Prospects for the Use of an Induction-Dynamic Drive in Technology

A. M. Elenkin, V. V. Ivashin

(Presented by the scientific seminar on electrical machines)

Induction-dynamic drives at present have not yet found wide application in various technical devices. This is explained, on the one hand, by the fact that IDDs require for their operation powerful switching devices for currents of hundreds or even thousands of amperes at voltages of hundreds and thousands of volts, which appeared only at the beginning of the 1960s in connection with advances in the development of thyristors. On the other hand, in most cases IDDs require for their operation pulsed power supplies in the form of capacitor banks charged through rectifier devices. The successes achieved in recent years both in capacitor technology and in semiconductor rectifier technology have substantially stimulated the development of IDDs. Nevertheless, work on IDDs is at present proceeding extremely slowly, and their capabilities are unknown to a broad circle of electrical engineers. Only in recent years has interest in IDDs begun to grow, and a number of works have appeared in which various devices using IDDs are described. The main advantage of an IDD is the ease of obtaining, over a short interval of time (on the order of tens and hundreds of microseconds), a considerable mechanical force, the magnitude of which is practically limited only by the strength margin of the drive structure and by the power of the power supply.

An IDD is a transformer operating in short-circuit mode with a movable secondary winding made in the form of one or several turns. Structurally, an IDD may be made without a magnetic core (Fig. 1, a), with an open magnetic core (Fig. 1, b), or with a closed magnetic core (Fig. 1, c).

When a current pulse is supplied to the primary winding, a current of opposite direction is induced in the secondary winding. Between the windings a magnetic field appears, by the lateral pressure of which the secondary winding is repelled from the primary and is accelerated to a certain velocity.

IDD designs according to Fig. 1, a are used to obtain mechanical forces reaching 300,000 N at an induction in the air gap of 10 T and higher. The kinetic energy of the moving part reaches 1000 J at a velocity of up to 200 m/sec. Such an IDD design is used in fast-acting short-circuiters, in fast-acting shutters of physical instruments, in magnetic stamping of metal products, etc. [1, 3, 4, 10—13].

The designs according to Fig. 1, b and c are used in mechanisms with reciprocating motion [2], in contact apparatus, and also

it was proposed to use an IDD in electric impact tools [5], when the secondary winding is mounted on the central part of the magnetic core, which is made movable. The presence of the magnetic core reduces the energy capacity of the power source and improves the magnetic coupling between the circuits. The displaced mass is on the order of several kg,

Fig. 1. Types of IDD designs

the kinetic energy reaches 100 J at a motion speed on the order of 10 m/sec. The use of such drives is known in high-speed contact apparatuses for switching large pulsed currents (\(\sim 100\) kA) at voltages of 10–15 kV. The operating time of these apparatuses reaches 1–3 msec; a mass of moving parts of 1.5–2 kg is displaced by a distance of 30 mm. Such a high speed of response is practically impossible to achieve using pulsed electromagnets, since in them the mechanical force is limited by the saturation of the steel and by the considerable inductance caused by the presence of large leakage fields. In addition, in an IDD the weight of the moving part is significantly smaller, since the secondary winding can be made in the form of a light duralumin turn [6–9].

In known IDD designs the efficiency, as a rule, ranges from 2 to 30%. In devices with a small number of operations per unit time (once every several minutes or tens of minutes), this drawback is not a serious obstacle to the use of an IDD. However, in devices with limited energy resources (when powered from autonomous sources) and an increased frequency of operation (aircraft, impact machines, etc.), low efficiency is a serious drawback, causing strong heating of the windings and considerable dimensions and weight of the power-supply circuit.

A known method of increasing efficiency is to short-circuit the working winding at the current maximum (Fig. 2) [1, 14]. In this case all the energy stored in the capacitor is converted into the energy of the magnetic field of the primary and secondary windings, which then passes into kinetic and thermal energy.

However, the current in the short-circuited working winding, under the action of active losses and the counter-emf that arises as a result of the change in flux linkage when the moving part is displaced, rapidly decays. As a result, the accelerating mechanical force rapidly decreases with time, which adversely affects the use of the active materials of the drive and worsens the efficiency.

Studies of the IDD have shown that, when the working winding is supplied with quasi-rectangular current pulses (shown by the dashed line in Fig. 2) by introducing additional energy into the short-circuited circuit during the displacement of the moving part, the mechanical force is main-

remains of considerable magnitude over the entire length of the working stroke. This determines a better utilization of the active materials and a higher efficiency. In addition, the efficiency can be substantially increased through a more careful development of the design, with the aim of increasing the rate of change of the magnetic conductance during displacement of the secondary circuit.

Fig. 2. Power-supply circuit of an IDD with short-circuiting of the working winding at the current maximum

Studies of several IDD specimens with a moving-part mass of 1, 4, and 100 kg show that their energy-conversion efficiency is 20–40%. Here efficiency is understood as the ratio of the kinetic energy of the moving part to the energy expended by the storage capacitance of the power-supply system during operation of the drive.

In view of the increased efficiency, and the advances in semiconductor technology and capacitor engineering, the use of IDDs becomes promising in devices with a limited reserve of energy, a high operating frequency, and a limited weight of active materials: in launch apparatus, in fast-acting contact apparatus, in various electrophysical devices, in the performance of various technological operations, in impact-action machines, etc. In terms of the weight of active materials and impact energy, IDDs become capable of competing both with pneumatic reciprocating engines (efficiency \(\sim 5\%\)) and with electromagnetic-type drives widely used at present in engineering. At the same time, IDDs surpass them in efficiency, speed of response, and ease of regulation of frequency and energy.

One of the principal tasks in further investigations of IDDs remains the increase of the coefficient of efficiency.

LITERATURE

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