On Reducing Losses in the Steel of a Solenoid Hammer
Abstract Generated abstract
This paper examines losses in the steel magnetic circuit of a solenoid hammer, with emphasis on eddy current heating of the striker and ways to improve efficiency without adding cooling equipment. Using calorimetric measurements on an MS-5 hammer, the authors compare striker materials, heat treatments, and coil supply circuits. The results indicate that pressed powdered Armco iron gives the lowest magnetic-circuit losses, annealing after machining reduces losses further, and hardened 5KhVS steel is preferable for the striker. A two-half-period rectifier supply is also recommended, since a half-wave circuit increased striker losses by about 20 to 25 percent in the tests.
Full Text
ON THE QUESTION OF REDUCING LOSSES IN THE STEEL OF A SOLENOID HAMMER
N. P. Ryashentsev, A. V. Frolov
When a magnetic flux passing through continuous conducting masses undergoes changes in them, eddy currents arise. In this case it is immaterial whether the change in flux is the result of the motion of conducting masses in a magnetic field or whether it occurs because the flux varies with time; that is, eddy currents are a special case of the phenomenon of mutual induction.
In accordance with Lenz’s rule, eddy currents tend to oppose the changes that cause them. Therefore the eddy currents’ own magnetic field always tends to weaken the change in the main magnetic flux. The presence of eddy currents is accompanied by the release of heat, and since this heat is not utilized, the power corresponding to it is called eddy-current loss.
In addition to eddy-current losses, there are hysteresis losses in the magnetic circuit. Without remagnetization of the magnetic circuit and when the coils of a solenoid hammer are supplied according to the circuit given in [1, 2], hysteresis losses amount to 3–5% of the losses during remagnetization of the magnetic circuit.
In existing designs of solenoid hammers, losses in the steel constitute a significant part of all losses.
As a result, the magnetic circuits of hammers, even during brief operation, heat up to 30–40°C, and the moving part—the striker—up to 100–120°C. The heating of a hammer can be reduced in two ways:
1) by intensive cooling of the hammer;
2) by reducing the losses in the steel of the magnetic circuit.
The first method leads to complication and increased weight of the design. The second is more desirable, since it means increasing the efficiency of solenoid hammers.
To reduce eddy-current losses, and also to reduce the induction of a counter electromotive force, one should strive for the greatest possible attenuation of eddy currents. The latter can be achieved by making the magnetic circuit from thin sheets of electrical steel, electrically insulated from one another. This simple and reliable method of reducing eddy-current losses is unacceptable for solenoid hammers because of the great labor intensity of manufacturing the magnetic circuit and the impossibility of making the striker from sheet electrical steel.
But there is another, no less effective way to reduce eddy-current losses—reducing the specific electrical conductivity of the magnetic-circuit material, or using ferromagnetic materials with lower specific electrical conductivity.
The heating of solenoid hammers, caused by large losses in the steel of the magnetic circuit, is of decisive importance in the design of hand tools. We have established that in a hammer the striker is subjected to the strongest heating.
The present work on selecting a ferromagnetic material and reducing losses in the steel of solenoid hammers was carried out at the Department of Mining Machines and Mine Transport of TPI. The losses in steel were determined by a calorimetric method.
Strikers of diameter \(d = 34\) mm and length \(l = 100\) mm, made from various materials, were tested.
To reduce the magnitude of the experimental error, the following was taken into account:
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To avoid additional heating of the striker due to the heat released when the striker struck the end of the pick and when the striker rubbed against the guide tube, the position of the striker during the experiments was strictly fixed.
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The temperature of the striker and of the guide tube of the solenoid hammer before the experiment were identical.
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The power consumed by the hammer was determined with a Weston Normal Wattmeter, with a relative error of \(\pm 0.2\%\).
The coils were supplied according to a two-half-period circuit [2].
