Some Features of Pulsed Energy Conversion in a Multisection Induction-Dynamic Motor with Reciprocating Armature Motion

V. V. Ivashin, A. M. Elenkiy

The development of a number of branches of science and technology requires the creation of efficient pulsed electromechanical energy converters with a reciprocating character of motion of the moving part. Converters of this kind are finding ever wider application in the equipment of physical experiments [1, 2, 3], for synchronous commutation of electrical installations [4, 5], in impact-action machines [6], etc. Owing to the rapid development of semiconductor technology in recent years, as well as advances in capacitor construction and insulation technology, prerequisites have appeared for the development and manufacture of motors for the indicated purposes with an intensified energy-conversion process and with a simple and compact pulsed power-supply system. Below, some results are described of studies of physical models of induction-dynamic motors (IDM).

Figure 1 shows the basic design of an IDM, consisting of a cylindrical ferromagnetic inductor 1, in which there are annular coils 2, and a ferromagnetic armature 3 with conducting rings 4 (short-circuited turns). The inductor is assembled from sheet electrical steel, while the armature core has longitudinal slots (but may also be made laminated). The armature can move inside the inductor in the axial direction. In the initial position (Fig. 1a), the armature is set in such a way that each short-circuited turn is opposite the corresponding inductor coil and is somewhat asymmetrical with respect to it in the axial direction. When a current pulse is applied to any coil, a current of the opposite direction is induced in the short-circuited turn that at that moment has strong magnetic coupling with this coil; the magnetic flux of this current damps the mag-

Fig. 1. Basic design of an IDM

the magnetic field of the coil. As a result of the interaction of the magnetic fields, an axial mechanical force appears (because of the asymmetric position of the turn relative to the coil), pushing the turn out of the coil. The interaction proceeds in a similar way in the other coils as well. As a result, the armature is pushed out into the position shown in Fig. 1,b, where the magnetic coupling of the short-circuited turns with the coils is zero, accelerating in the process to a certain velocity; i.e., magnetic energy is converted into mechanical energy. The armature is returned to its initial position under the action of another current pulse or by some external device.

The operating principle of the described IDM is based on the phenomenon discovered by Professor Thomson in 1887 [8], and is analogous to the drives described in [1, 4, 7]; however, the nature of the energy conversion differs substantially because of the features of the design. The mutual inductance of the primary and secondary circuits changes from a maximum to zero with a comparatively small displacement of the armature, which is reflected in a considerably larger derivative of the mutual inductance with respect to displacement. This determines a more rational use of the magnetic energy (since practically all of it is converted into mechanical energy) with a considerable mechanical force over the entire length of the working stroke of the IDM and a reduction in the energy capacity of the power supply.

Fig. 2. Results of investigation of model No. 1: a) dependence of the efficiency and kinetic energy of the armature \(W_m\) on the initial energy of the capacitor \(W_c\); b) circuit for forming current pulses in the IDM coils; c) oscillogram of the current \(i\) in the IDM coils (\(i_{\max}=1250\) A), of the armature velocity \(V\) (\(V_{\max}=8.3\) m/s) and of the voltage at the IDM terminals \(U_L\) (\(U_{L\max}=280\) V) with a capacitor capacitance of 6000 \(\mu\)F.

The construction of the IDM in the form of several pairs of coil—short-circuited turn (sections), as applied to its use in a specific mechanism, has a number of positive qualities. First, it is possible in a simple way to vary the ratio of the inductor length to its diameter for one and the same IDM power and response time. By changing the length of the inductor, the IDM power can also be regulated. Second, the armature itself, made in the form of a steel rod, can transmit mechanical energy to other parts of the mechanism in which it is used. And third, the multisection design of the IDM permits multistage acceleration of the armature, which increases the utilization of active materials and also makes it possible to brake the armature by applying additional current pulses to the winding at the appropriate instants of time.

Figure 2,a presents the results of experimental studies of IDM model No. 1, having the following parameters: inductor with outside diameter \(d=66\) mm, length \(l=200\) mm; coils of 20 turns each, with resistance \(r=0.010\) ohm, 4 units in all; armature with diamet—

armature with \(d_{\text{a}}=30\) mm with three copper short-circuited turns and weight \(m_{\text{a}}=1.1\) kg; the total weight of active materials \(m_a=3.5\) kg.

Current pulses were generated in the coils, connected in series, according to the circuit in Fig. 2,b. A characteristic oscillogram of the armature acceleration process is shown in Fig. 2,c.

In order to make fuller use of the active materials, the fourth section operated in the mode of an ordinary solenoidal electromagnet (see Fig. 1). The curves of the efficiency of the electromechanical conversion of the capacitor—IDM system and of the maximum kinetic energy of the armature are plotted as functions of the initial capacitor energy at capacitances of 3000, 6000, and 9200 μF, with the initial voltage varied from 100 to 450 V. The maximum kinetic energy of the armature reaches 160 J at a system efficiency of 17%; moreover, the efficiency increases up to an armature energy of 80 J, and then its growth ceases.

