ON IMPROVING THE COMMUTATION OF ELECTRICAL MACHINES BY MEANS OF VALVE DEVICES

I. I. Ivashin, I. A. Miloradov

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

The conditions of commutation determine to a considerable extent the quality of operation of a direct-current machine, its design, and its geometrical dimensions. Despite the enormous number of works and extensive investigations in the field of commutation, a number of questions remain unresolved both theoretically and practically. This applies above all to questions of eliminating sparking at the commutator, especially in machines operating under sharply varying load.

As is known, one of the main causes of sparking at the commutator is the interruption of the residual commutation current at the trailing edge of the brush. Sparking at the leading edge of the brush is observed much less often, and it is not difficult to eliminate.

Fig. 1a. E. Ettinger’s commutation circuit:
1, 1′, 2, 2′—thyratrons; 3–4—commutators; 5—armature.

The operating conditions of the trailing edge of the brush can be considerably facilitated if it is possible to de-energize the commutator segment leaving the brush at the final instant of commutation. This can be achieved by using valve devices for commutation.

Figure 1a shows a commutation circuit for a direct-current machine using valve devices, proposed by E. Ettinger [1].

The machine has two commutators; moreover, the brushes on each commutator do not short-circuit the commutator segments to one another, since they are narrower than the insulation between them. The beginnings of the armature sections are connected to one commutator, and the ends to the other. During commutation the sections are short-circuited through two brushes and two valves. The principle of operation of the circuit is clear from the figure: the commutating current is interrupted by the valve earlier (at the moment when it passes through zero) than the contact between the commutator segment and the brush is broken. A circuit by N. S. Klimov, shown in Fig. 1b [1], is constructed according to an analogous principle. Each brush of the machine is divided into two parts connected to one another through valves. The width of the brush is less than the thickness of the insulation between commutator segments. The width of the commutator segment and the distance between the brushes are selected so that the brushes overlap by one commutator segment.

Both circuits can be designed in such a way that the machine will not spark over a wide load range up to a short circuit; for this it is sufficient to provide a regime of strong overcommutation at rated current.

Fig. 1b. Commutation circuit of N. S. Klimov:
1—armature, 2–3—brushes, 4–5—valves, 6—insulating segment, 7—working commutator segment

Disadvantages of the circuits presented:

1. The valves must be rated for the full current of the machine.

2. When the brushes short-circuit the commutator segments, the transfer of current from one brush to the other occurs abruptly, which may be a cause of sparking.

A circuit with an auxiliary brush installed near the trailing edge of the main brush and connected to the main brush through a valve [2], though correct in its concept, is not capable of ensuring sparkless operation of the machine. Indeed, spark-extinguishing circuits with a valve shunting an electrical contact are widely used in apparatus construction and were used in their time to improve the commutation of mechanical current rectifiers. The application of such circuits will undoubtedly make it possible to improve substantially the commutation of DC machines. But their use in electrical machines is possible only if a number of design requirements are met; for example, the width of the auxiliary brush and the mutual arrangement of the brushes must be quite definite [3]. In work [2] these questions are not resolved to any extent; therefore the circuit presented will not give the expected effect, above all because the auxiliary brush is wider than the insulation between the commutator lamellae. This leads to the auxiliary brush short-circuiting the sections of the armature winding, and therefore its sparkless operation is impossible. It should be noted that the explanation of the circuit operation given in [2] does not correspond to the processes actually taking place.

Figure 2a presents a valve-mechanical commutation circuit free from some of the shortcomings inherent in the circuits described above. The idea of the circuit is that the functions of passing the current and interrupting it, which in the usual commutation circuit are performed by the brush, are divided between the brush and an auxiliary valve connected pa—

in parallel with the main brush. Commutation of the current from the main brush into the auxiliary valve circuit is carried out forcibly by means of a three-winding saturating reactor.

The saturating reactor \(D_2\), connected in series with the main commutator segment, and the reactor \(D_1\), by means of which the current is transferred into the auxiliary circuit with the valve, are made of magnetically soft steel with a rectangular hysteresis loop and low remagnetization losses.

The main commutator segment, the auxiliary commutator segment, and the reactors \(D_1\) and \(D_2\) are located on the rotating part of the machine; the main brush, the auxiliary brush, and the valve are on the stationary part of the machine.

On reactor \(D_1\) there are three windings: the main winding \(W_1\), the additional winding \(W_2\), and the premagnetizing winding.

The circuit operates as follows: suppose that the current in the coil of the commutator segment decreases from its maximum value to zero. If the brush were to interrupt the current circuit at the moment when it passed through zero, sparking would not occur. But, as is known, there is always over- or undercommutation current, which in fact is what the brush interrupts, and this is one of the main causes of sparking.

