EXPERIMENTAL STUDY OF A SYNCHRONOUS GENERATOR OF PERIODIC PULSES

G. A. SIPAILOV, A. V. LOOS, Yu. I. PAVLOV

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

Over the last decade, in various fields of technology, pulse methods have become increasingly widespread; these are characterized by the discontinuous delivery of individual portions of energy (energy pulses). These methods have found the broadest application in radar, pulse radio communication and television equipment, in telemetering, in computers, and in many other devices of automation and telemechanics.

However, periodic high-current pulses of relatively large energies are at present used mainly in electrical-discharge machining of metals and in pulse welding. To supply electrical-discharge machines, dozens of types of periodic-pulse generators have been developed in the USSR and abroad.

One of the first high-current pulse generators, used as early as the dawn of radio engineering, was the spark relaxation generator of alternating-sign pulses of the RC type, employed by B. R. and N. I. Lazarenko in 1943 for dimensional electro-spark machining. In 1939 M. S. Mikhailov-Mikulinskii [1] proposed a synchronous generator of alternating-sign pulses, which could be used for high-current circuits. In 1952–1953 Yu. V. Mordvinov [2] proposed an inductor generator of asymmetrical pulses for electrical-discharge machining.

In 1951 the first machine generator of unipolar pulses [3], based on the principle of unipolar induction, was proposed.

In 1952, commutator machine generators of unipolar pulses of the MGI type appeared; on their basis, in 1955–1956, serial production of machine generators began at the KhEMZ plant and continues to the present time.

A new stage in the improvement and enhancement of the characteristics of pulse generators began with the advent of semiconductor technology. Along with static generators using semiconductor elements, machine pulse generators continued to be improved [4].

Thanks to the synthesis of machine generators with semiconductor elements, it is possible to obtain pulse sources with characteristics unattainable when using only machine generators or only semiconductor generators.

However, in most machine generators, in order to obtain the pulsed nature of the e.m.f., narrow poles and concentrated windings are used, which reduces the utilization of active materials.

The production of periodic pulses from a synchronous generator is also possible with the usual geometry of the toothed zone [5, 6]. The principle of obtaining a pulsed e.m.f. is based on the regime of an incomplete two-phase short circuit. Figure 1 shows the schematic diagram of a synchronous generator of periodic pulses. Phases C and B can be short-circuited to one another by means of commutator K-1. Phase A is connected to the load \(R_{\text{н}}\) by commutator K-2. On the rotor there is a field winding and a damping winding along the \(q\)-axis, the ends of which are brought out to slip rings.

Fig. 1. Schematic diagram of a synchronous generator of periodic pulses

It is known that, in a two-phase short circuit, the pulsating magnetomotive force of the stator winding creates not only a direct and a reverse synchronously rotating field, which, with an asymmetrical rotor, induces a current of double frequency in the field winding and a current of the third harmonic in the stator winding. These currents, in turn, cause the appearance in the field winding of a series of even harmonic current components and in the stator winding of a series of odd harmonic current components. If an aperiodic component is present in the stator current, then it will produce odd harmonics in the field winding and even harmonic components of current in the stator winding. The current of the field winding induces in the free phase of the stator an e.m.f., the value of which at the initial instant after closing is determined by formula (1). [7]

\[ e_A=-E_{0m}\left[(1+b)(\sin x-3b\sin 3x+5b^2\sin 5x\pm \ldots)+ \right. \]
\[ \left. +2b\sin\alpha(2\cos 2x-4b\cos 4x+6b^2\cos 6x\pm \ldots)\right], \tag{1} \]

where \(x=\omega t+\alpha\);

\[ b = \frac{\sqrt{x_d} - \sqrt{x'_d}}{\sqrt{x_d} + \sqrt{x'_d}} . \]

All even and odd harmonics are present in the curve of the emf \(e_A\), owing to which it has the form of sharp peaks.

