a)
b)
Fig. 4.1
The single-turn inductor ó a) and the magnetic pulse installation ó b) are the complete set of the technical equipment for the tube work-pieces deforming.
Chapter 4. The Technical Equipment for Working of Metals by Pulse Magnetic Fields.
Speaking about the pulse magnetic fields usage for progressive working technologies realization we must pay attention to the technical provision questions.
In the basic scheme of the magnetic pulse metal working two main blocks can be distinguished according to the functional attributes. These are, so named, the inductor and high voltage systems.
The first of them consists of inductor what is the magnetic field generator, the workpiece to be stamped and the matrix if it is necessary in accordance with the technological operation conditions. As it was pointed out the inductor system is the magnetic pulse method tool. Its functional duty consists in the ordered production task execution. This is working or making of the articles of the according configuration.
The second block is high voltage system. It is purposed functionally for the electrical energy accumulation and power transmission in inductor system.
For better understanding of role of the parts distinguished in the magnetic pulse scheme some analogy can be conducted with metal cutting production where for execution of the technological operation ordered two main functional units are being required.
The first of them is a machine. The second unit is a concrete tool for the production task execution. It can be a cutting tool, a device for producing of a thread or some other gadget for fixed purpose.
The machine provides a regime what is necessary for the ordered operation execution. For example, it is generating of mechanical energy and its transmission to the complex "tool ó workpiece".
The functional purpose of tool consists in concrete action on an article being worked and in getting of the finished products.
In the magnetic pulse scheme the high voltage system provides general problem solving by analogy to machine in metal cutting production. This problem consists in energy transformation and its transmission to the inductor. The last one is the tool for the concrete production operation execution as, for instance, a cutting tool under mechanical working.
As in mechanics, the technological operation execution with a help of the pulse magnetic fields demands solving of two main problem, namely: an energy source creation and a tool carrying out.
The energy source is the high voltage system. In the special literature it was named as the magnetic pulse installation (or plant).
The tool is the complete set of "inductor ó workpiece". It is being named by the inductor system.
Thus, the magnetic pulse installation plus the inductor system are the technical equipment for metals working with a help of the pulse magnetic fields energy.
A singleóturn inductor for the technological operation "compression" making is shown on Fig.4.1a. It was being connected to the magnetic pulse installation what was presented on Fig.4.1b.
The given complete set of the technical equipment has been made at the Kharkov State Polytechnical University. It was being purposed for a tube workpieces working.
The installation represents an electrical system consisting of the separate blocks that are being placed in the same part of the construction. The control panel block is situated on the upper plane of bulging part of an installation technological table. All control instruments and signaling system are placed at upper part of the installation vertical panel.
The questions of the different systems creation have been considered in the previous chapter of the present monograph.
In this chapter we will quote the common information only about an arrangement and operating of installations for traditional magnetic pulse metals working.
A constructive solutions description, design schemes and other particularities of the magnetic pulse installations are not subjects of the present exposition.
A reader who is interested in the detailed information by the given
questions must address to any special technical literature of the according
subjects. Some of primary sources are represented in the bibliography of
this monograph.
4.1. The General Arrangement and Functional Scheme
of the Magnetic Pulse Installation.
The classical functional scheme of the magnetic pulse installation is given on Fig.4.2. In essence, the represented installation is a pulse current generator what consists of the following elements:
b)
Fig. 4.1
The single-turn inductor ó a) and the magnetic pulse installation ó b) are the complete set of the technical equipment for the tube work-pieces deforming.
The start regulator is designed for switch-on of the installation, regulation of a current regime in time of the capacitor bank charging and an ignition of the shielding devices in any emergency situations.
The charging device consists of the step-up and rectifying blocks. The first of them represents the high voltage transformer with inductance coil what clips off the current. The charging device must provide an ordered charging time of capacitor bank, the high reliability of used equipment and the minimum expenditure for one technological operation execution. The interrupted operation of this unit must not depend on any work conditions and a scheme of the magnetic pulse installation.