The losses in the steel of the striker were determined from the heat-balance equation by the formula
\[ P_{\mathrm{s.b}} = \frac{4.186}{t}\left(t_k^0 - t_0^0\right)\left(m_{\mathrm{v}} + C_k m_k\right)\ \mathrm{W}, \]
where \(P_{\mathrm{s.b}}\)—losses in the steel of the striker, kg;
\(t_0\)—time of operation of the hammer, min.;
\(t_k\)—maximum temperature of the water in the calorimeter after the experiment, \({}^{\circ}\mathrm{C}\);
\(t_0^0\)—initial temperature of the water in the calorimeter, \({}^{\circ}\mathrm{C}\);
$m_{\mathrm{w}}$ — mass of water in the calorimeter, g;
$C_{\mathrm{k}}$ — heat capacity of the calorimeter, cal/g·deg;
$m_{\mathrm{v}}$ — mass of the inner vessel of the calorimeter with stirrer, g.
Along with the choice of ferromagnetic material, attention was paid to the influence of its heat treatment and to the power-supply circuit of the hammer coils.
The test results are shown in Figs. 1 and 2.
In the curves presented (Figs. 1 and 2), the loss coefficient $K$ determines the ratio of the losses in the steel of the striker to the power consumed by the solenoid hammer, i.e., it characterizes the relative magnitude of the losses. The experiments were carried out on an MS-5 solenoid hammer.
Technical characteristics of the MS-5
| Parameter | Value |
|---|---|
| Energy of a single blow, kg·m | $A = 1.3$ |
| Number of blows per minute | $n = 1500$ |
| Power consumed, kW | $P = 1.2$ |
| Number of coils, pcs. | $m = 2$ |
| Number of turns in one coil, turns | $W = 1050$ |
| Weight of hammer, kg | $G = 11.5$ |
| Efficiency, % | $\eta = 26.7$ |
Figure 1 presents the results of tests of ferromagnetic materials that had not undergone heat treatment; Fig. 2, of heat-treated materials.
In testing strikers made of ferromagnetic materials without subsequent treatment, it was found that when the power consumed by the solenoid hammer was increased from 1.8 to 2.8 kW, the losses in the steel of the striker changed only slightly (Fig. 1). From the same figure it is evident that the smallest losses are exhibited by strikers made of Armco-type powdered iron, pressed under pressure. Of the structural steels tested, steel 45 has the smallest losses. But even in this steel the losses, compared with Armco powdered iron, are more than twice as large (at a hammer power consumption of 0.6–2.8 kW). This is explained by the following:
1) the density of Armco powdered iron is 1.57 times less than the density of steel 45;
2) the air gaps formed during pressing of the Armco iron powder significantly reduce eddy currents.
Thus, the magnetic circuit of solenoid hammers should be made of Armco powdered iron, especially since it lends itself well to machining. For the striker, steel 45 will be more suitable.
In the process of machining parts of the magnetic circuit, local changes in structure arise in the latter, leading to a deterioration of the magnetic properties of the material: the magnetic permeability decreases...
permeability decreases, the coercive force increases, and losses due to eddy currents and hysteresis rise. Therefore, after mechanical machining, the parts of the magnetic circuit must be annealed. When the hammer striker made of Armco powdered iron is annealed at a temperature of 900–950°C, the losses in it decrease by one and a half times (Figs. 1, 2).
The value of the coercive force after annealing Armco iron decreases from 1.3 to 1.08 oersted/cm.
Quenching 5KhVS steel to a hardness of \(H_{R_c} = 45\) reduces the losses in the striker steel by 50–65% (depending on the power consumed), while quenching steel 45 to \(H_{R_c} = 60\) reduces the losses in the striker steel by only 10% (Figs. 1, 2) in comparison with the same steel but without heat treatment.
Fig. 1. Dependence of losses in steel on the power consumed by the hammer for unhardened strikers:
1—pressed Armco-type iron; 2—steel 45; 3—5KhVS steel; 4—U10 steel; 5—steel 3.
Thus, from Fig. 2 it is seen that it is most expedient to use 5KhVS steel for manufacturing the striker, with subsequent quenching to a hardness of \(H_{R_c} = 45\), which can fully ensure the established operating regime of the striker and at the same time reduce the losses in it.