In the transition to the steady-state regime, the even harmonics disappear, and the emf of phase \(A\) is expressed by formula (2).

\[ e_A = - E_{0m} \frac{\sqrt{x'_d}}{\sqrt{x_d}} \left[(1+b)(\sin x - 3b \sin 3x + 5b^2 \sin 5x \pm \ldots)\right]. \tag{2} \]

In this case the emf curve has a sharply pronounced pulsed character. When a pulsed emf is connected to a load \(R_n\), a pulsed current can be obtained in the load.

To verify the theoretical propositions, an experimental model of a synchronous generator of periodic pulses was created on the basis of the AK51/4 motor, and certain relationships were clarified.

Figure 2a shows an oscillogram of the emf \(e_A\) in the steady-state regime with phases \(C\) and \(B\) short-circuited. The maximum value of the emf exceeds the no-load emf \(E_{0m}\) by \(\sqrt{x_d}/\sqrt{x'_d}\) times as in [7]. For \(x'_d = 0.2x_d\), the maximum overvoltage of phase \(A\) is equal to \(2.24 E_{0m}\).

Fig. 2. Oscillograms: a) emf \(e_A\); b) emf \(e_A\); c) current \(i_{\text{load}}\), obtained according to the circuit without diodes D1 and D2; d) current \(i_{\text{load}}\), obtained according to the circuit with diodes.

If a diode D-1 is inserted into the circuit closing phases C and B (shown by a dashed line in Fig. 1), then an overvoltage in phase A will occur in each half-period corresponding to the conducting state of the diode; the emf pulses \(e_A\) will have an almost unipolar character. An additional advantage of such a circuit is that the sudden two-phase short-circuit condition is periodically repeated. The maximum overvoltage of phase A exceeds the no-load emf \(E_{0m}\) by

\[ \left(\frac{2x_d}{x'_d}-1\right) \]

times [7]. For the case \(x'_d=0.2x_d\), the maximum phase overvoltage is equal to \(9E_{0m}\). The oscillogram of the emf \(e_A\) for this case is shown in Fig. 2,b. The pulse repetition frequency is reduced by a factor of two compared with the frequency of alternating-sign pulses.

If commutator K-2 is closed, a pulsed current will flow through the load \(R_n\). However, when the resistance \(R_n\) is decreased and the load current \(I_{\text{load}}\) is increased, the waveform of the current pulses is distorted, since the short-circuit condition becomes more symmetrical and the content of higher harmonics in the emf curve \(e_A\) decreases. In Fig. 3, curve 1 shows the dependence of the current-pulse amplitude factor

\[ k_a=\frac{I_{m\,\text{load}}}{I_{\text{load}}} \]

on the relative load resistance \(R_n/x_d\), and curve 2 shows the dependence of the ratio \(I_{\text{load}}/I_k\) on \(R_n/x_d\), where

\(I_{m\,\text{load}}\)—amplitude of the current pulses;
\(I_{\text{load}}\)—rms value of the pulsed current in the load;
\(I_k\)—rms value of the current in the short-circuited phases.

Fig. 3. Dependence of \(I_{m\,\text{load}}/I_{\text{load}}\) and \(I_{\text{load}}/I_k\) on \(R_n/x_d\), respectively for the circuit without diodes (1, 2) and with diodes (3, 4)

During the experiment, the rms value of the current \(I_k\) was maintained constant.

Figure 2,c presents an oscillogram of the current pulses at

\[ I_{\text{load}}/I_k=0.7. \]

The circuit using a diode in the circuit that short-circuits phases C and B makes it possible to obtain clearly defined current pulses in the load at lower values of \(R_{\mathrm{н}}/x_d\), since, under sudden short-circuiting, the content of higher harmonics in the emf curve of the free phase is higher.