As a rule, the energy capacitor bank is being charged in accordance with two schemes: 1) the charge from the invariant voltage source supply; 2) the charge from the invariant current source supply.
In the first scheme (the most simple) the charging device can be represent by the following main elements: a high voltage transformer, rectifying apparatus and resistance (active or inductive) for current limitation. The last element is being connected to circuit of rectifying apparatus in series.
The second scheme of the capacitor bank charge is more complicated because it has some additional systems for automatic regulation of the charge current (beside of the first scheme elements!). Unlike the scheme with the invariant voltage source the charge from the invariant current source permits to reach the essential higher factor of efficiency for all process in the whole.
The most important energy unit of installation is the capacitor bank what represents the battery of capacitors. They must satisfy the requirements that provide the installation efficiency in the first turn. We will talk it over in detail some later. Now we will stop on the main positions: the capacitor battery must have a small inductance and ability to stand the maximum possible number of pulse discharges, the minimum mass, overall dimensions, operation cost and other.
As a rule the low inductive capacitors with a high value of stored energy
is being used in the magnetic pulse plants.
Fig.4.2
(1) is the start regulator; (2) is the step-up device; (3) is the rectifying device; (4) is the energy storage capacity; (5) is the commuting device; (6) is the inductor system; (7) is the shielding device; (8) is the block of measuring equipment; (9) is the automation module; (10) is the controller of capacitor bank energy level; (11) is the discharging circuit commutator ignition block.
The capacitorís battery parameters determine the technical and economic characteristics of the whole technological process (but not installation only). We keep in mind the lifetime, the overall dimensions and the mass of installation, the product process cost in terms of one operation.
The lifetime of capacitors determine the whole economy of the technological process with magnetic pulse energy usage. This parameter depends on many factors. Not enumerating of all them, we will point out that the usual lifetime of capacitors is million cycles of "charge-discharge" approximately.
As devices for controlling of the capacitor bank charging and discharging they use some electrode discharges (ignitrons, thyratrones, for example), vacuum and air three-electrode discharges (trigatrones) and also various two-electrode mechanical commutators.
The commuting devices in the magnetic pulse installations must satisfy the following requirements: the minimum self-inductance; the low electrical resistance; the ability to switch the great currents any ordered number of commutations; the relative simplicity of construction.
A built-in-safety system consists in a short-circuiting device shunting of the capacitor bank under the controlling signal appearance, the different blocking elements and other units. In the first turn the built-in-safety system is intended for the safety provision of service personnel. Besides, there are several short-circuiting devices in the magnetic pulse installations for discharge of the capacitor bank under infringement of the safety regulations rules.
The measuring equipment consists of the high voltage divider and devices for monitoring of the magnetic pulse installation operation.
Finally, the automation system includes a controller, a strictly automation module and an igniting device for commutator in discharge circuit. This system is intended for a function provision of the magnetic pulse installation in any ordered working regimes.
The controller fixes an energy quantity what is by the capacitor bank being stored. It represents a multi-channel switch. Its every position corresponds to level determined of energy what is being stored by capacitors. The controller provides operation of the commutator ignition device in a moment when the capacitor bank energy reaches the maximum value in the time of charge process. As a rule this level of energy is necessary for the ordered technological operation execution.
All components of the magnetic pulse installation are connected by conductors known as busbars.
The busbar must satisfy the following requirements: a high mechanical strength and capability to withstand the large electrical dynamics forces; the minimum self-inductance and active resistance; ability to operate in demanded temperature regime.
The busbar of the magnetic pulse installation is made either from plane parallel closely arranged conductors or of standard coaxial cable. As the design experience showed the flat busbar has not a demanded flexibility. This property sets a limit on possibilities of its constructive execution what has to provide a minimum of the active and inductive resistance. The busbar what is made from coaxial cable is flexible. Ultimately, this circumstance allows to fall down an energy losses and to provide a high efficiency of the magnetic pulse installation in the whole.
Thus, we have considered the functional scheme of the magnetic pulse installation including its main units and defining their connections and interaction.
Now we shall pass to an enumeration of the installation main parameters.
