Study of operating modes of electromagnetic hammer with adjustable impact energy and blow frequency

ABSTRACT


INTRODUCTION
Experience gained during operation of electric machines (i.e., those using electromagnetic forces) in different industry sectors allows identifying the most promising areas of their further application.Many researchers [1]- [5] are currently studying issues related to effective use of linear electromagnetic drives in various production processes.Electromagnetic impact machines (electromagnetic hammers) are used for breaking rocks, driving piles, developing frozen soils, in offshore oilfield construction, and as well-bore vibration sources [6], [7].
One of the most crucial tasks in the development of such hammers is improvement of energy performance per cycle.Energy conversion issues in electromagnetic impact machines are among the most complex ones [8]- [13].Energy conversion efficiency is intimately connected with formation of the operational cycle of electromagnetic machines.Previous research work in this field reported many important recommendations as to the formation of the operational cycle of electromagnetic machines [14]- [18].Implementation of those recommendations helped in achieving either maximum speed performance or the best efficiency of the machine.Determination of the required single impact energy is an essential condition to be considered in the design of impact machines.This is due to the fact that rock fracturing does not assume constant correspondence between the material strength and drive loading, which is typical for static machines, such as machines that use cutting tools.If a static machine is fitted with an overpowered motor, the motor will still consume only the power conditioned by a given load (strength of specific rock and actual dulling of the cutting tool.The same behavior is not true in case of impact machines.Even if the single impact energy appears to be excessive for fracturing of a certain rock, all the energy stored by the striker will be consumed. The excess work will either be absorbed by the oversized material or will be used for over crushing of the treated material.Therefore, selection of the correct single impact energy is essential.Quantitative assessment of specific energy consumption for crushing of oversized rocks was studied in a large number of researches [19]- [21].Operating cycle of electromagnetic hammer consists of the following stages: − Build-up of current in the idle stroke winding up to the starting current; − Upward striker movement until stop (idle stroke); and − Downward striker movement until delivering a blow (working stroke).
The striker accumulates potential energy during its upward motion (idle stroke mode).This energy is converted into kinetic energy during the working stroke.Potential energy is proportional to the striker rising height (stroke), while cyclic time of the hammer (i.e., the impact frequency) depends on the movement time.Therefore, the striker travel is a controlled parameter, which determines the main performance characteristics of the hammer.
Deceleration and stoppage of the striker during its idle stroke may occur under the effect of braking forces acting opposite to the direction of motion (weight of the striker and friction forces), as well as due to the forces created by braking devices (mechanical springs, and air bumpers).At that point, the striker slows down, and kinetic energy accumulated by the striker is converted into potential energy, and can be partially used during the working stroke of the striker.The starting moment of the deceleration mode and its intensity defines the travel of the striker and the deceleration time and, therefore, the impact frequency and impact energy.
As the striker moves downward (working stroke mode), potential energy accumulated during the idle stroke is converted into kinetic energy.Besides, downward striker speed can be further increased due to the traction force created by the working stroke winding.Electromechanical systems with electromagnetic hammer were designed and built taking into account particular issues associated with the conversion of energy during machine operation and the required ability to control main energy parameters of the electromagnetic drive.The current study aims at obtaining quantitative estimation of energy parameters of electromagnetic hammer per operating cycle through a mathematical model of the machine.

THEORITICAL BACKGROUND
Analytical methods for calculating the dynamic behavior of electromagnetic machines are based on the magnetic circuit theory.During the calculations, a magnetic system is replaced by an equivalent electrical circuit.At this stage, the calculation of the electromagnet thrust force requires accurate accounting of the leakage flux distribution, which is considered to be quite difficult when using the existing approaches [22], [23].The research findings presented in this work were obtained using computer modelling in MATLABbased Simulink simulation environment.The initial differential equation system for each of the electromagnets is as follows: where: U: the applied voltage; R: active resistance of the winding; i: instantaneous winding current; ψ: flux linkage; δ: the air gap; m: mass; V: speed of the striker; Fe: the electromagnet traction force; and Fc: the resistive force to motion of the striker.