If, in this circuit, one more diode \(D\)-2 is inserted into the load circuit so that the load current is switched off during the half-period corresponding to the blocked state of diode \(D\)-1, then the current pulses will have the form shown in Fig. 2, г. In Fig. 3, curves 3 and 4 show, respectively, the dependences \(I_{m\,\mathrm{load}}/I_{\mathrm{load}}\) and \(I_{\mathrm{load}}/I_k\) on \(R_{\mathrm{н}}/x_d\). Comparing curves 1 and 3, it can be seen that the amplitude coefficient of the pulses obtained with the circuit including diodes \(D\)-1 and \(D\)-2 is higher. The pulse shape is not distorted when the load current is increased. Curves 2 and 4 coincide, i.e., the rms values of the currents are equal in both experiments, and no reduction in the delivered energy occurs, despite the twofold lower pulse repetition frequency in the second experiment.

Adjustment of the parameters of the emf (current) pulses is performed by introducing an adjustable inductance \(L_p\) into the circuit of winding \(D_q\). When the value of \(L_p\) is varied over wide limits, the degree of rotor asymmetry changes and, consequently, so does the content of higher harmonics in the emf curve.

A similar result is obtained when the composition of the higher harmonics in the excitation winding is limited by introducing an adjustable inductance \(L_p\) into its circuit. A good result can be obtained by limiting the composition of the higher harmonics in the circuit of the short-circuited phases C and B, but in this case a substantially larger value of \(L_p\) is required.

In the tested installation, the duty factor of alternating-sign pulses was adjusted from 1 to 4.5; that of unipolar pulses—from 1 to 10. The frequency of alternating-sign and unipolar pulses obtained with full-wave rectification is

\[ f_{\mathrm{p}} = 2pn \ \text{cps}. \]

The frequency of unipolar pulses obtained with the circuit using a diode in the short-circuiting circuit is

\[ f_{\mathrm{p}} = pn \ \text{cps}. \]

Frequency adjustment is performed by changing the rotational speed. The maximum pulse duration, at minimum pole number and a rotational speed of 50 rev/sec, does not exceed \(8\text{–}10\) msec.

A synchronous generator of periodic pulses can find application in electroerosion machining of metals and especially in pulse welding. Since in welding the generator operates into a spark gap having low resistance, it is more expedient to obtain pulsed current by the circuit with two phases short-circuited through diode \(D\)-1, which gives a more sharply defined pulse shape at reduced load resistance. For comparison, Fig. 2 gives oscillograms of current pulses obtained with the diode circuit (Fig. 2, г) and with the circuit without a diode (Fig. 2, в) at the same load.

The use of the diode circuit has the further advantage that, in order to obtain the same average power in the load, the circuit with the diode requires a substantially smaller excitation current. This is explained by the use, in the circuit with the short-circuiting diode, of the periodic sudden-short-circuit mode. The additional power is taken from the shaft of the primary motor.

The usual geometry of the tooth zone of the pulse generator makes it possible to obtain utilization of active materials no lower than in ordinary sinusoidal-current generators.

REFERENCES

1. M. S. Mikhailov-Minkulinskii. Pulse-current generator. Dissertation, Moscow Power Engineering Institute, 1939.

2. Yu. B. Mordvinov. Pulse synchronous generator, class 21 d², 5, inventor’s certificate No. 95095. Bulletin of Inventions, 25/VII 1954, No. 5.

3. A. L. Livshits. Unipolar direct-current generator, class 21 d, 6, inventor’s certificate No. 104347. Bulletin of Inventions, 25/XII 1956, No. 10.

4. A. L. Livshits, N. S. Rogachev, M. Sh. Otto. Pulse generators. Moscow, “Energiya,” 1970.

5. A. V. Loos, G. A. Sipaylov et al. Generator of adjustable periodic pulses. Class H 02K 39/00, inventor’s certificate No. 376859. Bulletin of Inventions, 05/IV 1973, No. 11.

6. G. A. Sipaylov, A. V. Loos et al. Generator of periodic, unipolar pulses. Class H02K, inventor’s certificate No. 375738. Bulletin of Inventions, 22/III 1974, No. 16.

7. K. P. Kovach, I. Rats. Transient processes in alternating-current machines. Moscow–Leningrad, GEI, 1963.