Among them we will distinguish the following ones: the energy stored, the working voltage, the self-frequency of the charge circuit and , at last, productivity.
The magnetic pulse installations are being divided into three basic groups in accordance with their frequency: 1) the low frequency installations with f = (5 - 20) kHz; 2) the installations of the average level of frequency, where f = (20 ó 50) kHz; 3) the high frequency installations with f = (50 ó 200) kHz.
The real self-frequency of discharge in the magnetic pulse installations of distinguished groups will be more than mentioned values naturally. It should be reminded that the self-frequency is being defined under short-circuiting of the installation electrical output.
The charged voltages of installations in the traditional magnetic pulse metalworking were (5 ó 20) kV usually.
There is a division of the magnetic pulse installations depending on the technological duty.
The installations from the first group with productivity 360 operations/hour were purposed for the non-ferrous metals working in a small batch production. The second group included the installations for operation in automation regime with a maximum quantity of discharges of 720/hour.
The main property of the magnetic pulse installations from third group consists in the high value of self-frequency and high productivity (frequency till 300 kHz, productivity till 2400 operations/hour).
As a rule, the installations from this group are automated completely and can operate in any automatic lines.
Besides of the high-mentioned magnetic pulse installations for general duty there are a great quantity of, so named, combined plants what are intended for solving of the special technological problems. Among them some magnetic pulse installations should be distinguished separately. They are realizing step-by-step high frequency heating of the work-pieces and than ó their magnetic pulse forming. The installations like those were created for pressure working of some hard wrought and brittle metals, for example, titanium, magnesium alloys and other.
Finishing the principal basis exposition of the high voltage systems in the magnetic pulse metals working it is necessary to throw light on problem of the strong magnetic pulse fields getting from an energy view point.
An approach what was offered by the scientists of the Kharkov State and the former Leningrad Polytechnical Universities is the more simple and convincing from many methods of the installations optimization for an energy index.
Generalizing the main conclusions of these suggestions and according scientific works we may account that transforming (energy stored by capacitor bank!) into a deformation work is being determined by the following relationship:
(4.1)
where
is the installation usage
coefficient, what determines an energy transfer from capacitor bank to
inductor system;
is
the coefficient of the inductor magnetic field usage what is being determined
as a ratio of the energy in the working space to full energy of the inductor
magnetic field.
(4.2)
is the relative attenuation
coefficient what is equal to ratio of the damping decrement in charging
circuit to the current frequency.
)
under magnetic pulse metal working can be represented by the following
relationships:
(4.3)
The first inequality in the (4.3) determines a relation of the inductor system inductance and the magnetic pulse installation self-inductance. This condition fixes the application expedience of the multi-turn inductors with high value of inductance. If single-turn inductor is necessary in accordance with technological operation conditions (for example, strength and frequency!), then artifact increasing of the installation usage coefficient has been reached in the magnetic pulse metals working at the expense of the pulse transformer connection (the matching device).
The calculation of the inductor field usage coefficient is not simple problem in a total case. In the scientific literature there are some different suggestions how to evaluate this parameter. But all they have adequately limited area of application.
For example, in the case of the sheet stamping the flat inductor field usage coefficient can be found as a ratio of the inductor surface area from the workpiece side to the all working zone area what is being determined by overall dimensions of workpiece to be stamped. But it is a rough estimate enough.
Finally, the second correlation in the (4.3) fixes the obvious fact
(from the physical view point): for decreasing of an energy dissipation
in the magnetic pulse busbar the last one must be made from metals with
a high value of electrical conductivity. The same recommendation is justify
for the inductor systems windings.
4.2. The Characteristics of the Magnetic Pulse
Installations main Elements.
A brief characteristic of the magnetic pulse installations main units was givenin preceding part under description of its function scheme. Now we will stop in a detail and consider those elements what fix an efficiency and economy of the whole production operation with the magnetic pulse energy application.
The most important unit of any installation is the CAPACITOR BANK. It represents an electrical battery of capacitors.