Analysis of curves 1 and 3 (Fig. 2) shows that the power-supply circuit of the solenoid-hammer coils also affects the magnitude of losses in the striker steel. When the coils of the solenoid hammer are supplied according to a half-wave circuit, the losses in the striker steel
increase by 20–25% in comparison with the two-half-period circuit for supplying the coils [1, 2]. This is explained by the fact that eddy-current losses are proportional to the square of the form factor of the voltage curve.
The investigations carried out on the choice of materials and on reducing losses in the steel of the magnetic circuit make it possible to draw the following conclusions:
Fig. 2. Dependence of losses in steel on the power consumed by the hammer for hardened rams:
1 — pressed Armco iron with subsequent annealing; 2 — steel 5KhVS \((H_{R_c}=45)\); 3 — pressed Armco iron when the hammer is supplied according to a one-half-period circuit; 4 — steel U10 \((H_{R_c}=58)\); 5 — steel 45 \((H_{R_c}=60)\).
1) after machining, the parts of the magnetic circuit of the solenoid hammer should be subjected to heat treatment—annealing, and the ram should be hardened along its entire length;
2) as the material for the magnetic circuit, powdered Armco iron should be used;
3) it is advisable to manufacture the ram from steel 5KhVS;
4) for supplying the coils of the solenoid hammer, it is desirable to use a circuit with two-half-period current rectification [2].
The investigations carried out show that the efficiency of solenoid hammers can be increased by reducing losses in the steel of the magnetic circuit and thereby significantly decreasing the heating of the hammers.
REFERENCES
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P. M. Alabuzhev, N. P. Ryashentsev. On the question of creating solenoid hammers. Izvestiya TPI, vol. 106. Metallurgizdat, 1958.
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N. P. Ryashentsev. Synchronous sparkless switch. Izvestiya TPI, vol. 106. Metallurgizdat, 1958.
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F. A. Stupel. Electromechanical Relays. Gosenergoizdat, 1950.
Noted Misprints
| Page | Line | Printed | Should be | By fault of |
|---|---|---|---|---|
| 96 | Fig. 2 | $n = 0.6 \sqrt{\dfrac{Q \cdot p}{m \cdot S}} +$ | $n = 0.6 \sqrt{\dfrac{Q \cdot p}{m \cdot S}}.$ | author |
| 153 | 21 from top | $l_k - 70\ \text{mm},$ | $l_k = 700\ \text{mm},$ | typ. |
| 160 | 12 from top | with wear | with wear | » |
| 182 | Fig. 2,a | interchange the numerals 2 and 3 | interchange the numerals 2 and 3 | author |
| 189 | 8 from top | fixation | fixed | » |
| 198 and 199 | Formulas (14) and (16a) | $q$ | $q_n$ | » |
| 200 | 14 from top | coefficient of the amplitude of the curve | coefficient of form | » |
| 205 | 12 from top | $\sigma$ and $\sigma$ | $\sigma_b$ and $\sigma_g$ | » |
| 208 | 2 from top | $d_{n1} = \dfrac{4.186}{t}$ | $D_{n1} = \dfrac{4.186}{t_0}$ | » |
| 212 | 6 from bottom | of the second stage | gears of the second stage at | » |
| 276 | 15 from bottom | $v_1$, $v_2$ and $v_3$ | $V_1$, $V_2$ and $V_3$ | » |
| 287 | 14 from bottom | $\dfrac{2t_0 + t'_{\mathrm{sh}} + t'_{\mathrm{sh}}}{L}$ | $\dfrac{2t_0 + t'_{\mathrm{sh}}}{L}$ | typ. |
| 287 | Formula (12) | $\dfrac{2t_0 + t''_{\mathrm{sh}}}{L}$ | $\dfrac{2t_0 + t'_{\mathrm{sh}} + t''_{\mathrm{sh}}}{L}$ | author |
| 287 | Formula (13) | » | ||
| 292 | 2 from top | (curve 1, | (curve 1′, | » |
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