Fig. 2a. Commutation circuit with saturating reactors. \(BKP\) — auxiliary commutator segment, \(BSh\) — auxiliary brush, \(GKP\) — main commutator segment, \(GSh\) — main brush, \(V\) — valve.

If reactor \(D_1\) is premagnetized so that, at current \(i_1 = I_1\) (Fig. 2b), it begins to remagnetize in winding \(W_1\), then a voltage \(U_2\) will be induced in winding \(W_2\), which is applied to the valve in the forward direction. In this case the winding is closed through the auxiliary commutator segment, the auxiliary brush, the valve, the main commutator segment, the main brush, and reactor \(D_2\).

In a first approximation it may be assumed that winding \(W_2\) is short-circuited, i.e., its flux linkage must remain constant, and this means that reactor \(D_1\) temporarily ceases to be remagnetized.

During this time a current \(i_v\) flows in the circuit of winding \(W_2\); under its action the current \(i_1\) through the main brush decreases from the value \(I_1\) to zero (time interval \(t_2 - t_1\)). If a saturating reactor \(D_2\) were not connected in series with the main commutator segment, then the current \(i_1\) through the main brush would not only decrease to zero, but would also change direction.

But reactor \(D_2\) is remagnetized at zero current, and consequently the current through it and through the main commutator segment cannot change direction until the magnetic circuit of reactor \(D_2\) has been remagnetized.

Together with the change of current in the main commutator segment from \(I_1\) to zero, the current in the valve and in the auxiliary commutator segment increases from zero to \(I_2\). At time \(t_2\), reactor \(D_2\) begins to be remagnetized and reactor \(D_1\) continues to be remagnetized, since at this moment the circuit short-circuiting the turns \(W_2\) is opened.

During the interval \(t_1 - t_2\), the main commutator segment is de-energized, since reactor \(D_2\) is being remagnetized and the main brush can interrupt the circuit without a spark.

When the current passes through zero, i.e., at the instant \(t_4\), the valve locks the circuit, after which the auxiliary brush breaks the contact auxiliary brush—auxiliary commutator plate, i.e., when this circuit has already been practically opened by the valve.

Fig. 2b. Change of current during commutation according to the circuit of Fig. 2a

Thus, the commutation process is divided into a number of periods:

1. The current passes through winding \(W_1\), choke \(D_1\), choke \(D_2\), and the main brush, \(t<t_0\).

2. The current passes in the same way as for \(t<t_0\), but choke \(D_1\) begins to be remagnetized, \(t_0<t<t_1\).

3. The current passes both through the main and through the auxiliary brushes, \(t_1<t<t_2\).

4. The current passes through windings \(W_1\) and \(W_2\), choke \(D_1\), the auxiliary brush, and the valve. Both chokes are remagnetized, \(t_2<t<t_3\).

5. The current passes in the same way as for \(t_2<t<t_3\), but choke \(D_1\) has completed remagnetization; the main-brush circuit is de-energized and ready for sparkless interruption, \(t_3<t<t_4\). At \(t=t_p\) the contact “main brush—main commutator plate” is broken.

6. At \(t=t_4\) the valve locks the circuit; the auxiliary brush and the auxiliary commutator plate are de-energized. At \(t=t_5\) the auxiliary brush breaks the de-energized valve circuit.

When the main circuit is opened, a voltage is applied to the contact gap “main brush—main commutator plate” equal to the sum of the voltage drops across the active resistance of winding \(W_2\), the auxiliary brush, and the valve. All these voltages are very small in magnitude, are active in character, and cannot lead to sparking under the main brush.

To carry out commutation according to the scheme presented, two commutators are required. The main commutator is designed for a current load of the order of 85–90%; the auxiliary commutator for a load of 10–15%. It is obvious that the valve must also be rated for a load of 10–15% of nominal. The number of valves and auxiliary brushes (for nonreversible machines) is equal to the number of poles of the machine. On poles of different polarities the circuit differs by the inclusion of the valve and of the bias winding of choke \(D_1\).

Valve-mechanical commutation circuits make it possible to significantly increase the reactive electromotive force of commutation; and since the machine will not spark in practically all operating modes, the voltage between commutator segments can also be increased.

An increase in the reactive e.m.f. and in the permissible voltage between commutator segments will make it possible to raise the limiting power, reduce the dimensions, and simplify the design of direct-current machines.

REFERENCES

1. N. S. Klimov. On improving commutation conditions in direct-current machines. Elektrichestvo, 1959, No. 8.

2. I. A. Didorenko, I. M. Kovtun. Expansion of the sparkless commutation zone of direct-current electrical machines. Vestnik elektropromyshlennosti, 1957, No. 10.

3. G. T. Vasil’eva. Electrical machine. USSR Author’s Certificate No. 79604, class 21 d¹, 39.