As a rule, the capacitors being applied in the magnetic pulse installations operate in a regime of an oscillation discharge with frequency (5 ó 200) kHz. The high voltage capacitors must satisfy the main requirement what consists in a possibility to store the maximum energy in a volume unit. The field working intensity in the insulation and the electrical permeability of the dielectric material fix this parameter value.
The working intensity of capacitor is being limited by the following circumstances in the main: a development of the partial discharges what destroys a dielectric insulation and a capacitor heating in time of its operating.
Depending upon the exploitation regime the working intensity choice is being carried out in accordance with the first distinguished circumstance under few repetition of discharges or with the second limitation under great frequency of the pulse repetition.
As the experience shows an inner inductance of capacitor must be limited by the reasonable ultimate value. Usually, the total inductance of capacitor battery in the magnetic pulse installation must be no less 10% of the whole scheme total inductance in the short-circuiting regime.
The capacitors have to be designed for the currents with amplitudes 100 ó 500 kA. Therefore, all contact connections and electrical outputs must have a high dynamical stability. This requirement is being provided by the well known contact pairs "copper ó copper", "copper ó aluminum", "aluminum ó steel" and other.
Now we shall pass to description of the capacitors main parameters. Their values permit to establish an accordance of capacitors to the operation requirements.
A working intensity of capacitor. Practically, a capacitor heat regime can not be broken under a little frequency of pulse repetition. In this case the processes of electrical deterioration with time will fix a term of operation solely. In these processes of the insulation destruction, so named, partial discharges play the main role. On the electrode edges they have a view of the crown or glancing discharges.
The initial and critical partial discharges are being distinguished. The critical partial discharges determine some level of the capacitor permissible intensity. It has to be lower than the electrical field intensity amplitude what is sufficient for existence of the critical partial discharges what lead to the fast destruction of the capacitor insulation.
If capacitor is being intended for operation with high frequency repetition of cycles "charge-discharge" then its working intensity level must be revised with additional allowance for a gas emission or heat processes.
Ultimately, the working intensity of capacitor establishes a voltage nominal level and its operation term.
A mutual connection of these parameters was got experimentally. Its graphical dependence is presented on Fig.4.3. The curve is built in the relative units. The 100% operation term accords to the nominal voltage of the capacitor.
Because the capacitor bank is the main functional block, its service term fixes the magnetic pulse installation operation life in the whole. Therefore, the curve on Fig.4.3 is useful in the visualization of the service term in a functional dependence on the working regimes for the time of exploitation.
A specific energy of capacitor. This parameter characterizes the ability of capacitor to store the energy. In the time of practical calculations they distinguish the specific energy of active volume W1 and the specific energy of the capacitor full volume W2.
There is known formula for calculation of the first parameter:
(4.4)
where
The specific energy of the capacitor full volume is being fixed by the relationship: is the absolute electrical permeability;
E is the working intensity of electrical field.
(4.5)
where V is the volume of capacitor with capacitance C and working voltage U. The value of W2 is usually less than W1.
For the best capacitors, which fit for usage in the pulse current generators, it accounts for around (50 ó 70)% of W1.
An inductance of capacitor. As a rule the pulse capacitors for the magnetic pulse installations operate in the oscillator regime of capacitor discharge into inductive load. During this process the equivalent frequency of discharge represents always a resonance frequency in a circuit with this capacitance and total inductance what includes the internal and external components.
In the whole a capacitor self-inductance value consists of separate elements inductance: the sections, the internal connections and the external outputs. Ultimately, the self-inductance is being defined by constructions of these elements and also by the capacitors electrical scheme of connection in the battery.
Fig. 4.3
The capacitor operation term (the discharges quantity Z) dependence on the charged voltage under and the frequency 30 kHz.
We will not dwell on these problems, because in the main they are problems for specialists in the capacitorís designing. They are being solved in accordance with a technical task for capacitor bank elaboration in the every concrete case.
In the whole, practice of the magnetic pulse metals working shows the more advantageous usage of the little inductive capacitors. In this case some possibilities appear for variation of the working frequencies within wide range quite.