PROPOSED METHOD
As shown in Figure 1, the hammer consists of dummy (LM1) and working (LM2) coils enclosed in a magnetic tube, and a ferromagnetic striker, which performs reciprocating motion.The coils are powered by  The hammer model as shown in Figure 3 is designed according to the function block diagram of the hammer itself (Figure 1) and the system of (1) used for each of the machine coils.The parameters of the dummy coils and the value of current, flux linkage, and traction force created by this coil are indicated in the model with index 1.The same parameters and values created by the working coil are indicated in the model with index 2.The coils operate in turn, according to signals from the input device (Um1, Um2) and from striker position sensors (S1, S2).Initially, the striker is in the lower position (Figure 1), it is completely placed in the coil of the lower second electromagnet and partially enters the coil of the upper first electromagnet.The length of the ferromagnetic striker is about one third longer than the length of the electromagnet coil.To start the hammer and its model, the Rect1 rectifier mode is turned on and voltage is supplied to the dummy coil, a current i1 appears, the value of which depends on the active and inductive resistance of the coil (block B1).
The current i1 creates the traction force of the coil F1.The magnitude of this force depends on the striker position inside the dummy coil.If the force F1 is greater than the movement resistance forces Fc applied on the adder in the model, then the striker will start moving upwards.The speed of movement and position of the striker are determined in the model using integration blocks.When moving the striker, the values of flux linkage, inductance, current and traction force of the electromagnet change.These changes are taken into account and calculated in the model using blocks B3 and B4.When the striker reaches the upper position sensor S1, a signal is transferred to the control unit B, converting the rectifier that feeds the dummy winding to the inverter mode.The signal Rect1 disappears in the model and signal Inv1 appears.The current i1 is reduced to zero and the field of the upper electromagnet is damped.The striker starts moving down.At the same time, according to the signal from the sensor S1, the Rect2 rectifier mode is turned on and the voltage is supplied to the working coil, the current i2 appears, the value of which depends on the active and inductive resistance of the coil (block B2).The current i2 creates the traction force of the coil F2.The magnitude of this force depends on the position of the striker inside the working coil.The striker accelerates due to the appearance of the F2 force.When moving the striker, the values of flux linkage, inductance, current and traction force of the electromagnet change.These changes are taken into account and calculated in the model using blocks B5 and B6.When the striker reaches the lower position sensor S2, a signal is transferred to the control unit B, converting the rectifier that feeds the dummy winding to the inverter mode.The Rect2 signal disappears in the model and signal Inv2 appears.The current i2 is reduced to zero and the field of the lower electromagnet is damped.The hammer strikes.At the same time, according to the signal from sensor S2, the Rect1 rectifier mode is turned on and voltage is supplied to the dummy coil, current i1 appears.The cycle of operation of the hammer model is repeated.

69
BM measurement block allows determining instantaneous mains power consumption Р1 = Ui; copper losses in the hammer windings Рm = i2R; mechanical power Рmech = F2 V. Mains power consumption W1, copper losses in the hammer windings Wm, mechanical power of the hammer Wmech are obtained by integrating the corresponding power values.
The proposed analytical methods for calculating such systems, when deriving the main calculation expressions, abound in many assumptions and limitations concerning the consideration of magnetic resistances and parasitic gaps, dissipation and convex fluxes, and the range of variation of the operating gap [24].Most importantly, the results of calculations using known expressions and experimental data for newly created devices in practice can have large discrepancies.This is primarily because the existing methods for calculating electromagnetic systems are based on the theory of magnetic circuits.In the calculations, the magnetic system is replaced by an electrical substitution diagram, and the values of magnetic resistances are determined from a simplified field picture.The working air gap, where the energy conversion process takes place, also appears to be one of the sections of the magnetic circuit, the resistance of which depends on the dimensions and surfaces of the striker and stationary magnetic core arrangement with some formal consideration of the magnetic flux distribution.In fact, the calculation of the thrust force in long stroke systems is associated with the exact consideration of the distribution of the dissipation fluxes, which causes great difficulties when using the existing approaches.Consequently, there is no simple and sufficiently accurate analytical method for calculating long stroke plunger systems.
The dedicated mathematical program for calculating the two-dimensional magnetic field FEMM is based on the finite element method [25].There are three main functional units in FEMM: the unit for creating the geometry of the computational domain, identifying and assigning physical properties to its individual parts (preprocessor); the unit for calculating model parameters by the finite element method (processor); the unit for displaying the results of the calculation (postprocessor).The method of computer modeling proposed in our article using the MATLAB-based Simulink simulation environment is currently used to study objects in systems of almost any complexity.The important advantages of this method include the possibility to observe the process in time.