Figure 3 presents curves of the dependence of the efficiency on the magnitude of the accelerated mass \(m\) (curves \(a\)), which was created by attaching additional loads to the IDM armature; and on the number of sections \(n\) to which a current pulse was simultaneously applied (curves \(b\)), with each curve plotted for one and the same initial capacitor energy. It can be seen that the efficiency decreases as the accelerated mass increases and as the number of sections decreases.

Fig. 3. Dependences of efficiency on the magnitude of the mass \(m\) (curves \(a\)) and on the number of simultaneously switched-on coils \(n\) (curves \(b\)) at \(m=1.1\) kg.

Figure 4,a presents the results of experimental studies of IDM model No. 2, having the following parameters: inductor with \(d=160\) mm; \(l=140\) mm; coils of 68 turns each with \(r=0.070\) ohm, two coils in all; armature with \(d=60\) mm with two copper short-circuited turns and \(m_{\text{a}}=4\) kg, total weight \(m_a=14\) kg. In comparison with model No. 1, here the copper area of the coil and short-circuited turn is increased threefold. The capacitor capacitance was varied from 3000 μF to 11000 μF at an initial voltage from 100 to 450 V. A maximum armature energy of 210 J was obtained, while the system efficiency at first increased (up to 24%) and then slowly decreased (the current pulses were supplied according to the circuit in Fig. 2,b). At high energies, in the final stage of armature acceleration, strong saturation of the steel was observed; therefore, in the IDM a certain part of the magnetic energy remained unused. In order to clarify the fundamental possibility of evacuating this energy, the circuit in Fig. 4,b was used, where capacitor \(C_2\), chosen to have a significantly smaller capacitance than the main capacitor \(C_1\), was charged to the same initial voltage as the latter and was connected to the IDM winding through the corresponding thyristor at the final stage of armature acceleration. A rapid recharge of capacitor \(C_2\) to negative polarity occurred, with the magnetic energy returned from the IDM. A characteristic oscillogram of such a process is shown in Fig. 4,c. Taking into account the returned energy, the system efficiency increased to 32% (shown by the dashed line in Fig. 4,a).

From consideration of the curves in Figs. 2,a and 4,a it may be noted that, re-

despite a certain variation in the parameters of the power source (capacitance and capacitor voltage), the efficiency and kinetic energy of the armature depend primarily on the initial energy of the capacitor. As the energy increases, in both cases an increase in efficiency is observed. The limitation on the growth of the efficiency is apparently explained by strong saturation of substantial portions of the magnetic circuit in the region of the motor working gap. Thus, for faster energy-conversion processes, characterized by a larger ratio of the initial energy of the capacitor to the accelerated mass, the efficiency is higher, which is consistent with [9, 10].

Fig. 4. Results of the study of model No. 2: a) dependences of the efficiency and \(W_m\) of the armature on \(W_c\). The dashed line shows the efficiency taking into account the returned energy; b) circuit for forming current pulses in the coils of the induction-dynamic motor with return of the residual magnetic energy to capacitor \(C_2\); c) oscillogram of the current (\(i_{\max}=555\) A), armature velocity (\(V_{\max}=8.1\) m/sec), and voltage (\(U_{L\max}=400\) V); \(C_1=5900\) μF, \(C_2=1740\) μF.

In the experiments it was observed that supplying the excitation coils with current pulses of alternating polarity is accompanied by a small (\(\sim 5\%\)) increase in the motor efficiency.

By applying the second current pulse at the appropriate instants of time to the IDM coils while the armature was moving, two-stage acceleration and smooth braking of the armature were experimentally achieved. Owing to the higher average velocity in the second acceleration stage, the efficiency increased twofold. On the basis of model No. 1, a rebound hammer with adjustable frequency and impact energy was manufactured [11], and on the basis of model No. 2, a device for studying the dynamic properties of soils with adjustable pulse energy and frequency.

Conclusions

1. In comparison with an electromagnetic drive (for example, a solenoid drive), the induction-dynamic drive makes it possible to obtain larger values of the ratio of motion energy to motor weight, and its application is promising when it is necessary to create drive devices of minimal weight.

2. The pulsed power-supply circuit of the drive makes it possible to obtain, over a wide range, an adjustable repetition frequency of the operating cycles—in principle from 0 to 3000–5000 per minute.

3. The efficiency of the motor can be no less than 30%, which opens up broad possibilities for its application in various devices of the vibrator type, impact units, drives for switching apparatus with a high switching frequency, etc.

4. Multistage acceleration of the moving part by feeding successive current pulses to the excitation coils makes it possible to substantially increase the efficiency of energy conversion. The use of a drive power-supply circuit with recovery of residual energy from the technological gaps and saturated sections of the inductor magnetic circuit also increases the efficiency.

References

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