A working voltage of capacitor bank. A choice of this parameter is being fixed by the value of energy stored and by the magnitude of necessary magnetic pressure amplitude on the workpiece to be deformed.
The stored energy can be found as a ratio of the deformation work what is necessary according to the production operation conditions and to the efficiency in the formula (4.1).
The value of the magnetic pressure towards the workpiece is connected with the deformation work too and it can be calculated if the concrete production operation conditions are predetermined.
The working voltage - Uw determines a frequency of the installation discharge circuit. If the stored energy is fixed, the following dependence is justifiable:
where
(4.6)
The value is the working angular frequency;
W3 is the stored energy of the capacitor bank;
L is the total inductance of the discharge circuit.
The stored energy, the working voltage and the nominal energy of one capacitor permit to fix their total quantity and to calculate all necessary parameters of the battery.
In the whole the capacitor bank calculation is being realized according to known well iteration scheme. Its essence consists in the following: the demanded data are being specified beforehand, then the approximate evaluations of the working process are being executed, after this the calculation results are being taken as initial data and so on till getting of the necessary values of the final parameters.
A CHARGING DEVICE providing the capacitor bank operation is the second important element of the magnetic pulse installation.
In the general we have said about the requirements to the charging devices and about the different variants of their scheme solution. We will add the previous information by some data and recommendations following from the practical experience of work with devices like those.
So a comparison of the different charging schemes shows that efficiency will be equal to 0.3 ó 0.5 under located voltage of source. But in the case of charge under invariant current power supply this parameter will be equal to 0.5 ó 0.9.
However, from economical point of view the last scheme with fixed current source is not optimum one always. The design and exploitation of the different charging devices in the Kharkov State Polytechnical University have shown that the single phase circuit with constant charge voltage is preferable in selecting the charging devices for the magnetic pulse installations.
As a rule, silicon, germanium, selenium semi-conductive elements and also thyratrons, gasotrons, high voltage kenotrons are used as rectifiers in the charging devices. As the design experience of the magnetic pulse installations showed the silicon semi-conductive elements are preferable from the practical point of view. The charging device with present rectifiers are the most compact, effective and not expensive systems.
As it was shown before the control process in the discharge circuits of the magnetic pulse installations is being executed by the different COMMUTING DEVICES. The three-electrode air discharges (trigatrones) and the ignitrone switches have found the widest application. The vacuum discharges have not found application in the magnetic pulse installations despite of obvious advantages if to compare with the air discharges (for example, this is a longer service life, a low self-inductance and other!). There are some defining reasons here. The main of them is necessity in the careful and constant technical service what prevents from the such commutators application in the magnetic pulse installations intended for the mass and high productive manufacture.
As a rule, in the magnetic pulse installations the discharges are being used which were specially manufactured in accordance with the technical task conditions. At the Kharkov State Polytechnical University the different constructions of the three-electrode air discharges were elaborated.
Not fixing on a detailed description of such commuting devices we will enumerate their basic electrical parameters only:
- the self-inductance 50 nH; - the working voltage (4 ó 25) kV;
- the output capacity up to 360 operations/hour;
-the maximum discharge energy up to 30 kJ.
These figures can serve as a rough guide for evaluations of the magnetic pulse installations characteristics.An operation of discharges in the magnetic pulse installations is being provided by some special control systems named as ignition device. Their principle action consists in feeding of a voltage pulse to one of discharge electrodes (the ignition electrode). This pulse is being formed in circuit of the control system with help of special capacitor and the step-up transformer. Usually, in the magnetic pulse installations the control system is equipped with an automation block. This device stops the charge process and gives simultaneously the command to fire the discharge when the stored energy of capacitor bank achieves its ordered level.
The commutator shorts out the installation discharge circuit. The capacitor bank is discharging in the inductor winding where the magnetic pulse field is being executed. It is acting on the workpiece to be deformed.
4.3. The Magnetic Pulse metals Working Installations
designed by the leading Firms in the World.
The presented hereunder material must illustrate the state of the magnetic pulse technology development in the modern stage of the science-technical progress by the concrete examples of installations which were designed in different time by the leading electrical engineering firms of the world.
