speed control by weakening the field of the main poles. Izvestiya TPI, vol. 212, 1971.

3. V. A. Ivannikov, I. A. Miloradov. Direct-current electrical machine. Author’s certificate No. 351284. Bulletin of Inventions, No. 27, 1972.

4. V. A. Ivannikov, I. A. Miloradov. Stable operation of a direct-current motor without a stabilizing winding. Izvestiya TPI, vol. 242, 1972.

1. The electric motors were manufactured by the Elektromash plant (Prokopyevsk). ↩