RESULTS AND DISCUSSION
Transient processes in terms of coil current, striker speed and stroke, energy consumed from the grid, power losses in copper and impact energy, as well as efficiency have been obtained for the hammer operation cycle.Studies have shown that with the maximum voltage of the working coil and a change in the voltage of the dummy coil, the impact energy does not change, and the frequency of impacts varies from 0 to 187 strokes per minute.With the maximum voltage of the dummy coils and an increase in the voltage of the working coil in the range from 0 to the maximum value, the impact energy increases by 4.3 times, and the frequency of impacts decreases by 1.96 times.The largest value for the operation cycle is the electrical losses in the hammer coils, to reduce them, it is necessary to reduce the magnitude of the coil currents, and to do this it is required to increase the striker speed.
Figure 5 shows Oscillograms of the upper winding current 1, linear speed V and stroke X of the striker at voltages U1 = 210 V and U2 = 0 V (as shown in Figure 5(a)); and oscillograms of the upper winding current, linear speed V and stroke X of the striker, lower winding current i2 at voltages U1 = 210 V and U2 = 210 V (Figure 5(b)).Figure 5(a) shows oscillograms of the upper winding current, linear speed V and stroke X of the striker, obtained from the model, when only the upper winding (U1 = 210 V and U2 = 0 V) is used.Figure 5(b) shows oscillograms of the upper winding current, linear speed V and stroke X of the striker, as well as the lower winding current at voltages U1 = 210 V and U2 = 210 V.
Analysis of the obtained oscillograms allowed the determination the following: − When only the upper winding is used (U2 = 0 V), time from the zero-point moment to the moment when the striker starts moving is 0.03 s, current at the striker starting moment is 28 A, and the total time of upward travel is 0.33 s − At voltages U1 = 210 V and U2 = 210 V, time from the zero-point moment to the moment when the striker starts moving is 0.03 s, current at the striker starting moment is 28 A, and the total time of upward travel is 0.

CONCLUSION
Mathematical model of the hammer has been developed using the measured static characteristics of flux linkage and traction force for each of the windings.Oscillograms of the idle winding current, working winding current, striker speed and stroke were obtained.Figures for the power consumed from the mains, copper losses and impact energy, as well as the hammer efficiency per operation cycle were determined.
The study has shown that at working stroke winding voltage U2 = 210 V and varying idle stroke winding voltage, the impact energy does not change, while the impact frequency varies between 0 and 187 blows per minute.At idle stroke winding voltage U1 = 210 V and working stroke winding voltage varying in the range from 0 to 210 V, the impact energy varies between 84 and 360 J, and the impact frequency varies between 96 and 187 blows per minute.Maximum losses over the working cycle are associated with electric losses in the hammer windings; those loses may be reduced by reducing winding currents, which in turn requires an increase in the striker speed.
To obtain the maximum machine efficiency in the idle phase, the striker shall be braked by coasting to stop, as such braking does not require consumption of energy by the machine, and the kinetic energy will be completely (excluding losses) converted into potential energy.As can be seen from the above, impact machine cycle includes three operating modes, each of which determines the main energy parameters of the machine: efficiency, impact energy and cycle time.At that, impact energy and frequency of blows of the machine should be selected in accordance with the requirements of the associated process.


ISSN: 2088-8694 Int J Pow Elec & Dri Syst, Vol. 15, No. 1, March 2024: 64-73 66 controlled rectifiers V1 and V2, that operate according to the commands from the control unit B. This unit receives the signals of set points Um1 and Um2, as well as signals from the sensors of the lower S1 and upper S2 positions of the striker, on the basis of which the unit controls the working cycle of the hammer.

Figure 5 .−Figure 6 .
Figure 5. Oscillograms of the upper winding current i1, linear speed V and stroke X of the striker at voltages U1 = 210 V and (a) U2 = 0 V in and (b) lower winding current i2 at voltages U1 = 210 V and U2 = 210 V