Let us start from the history.
The soviet academician Kapitza P.L. was the first to suggest an idea of the magnetic pulse fields forces usage for some production purposes, in the 1920's. He has paid an attention of the scientific responsible opinion to phenomena of the power forces appearance in the electrical engineering plants for generation of the pulse fields with high amplitudes of intensities. In that time these fields were being used for investigations of the substances physical properties change under the strong magnetic fields action.
The first efforts where the magnetic pulse fields were used for some technological aims should be related to the early 1960's. In this time the pulse current sources with the low inductive capacitors existed already. That is why the plants according of purpose was appearing and becoming widely spread relatively.
The first magnetic pulse installation has been demonstrated by the US firm "General Dynamics Corporation" during the Second International Conference (The Nuclear Energy for The Piece Goals) in Geneva in 1958 year. It was maintained by the inductor systems set for the metal cylindrical pipe working.
In the former USSR the similar magnetic pulse installation has been designed and adopted in the industry by the scientists of the Lenin V.I. Kharkov Polytechnical Institute (it is the Kharkov State Polytechnical University for the time being) in 1964.
In the different time the leading electrical engineering firms of the world were conducting the works as to creation of the technological processes where the magnetic pulse fields energy had been used.
In the mid-1960's the firms of Germany (such as Siemens, Hahn and Kolb), the Laboratories of Great Britain (Wickmen, Machine Tools), the French plants (Generale DíElectricity), The Switzerland Scientific Center (General Atomic Europe), the Japan companies (Mitsubisi Danky, Dzankusu Cabusiky Kise) and others.
The greatest successes in area of the magnetic pulse technique creation (for the production goals) have been achieved in the USA. So, the first industrial sample installations were even designed by the firm "General Dynamics Corporation" in 1962. These installations have been working at the plants of aviation, automobile and electrical engineering industries.
The main technical characteristics of the magnetic pulse installations are presented in the tables 4.1 ó 4.3. In the different time they were designed in the former USSR, in the eastern and western European countries and, finally, in the USA.
Finishing our historical commentaries we would like to mark that the presented material does not contain all interesting information, and rejects a little part of the most successful constructive solutions for creation of the magnetic pulse installations for different purposes.
Nevertheless, from our point of view the presented data give the full enough imagination about the stage and capacity of the modern pulse engineering for realization of the progressive working technology. From our view point the conditionally "averaged" characteristics permit even to predict the stage of the technical equipment (the simplicity, the reliability, the cost) which can be taken for wide-spread application in industry where magnetic pulse fields energy are being used.
|
-designer |
type |
stored, kJ |
voltage, kV |
of discharge circuit, kHz |
operations/ /hour |
type |
switches |
type |
dimensions, mm3 |
kg |
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1950/1650/1850 Technological block: 720/1300/900 |
4200 |
general |
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1420/1280/1700 Charging device: 1000/370/1700 Control block: 390/250/1000 |
2380 |
special |
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1420/1280/1700 Charging device: 1000/370/1700 Control block: 390/250/1000 |
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ENCK |
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2660/1810/2150 Charging device: 1240/770/2150 Control block: 390/250/1000 |
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4000/1810/2150 Charging device: 1240/770/1700 Control block: 390/250/1000 |
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Extension to table 4.1.
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VNIIElectro |
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busbar |
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commutator |
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busbar |
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pribor |
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917/810/1980 Power and control block: 790/480/1980 |
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Table 4.2. The magnetic pulse installations designed by
some European firms.
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type |
stored, kJ |
voltage, kV |
of discharge circuit, kHz |
type |
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Germany |
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technics, Germany |
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Institut, Prague, Czechoslovakia |
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Institut, Warsaw, Poland |
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Table 4.3. The magnetic pulse installations designed by
US firms.
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type |
stored, kJ |
voltage, kV |
of discharge circuit, kHz |
operations/ /hour |
type |
switches |
type |
dimensions, mm3 |
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installation for welding |
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busbar |
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