Pulsed Induction-Dynamic Device with Adjustable Impact Energy and Frequency

V. V. Ivashin, A. M. Elenkin

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

In recent years, increasing attention has been devoted to the development and study of electrical machines with a reciprocating character of armature motion, used in impact mechanisms [1]. As a result of research on induction-dynamic motors carried out at the Research Institute of Nuclear Physics of TPI over a number of years [2, 3], a pulsed device was developed and manufactured for creating large impact loads on a tested soil specimen in order to determine its vibrational characteristics.

Device parameters

The device consists of the impact unit itself (Fig. 1) and a power-supply circuit made as a separate block (Fig. 2).

The impact unit consists of armature 1, damping gasket 2, an inductor with two coils 3, and return spring with adjustable force 4. The inductor is assembled from sheets of electrical steel 0.5 mm thick, has an outside diameter of 160 mm and a length of 140 mm; the coils are wound with PSD busbar, 68 turns each, connected in series with a total active resistance of 0.14 ohm; the inductor sheets and coils are potted with epoxy resin; the armature is made of 9KhS steel, 60 mm in diameter, has eight longitudinal slots 15 mm deep; two copper rings, reinforced with bronze bands, are soldered into the body of the armature.

In Fig. 1b the inductance curve of the inductor is presented as a function of armature displacement, obtained experimentally with an unsaturated magnetic circuit of the impact unit. In the initial position the armature is held by the return spring in the extreme left-hand position. During поступ-

Fig. 1. a) Impact unit. b) Dependence of the inductor inductance on the position of the armature.

Fig. 2. a) Power-supply circuit. b) Control circuit of the impact device.

When a current pulse is applied to the stator coil from the power-supply circuit, the armature begins to move to the right under the action of the electromagnetic forces arising between the coils and the short-circuited copper turns of the armature, accelerating to a certain velocity, and then strikes the specimen being tested or the damping pad. The armature is returned to its initial position by the force of the return spring. In this case a spring with a force of 8–12 kg was used, which is quite sufficient to return the armature in approximately 30 msec. If necessary, the armature can be returned under the action of magnetic forces in a shorter time, which will make it possible to increase the repetition frequency of the blows to 50–60 Hz.

The power-supply circuit of the impact device (Fig. 2, a) consists of two capacitor banks \(C_1\) and \(C_2\), each of 3000 μF, a thyristor \(T\), the working winding of the impact device \(B\), a blocking diode \(D\) with a commutating choke \(Dr_2\), a charging choke \(Dr_1\), charging diodes \(D_m\), a magnetic starter \(MP\) with control buttons, blocking contacts \(K\), and a resistor \(R\). By means of the packet switch \(P\), capacitor bank \(C_2\) can, if necessary, be disconnected. When the “start” button is pressed, the magnetic starter \(MP\) is switched on and the capacitor bank is charged through the diodes \(D_m\) and the choke \(Dr_1\). When a control pulse is applied to the thyristor \(T\), the capacitor bank discharges through the working winding of the impact device. At the moment when the voltage on the capacitor passes through zero, the diode \(D\) opens, and after the choke \(Dr_2\) has been remagnetized, the thyristor \(T\) is blocked under the action of a small negative voltage on the capacitor bank. Then the current in the circuit—the working winding \(B\), choke \(Dr_2\), diode \(D\)—dies away, and the capacitor bank begins to charge again from the electric mains. When the next control pulse is applied to the thyristor \(T\), the process is repeated. When the “stop” button is pressed, the magnetic starter disconnects the charging diodes \(D_m\) from the electric mains and closes the contacts \(K\). The capacitor bank is rapidly discharged through the choke \(Dr_1\) and the resistor \(R\).

The operating voltage on capacitor bank \(C_1\), composed of K50-3I capacitors, is 310 volts. If necessary, the circuit permits the impact force to be doubled by forcing the operating mode, by increasing the voltage on the capacitors to 400–450 V.

The circuit for controlling the operation of the thyristor (Fig. 2, b) consists of toggle switch \(T_m\), diode \(D_1\), limiting resistor \(R\), three potentiometers \(P_1\), \(P_2\), and \(P_3\), capacitor \(C\), switching diode \(D_2\), and pulse transformer \(Tr\).

When toggle switch \(T_m\) is switched on, capacitor \(C\) begins to charge from the electric mains through diode \(D_1\), resistor \(R\), and potentiometers \(P_1\)–\(P_3\). When the voltage on the capacitor reaches the switching voltage of diode \(D_2\), the latter opens, and the capacitor discharges through the winding of pulse transformer \(Tr\). From the secondary winding of the transformer, the control pulse is applied through a diode to thyristor \(T\) of the power-supply circuit of the impact device. The pulse repetition frequency can be adjusted by means of potentiometers \(P_1\)–\(P_3\).

Figure 3 presents an oscillogram of the armature acceleration process. The oscillogram was taken with the armature striking the damping pad. At the end of the armature stroke, magnetic braking of the armature is observed, accompanied by the appearance of a flat portion in the current curve. This braking reduces the impact load on the housing of the impact assembly during no-load blows and is caused by the insertion of the copper ring into the next—

during motion of the coil. Fig. 4 shows the general appearance of the impact device.

Fig. 3. Oscillogram of the armature acceleration process with a battery capacitance of 6000 μF. 1 — current in the working winding of the inductor (current amplitude 420 A). 2 — voltage at the inductor terminals. 3 — armature velocity. Time scale along the ordinate axis: 1 large division corresponds to 2 ms.

Fig. 4. General view of the impact assembly with the armature extended

When testing the device with a duty cycle of 10–15%, it was found that all elements of the power-supply circuit and the design have sufficient reliability. In testing the impact assembly on a stand, an impact energy of 200 J was achieved with an electromechanical conversion efficiency of the capacitor energy of 34%. The developed impact device can be used in impact machines of various purposes. Its distinctive feature is the possibility of regulating the impact frequency in the range 0–15 (50) Hz and its high specific power, substantially exceeding the specific power of known electromagnetic solenoid machines [1]. In a number of cases, an impact-action device of the described type can successfully compete with pneumatic impact machines, from which it differs in not requiring bulky and expensive auxiliary compressor equipment.

REFERENCES

1. N. P. Ryashentsev, P. M. Alabuzhev et al. Hand-Held Electric Impact Machines, Moscow, Nedra, 1970.

2. V. V. Ivashin, A. I. Bludov, A. M. Elenkin. Valve-mechanical apparatus for switching high pulsed powers. Publ. TPI, vol. 160, 1966.

3. V. V. Ivashin, A. M. Elenkin. Development and investigation of an induction-dynamic d.c. motor for impact machines. Research report of the Research Institute of Nuclear Physics, TPI, Collection of Abstracts of Research Projects, No. 17–18, 1971.